mama-trend: trends, challenges and emergent new phenomena...

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"MAMA-Trend: Trends, challenges and emergent new phenomena in multi-

functional materials”!Sorrento (NA), May 20th-23rd, 2013

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MAMA-Trend “Trends, challenges, and new emergent phenomena in multifunctional materials” is the last and most important event organised within the EU-MAMA project “MAMA: unlocking research potential for multifunctional advanced materials and nanoscale phenomena" (Grant Agreement No. 264098.). It follows three topical workshops (at Ercolano, “MAMA-synt” Sept. 2011, in Vietri sul Mare, “MAMA-Protheo” March 2012, and Ischia, “MAMA-Hybrids”, Oct 2012) that have collected a large interest from the scientific community and a wide number of participants.

The conference has been planned to promote a strong synergy between researchers involved in the synthesis, advanced analyses and modelling of functional materials and related heterostructures with a special emphasis on those materials which exhibit a wide variety of physical phenomena ranging from superconductivity, magnetism to multiferroicity, spin-orbital-lattice orderings, quantum topological order, etc. To amplify the interest for the event and in order to cover at large the objectives of the MAMA project, the conference has been designed in four symposia:

• Unconventional superconductivity: materials, pairing mechanisms and physical properties; • Spin, charge and orbital ordering, phase transitions and quantum critical behaviour in

correlated electron systems; • Dielectrics, ferroelectrics and multiferroics; • Emerging phenomena at surfaces and interfaces in correlated electron systems.

Due to the MAMA-Trend constituents, we expect such event to have all the marks for being a lively event with a stimulating interaction between the involved parts and focused on the latest breakthroughs in the exciting and highly dynamic research field of novel (multi)functional materials.

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Conference Chairmen

M. Cuoco (CNR SPIN, Italy)

F. Miletto Granozio (CNR SPIN, Italy)

A. Vecchione (CNR SPIN, Italy)

Symposia Chairmen

Symposium 1 - Unconventional superconductivity: materials, pairing mechanisms and physical properties

C. Ferdeghini (CNR SPIN, Italy)

D. Larbalestier (Florida State University, Tallahassee, USA)

A. Ustinov (Karlsruhe Institute of Technology, Germany)

Symposium 2 - Dielectrics, ferroelectrics and multiferroics

S. Picozzi (CNR SPIN, Italy)

M. Bibes (Unité Mixte de Physique CNRS/Thales, Palaiseau Cedex, France)

J. Fontcuberta (Institut de Ciència de Materials de Barcelona, Consejo Superior de Investigaciones, Barcelona, Spain)

Symposium 3 - Spin, charge and orbital ordering, phase transitions and quantum critical behaviour in correlated electron systems

J. Van den Brink (IFW Dresden, Germany)

P. Calvani (Università La Sapienza, Roma, Italy)

R. De Renzi (Università di Parma, Italy)

Symposium 4 - Emerging phenomena at surfaces and interfaces in correlated electron systems

G. Balestrino (Università di Roma Tor Vergata, Roma, Italy)

A. Kalaboukhov (Chalmers University of Technology, Göteborg, Sweden)

P. Rudolf (Rijksuniversiteit Groningen, The Netherlands)

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Organizing Committee

E. Di Gennaro (CNR SPIN and Università di Napoli, Italy)

R. Fittipaldi (CNR SPIN, Italy)

P. Gentile (CNR SPIN, Italy)

L. Parlato (CNR SPIN and Università di Napoli, Italy)

MAMA Workshop Coordinator

C. Cirillo (CNR SPIN, Italy)

Workshop Organizing Agency

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WEB Site

Maurizio Cembalo (CNR SPIN, Italy)

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Attending Companies

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Table of Contents

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Conference Program

!

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Monday, May 20th, 2013 08.45

Opening

09.00

Van

den

Br

ink

P lenary Session D. Khomskii Coupled electricity and magnetism in solids: microscopic mechanisms and some novel effects

Chairs

Symposium 1

Chairs

Symposium 2

Superconductivity: materials, physical properties and applications

Dielectrics, ferroelectrics and multiferroics

09.40

Larb

ales

trie

r

M. Beasley

Font

cube

rta

Y. Taguchi Unconventional Proximity Effects Relevant to High-Tc Superconductivity

Strong coupling between magnetism and polarization in multiferroics

10.10

R. Green P. Barone What do electron-doped cuprates tell us about Cooper pairing in high-Tc superconductors?

Ferroelectric and multiferroic twin walls in CaTiO3 and CaMnO3

10.30

D. Di Castro V. Kocsis High Tc superconductivity in single interface of cuprate/titanate heterostrucures

Spin and lattice dynamics in multiferroic TbFe3(BO3)4

10.50 Coffee break

11.10

Larb

ales

trie

r

M. Snelder

Bala

kris

hnan

I. Fina Josephson supercurrent through a topological insulator surface state

Phase coexistence and magnetically-tuneable polarization in cycloidal multiferroics

11.40

W. Belzig D. Szaller Thermoelectric effects and nonlocal Onsager relations in a proximity-coupled superconductor-ferromagnet

Magnetic field enhanced directional dichroism in the spin resonances of the multiferroic Ca2CoSi2O7

12.00 A. Golubov A.K. Zvezdin Designing phase-sensitive tests for Fe-based superconductors

Spin flexoelectricity in multiferroic films

! !

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12.20 Lunch

14.30 U

stin

ov

F. Parmigiani

Rudo

lf

P. Lecoeur Out of Equilibrium Gap and Pseudo-gap behavior in Copper-Oxide based Superconductors

Strain control of domain wall pinning in hybrid piezoelectric-magneto-resistive nanodevices

15.00

P. Marra A. Petraru RIXS as a Probe of the Phase and Excitations of the Superconducting Order Parameter

Tuning the magnetoelectric properties of heteroepitaxial CFO/SRO/PZT/SRO films grown on piezoelectric PMN-PT substrates via strain

15.20

P. Orgiani D. Robbes Spin fluctuations regime extended up to room temperature in electron doped cuprates

PZT thin films capacitors: a way to sense Electric field ?

15.40

F. Romeo J. Schwarzkopf Magnetoresistance and spin-transfer torque in hybrid superconductor/ferromagnet structures

Anisotropic domain formation in NaNbO3thin films under tensile lattice strain

16.00 Coffee break 16.30 Poster session

! !

! !

!2"#!

Tuesday, May 21st, 2013

09.00 Ust

inov

P lenary Session Y. Maeno

Topological Superconductivity

Chairs

Symposium 1

Chairs

Symposium 2



09.40

Vag

lio

A. Ryazanov

Pico

zzi

V. Garcia Spin-splitting and magnetic switching in hybrid superconductor-ferromagnet structures

Large tunnel electroresistance controlled by ferroelectric domain switching in BaTiO3 and BiFeO3- based ferroelectric tunnel junctions

10.10

G. Ovsyannikov T. Archer Triplet superconductivity in heterostructures with oxide magnetic interlayers

Magneto-electric effect in multiferroic tunnel junctions from first principals

10.30

A. Sidorenko E. Plekhanov Detection of the Triplet Spin-Valve-Effect in Superconductor/Ferromagnet Proximity Coupled Layered Heterostructures

Co/Fe/BaTiO3 junctions - a new type of multiferroic interface

10.50 Coffee break

11.10

Att

anas

io

T. Dahm

Mar

itat

o

D. Sando Influence of surface Andreev Bound States to the Nonlinear Meissner Effect in d-wave superconductors

Electro-Optical Response and Strain-Dependent Optical Properties of Multiferroic BiFeO3 Thin Films

11.40

D. Manske J. Aarts Theory for Collective Modes and Interface Effects of non-centrosymmetric Superconductors

Low Energy Electron Microscopy for the study of oxides surfaces

12.00

P. Gentile V. Fiorentini Magnetic edge states in spin triplet superconductors

Dimensional crossover and thermopower burst in Nb:SrTiO3 superlattices

! !

!2#!

12.20 Lunch

14.30 Pe

pe

K. Machida

Sall

uzzo

C. Mazzoli Strongly correlated material UPt3 as a topological superconductor

EuTiO3 magnetoelectric properties investigated by neutrons and X-rays scattering experiments

14.50

P. Lucignano J. Hlinka Majorana Fermions in high Tc superconducting hybrid devices

Assigment of polar modes of BiFeO3resolved

15.10

S. Charpentier A. Caretta Superconducting proximity effect in the Topological Insulator Bi2Te3

Dynamic energy flow in a hybrid organic-inorganic crystal

15.30 Coffee break

15.50

Ferd

eghi

ni

R. De Renzi

Pico

zzi

J. Íñiguez The onset of magnetism around x=1/4 in LnFe1-xRuxAsO1-yFy (Ln = Sm,Nd or La) optimally electron-doped superconductors

Ferroelectric domain walls and other complex structures in BiFeO3 and related materials

16.20

L. Simonelli J. L. García-Muñoz Coexistence of different electronic phases in the K0.8Fe1.6Se2 Fe-based superconductor

Competing Ferroelectric Phases and Incommensurate Magnetic Orders in Multiferroic Mn1-

xCoxWO4

16.40

S. Pagano F. Kadlec Perspectives for electronic applications of iron-based superconductors

Electromagnon in the pyroelectric ferrimagnet !-Fe2O3

17.00

V. Braccini J. M. Rondinelli Tuning superconductivity and pinning mechanisms of Fe(Se,Te) thin films through different substrates

Crystal Engineering of Ferroelectric 214-Ruddlesden-Popper Phases

Social dinner

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Wednesday, May 22nd, 2013 !

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09.00

Rudo

lf P lenary Session

D. Schlom

Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics with record

Chairs

Symposium 3

Chairs

Symposium 4

Magnetic oxides and strongly correlated systems Interfaces and heterostructures

09.40

Cal

vani

R. Cava Ka

labo

ukho

v B. Kalisky

Crystal Chemistry and Structures of Topological Insulators

Enhanced conductivity due to tetragonal domain structure in LAO/STO heterointerfaces

10.10

S. Lupi M. Dildar Plasmonic Excitations in a Topological Insulator

Conductance and magnetism of LaAlO3/SrTiO3 grown in high oxygen pressure

10.30

I. Vobornik G. Herranz Role and electronic character of magnetic impurities

High mobility conduction and superconductivity at (110) and (111) LaAlO3/SrTiO3 interfaces

10.50 Coffee break

11.10

Cir

illo

J. Fontcuberta

Mil

etto

Gra

nozi

o

A. Rosenberg Electron distribution among 3d-eg orbitals in manganite thin films

Investigation of Current Channels at the Interface between Complex Oxide Heterostructures

11.30

N.M. Nemes Y. Chen Orbital polarization at complex oxide hetero-interfaces, an angular magnetoresistance study

Two-dimensional electron gases at a spinel/perovskite complex oxide heterointerface with electron mobilities exceeding 100,000 cm2V-1s-1

11.50

A. Nucara S. Gariglio Destabilizing an ordered manganite: optical properties of self-doped and Ga substituted thin films

Phase coherent transport in the 2DEG at the LaAlO3/SrTiO3 interface

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12.10 Lunch

14.30

Van

den

Bri

nk

G. Ghiringhelli

Win

kler

A. Santander Magnetic and orbital properties of interfaces and superlattices probed by resonant soft x-ray spectroscopy

Novel 2D electron gases at the surface of transition-metal oxides

15.00

J. Teyssier B. Cedric Optical and structural evidences of exotic spin-orbital order in tetragonal Sr2VO4

A two-dimensional electron gas with honeycomb electronic structure at the (111) surface of KTaO3

15.20

M. Baghaie Yazdi N. Plumb Study of the Verwey transition in magnetite thin films

Mixed Dimensionality of Confined Conducting State on SrTiO3 Tied to Ferroelectric Surface Distortion

15.40

C. Hicks U. Scotti di Uccio Quantum oscillations and high carrier mobility in the delafossites PdCoO2 and PdCrO2

Origin of the 2DEG at the LAO/STO Interface

16.00 Coffee break

16.20

De

Renz

i

J. Geck

Pryd

s

N. Reyren Determining Short-Range Spin Correlations in Cuprate Chain Materials with RIXS

Spin injection at LaAlO3/SrTiO3 interfaces

16.50

C. Autieri D. W. Jeong Structural and electronic properties of Sr2RuO4-Sr3Ru2O7 heterostructures

Dimensionality control of spin state in LaCoO3/LaAlO3 superlattices

17.10

S. Caprara A. Leveratto Rashba spin-orbit driven intrinsic charge instability in oxide heterostructures

2-dimensional electron gas at the interface of oxide semiconductors: (Mg, Zn)O/ZnO heterostructures

17.30

E. Gorelov U. Leuders Magnetism in layered ruthenates: LDA+DMFT study

Geometrically confined doping in LaVO3/SrVO3 superlattices

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Thursday, May 23rd, 2013 !

09.00 Larb

ales

trie

r

P lenary Session G. Sawatzky

Magnetic materials based on defects, interfaces and O2p holes

Chairs

Symposium 3

Chairs

Symposium 4

Magnetic oxides and strongly correlated systems Interfaces and heterostructures

09.40

Cla

eson

A. T. Boothroyd

Papa

ro

W. Prellier Unravelling emergent order in charge-ordered oxides

Functional oxides films: from single crystals to polycrystalline substrates

10.10

M. Mazzani S. Amoruso Stripe dynamics in the La2-

xSrxCoO4layered cobaltates by 59Co and 139La NMR and "SR

PLD of amorphous-LaAlO3/SrTiO3interfaces: correlating plasma plume and interface conduction

10.30

E. Di Gennaro P. Aurino Coexistence of superconductivity and magnetism in 2-dimensional electron gases at oxide interfaces

Metal-insulator transition in LaAlO3/SrTiO3 interface induced by Ar+ ion-beam irradiation

10.50 Coffee break

11.10

Noc

e

S. Boseggia

Aru

ta

C. Cancellieri Robustness of basal-plane antiferromagnetic order and the

Interface Fermi states of LaAlO3/SrTiO3and related heterostructure

Jeff = 1/2 state in single layer iridates

11.30

W. Brzezicki A. Koitzcsh Exotic spin orders driven by orbital fluctuations in the spin-orbital model for d electrons

Soft x-ray photoemission studies of LaAlO3/SrTiO3 and NdGaO3/SrTiO3interfaces

11.50

G. Zhang Z. Zhong Importance of exchange anisotropy and superexchange for the spin-state transitions in RCoO3

Theory of spin-orbit coupling at LaAlO3/SrTiO3 interfaces and SrTiO3surfaces

12.10 Lunch

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14.30

Vec

chio

ne

J. Lorenzana

Kala

bouk

hov

K. Garello Magnetic phases and high-Tc superconductivity: A love/hate story

Symmetry and magnitude of spin-orbit torques in ferromagnetic heterostructures

15.00

A. Avella Can Onur Auci Defect states and excitations in a Mott insulator with orbital degrees of freedom

Annealing study of spin-orbit torques in perpendicularly magnetized Ta/CoFeB/MgO layers

15.20

G. Giovannetti G. Koster Orbital Selectivity in heavy-fermion d-electron materials

High-Temperature Magnetic Insulating Phase in Ultrathin La0.67Sr0.33MnO3 Films

15.40

A. Oles Y. Boikov Fingerprints of spin-orbital entanglement in transition metal oxides

Influence of Double-TiO2 Terminated (001)SrTiO3 Surface on the Nucleation and Growth of La(Al,Mn)O3

16.00 Coffee break

16.20

Cuo

co

C. Ortix M

ilet

to G

rano

zio

G. Gregori Surface and edge states in topological insulators

Interface effects in nanocrystalline electronically and ionically conducting oxides

16.50

L. Baldassarre Q. Li Insulator to metal transition in Cs3C60under pressure

Capacitance Enhancement in LaNiO3-based Strongly Correlated Electrode System

17.10

B. Martinez L. Begon-Lours Intrinsic robust antiferromagnetism at manganite interfaces

Field-effect modulation of exchange bias and conductivity at ferroelectric/ferromagnetic interfaces

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Monday, May 20th 2013

Plenary

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Coupled electricity and magnetism in solids: microscopic mechanisms and some novel effects D.Khomskii Koeln University, Germany

Close connection between electricity and magnetism is one of the cornerstone of modern physics. In application to solids this connection takes different forms. In particular, there exist materials with linear magnetoelectric effect, and multiferroics - the systems which simultaneously have magnetic and ferroelectric ordering In my talk I will give general survey of some of these phenomena, paying main attention to the microscopic mechanism of corresponding effects. In particular, different mechanism of magnetoelectric response and of multiferroic behaviors will be discussed. I will pay special attention to the purely electronic mechanisms of such phenomena and will discuss the appearance of spontaneous currents, electric dipoles and magnetic monopoles in frustrated Mott insulators. Nontrivial magnetoelectric properties can exist at some special magnetic textures, such as domain walls, magnetic vortices, etc, even in conventional magnets; some of these effects will be also discussed in my talk.

The standard point of view is that at low energies Mott insulators exhibit only magnetic properties, while charge degrees of freedom are frozen out, because electrons are localized. I demonstrate that in general this is not true [1, 2]: for certain spin textures there exist quite nontrivial effects in the ground and lowest excited states, connected with charge degrees of freedom. In particular this may happen in frustrated systems, e.g. containing triangles or tetrahedra as building blocks. I will show that in some cases there may exist spontaneous circular currents in the ground state of insulators, proportional to the scalar chirality; this clarifies the meaning of the latter and opens the ways to directly experimentally access it. For other spin structures there may exist spontaneous charge redistribution, so that average charge at a site may be different from 1. This can lead to the appearance of dipole moments and possibly of the net spontaneous polarization. This is a novel, purely electronic mechanism of multiferroic behaviour. In particular I show [3] that such electric dipoles should exist in spin ice materials at every tetrahedra with three-in/one-out or one-in /three-out spin configurations, which are equivalent to magnetic monopoles [4]. Thus there should be an electric dipole attached to each magnetic monopole in spin ice. This leads to electric activity of magnetic monopoles, and opens the possibility to control magnetic monopoles by electric field. The possibility to use chirality as qubits will be also discussed.

[1] L.N.Bulaevskii, C.D.Batista, M.V.Mostovoy and D.I.Khomskii, Phys.Rev.B 78, 028402 (2008)

[2] D.I.Khomskii, JPCM 22, 164209 (2010)

[3] D.I.Khomskii, to be publ.

[4] C.Castelnovo, R.Moessner and S.L.Sondhi, Nature 451, 42 (2008)

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Symposium 1


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1.1 Unconventional Proximity Effects Relevant to High-Tc Superconductivity

M.R. Beasley Geballe Laboratory for Advanced Materials, Stanford University

Corresponding author: M.R. Beasley, Geballe Laboratory for Advanced Materials, Stanford University, Stanford CA, 94305, USA

Interface phenomena involving superconductivity have drawn considerable interest in recent years. In this talk, we present the results of studies of two such proximity effects relevant (or possibly relevant) to superconductivity:

1) There are persistent reports of very high temperature superconducting anomalies at the interface between Cu and CuO. Using thin film bilayers, we have examined this possibility. No superconductivity has been observed to date, but a new proximity effect between a metal and a Mott (charge transfer) insulator has been observed in which antiferromagnetic correlations are induced in the metal by the antiferromagnetism of the Mott insulator. The correlation is reflected in the quenching of spin flips of the residual magnetic impurities in the Cu, as observed through their affect on the Anderson localization of the electrons in the Cu.

2) Kivelson and coworkers have proposed a new route to high Tc superconductivity based on a proximity effect between a metal and a negative U insulator. We have synthesized and studied bilayers of BaBiO3 (a negative U insulator) and BaPbO3 (a metal) in order to test this proposal. Superconductivity has been observed but may be due to inter-diffusion of Bi into the BaPbO3 layer. Both the materials science of the bilayers and the nature of the observed superconductivity will be discussed.

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1.2 What do electron-doped cuprates tell us about Cooper pairing in high-Tc superconductors? R. L. Greene, K. Jin, N. Butch, and J. Paglione Dept. of Physics, University of Maryland, College Park, MD 20742, USA

Corresponding author: [email protected]

The origin of the electron pairing in high-Tc cuprate superconductors is still unresolved, in spite of the ~25 years of intense research since their discovery. Most research has focused on hole-doped cuprates, which have a “pseudogap” phase of unknown origin in the under-doped region of the phase diagram. In contrast, electron-doped cuprates have a “pseudogap” whose origin is known to come from antiferromagnetic spin density wave (SDW) fluctuations [1]. In this talk/poster, I will discuss a detailed analysis of very low temperature transport measurements (30mK < T< 2K) on electron-doped La2-xCexCuO4 [LCCO](0.10 < x < 0.21) films in the normal state (H >Hc2). We find a direct correlation between the strength of the low temperature linear-in-temperature resistivity and the superconducting transition temperature (Tc) [2]. This suggests that the Cooper pairing and the normal state scattering are both caused by the same coupling. Comparison with similar behavior found in organic superconductors [3] strongly suggests that the T-linear resistivity is caused by spin fluctuation scattering. A linear-in-T resistivity has also been linked to Tc in some hole-doped cuprates [3]. But, because of the unknown nature of the “pseudogap” phase in hole-doped cuprates, the scattering responsible for the T-linear resistivity in these cuprates is not known. However, our results [2] suggest that there is a fundamental connection between spin fluctuations and the Cooper pairing in all cuprates! I will also discuss the magnetic field dependence of the low-temperature resistivity in the overdoped region of the LCCO phase diagram. This data suggests there is quantum critical behavior at the edge of the superconducting dome, a behavior whose origin is unexpected and unexplained [4]. [1] P. Armitage, P. Fournier, and R. L. Greene, Rev. Mod. Phys. 82, 2421 (2010). [2] K. Jin et al., Nature 476, 73 (2011). [3] L. Taillefer, Ann. Rev. Cond. Matter Phys. 1, 51 (2010). [4] N.P.Butch et al., PNAS 109, 8440 (2012).

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1.3 High Tc superconductivity in single interface of cuprate/titanate heterostrucures D. Di Castroa, D. Innocentia, A. Tebanoa, C. Arutab, G. Balestrinoa

aCNR-SPIN and Dip. di Ing. Civile e Ing. Informatica, Università di Roma Tor Vergata, Italy bCNR-SPIN, Via del Politecnico 1, 00133-Roma, Italy

Corresponding author: Daniele Di Castro. CNR-SPIN and Dipartimento di Ingegneria Civile e Ingegneria Informatica Università di Roma Tor Vergata, Via del Politecnico 1; 00133-Roma. Tel:+39 06 72597230. Email:[email protected]

In the recently discovered superconducting superlattice CaCuO2/SrTiO3 [1], significant experimental evidences suggest the confinement of superconductivity within few unit cells at the CaCuO2/SrTiO3 interface and the important role of additional oxygen atoms entering the interfaces during growth in oxygen rich environment. In particular, the overall experimental data points toward an interfacial reconstruction by oxygen redistribution that, in case of excess oxygen, generates holes in the CuO2 planes. Therefore, the interfaces in these SLs behave as a charge reservoir for the CuO2 planes, allowing superconductivity in the CCO layer. Starting from these results, we deeply investigated the behavior of a single CCO/STO interface in CaCuO2/SrTiO3 bilayers. We found that the transport properties are strongly dependent on the layer sequence. In particular, on NdGaO3 substrate, the sequence NdGaO3/CaCuO2/SrTiO3 is superconducting, whereas the sequence NdGaO3/SrTiO3/ CaCuO2 is non-superconducting. The transport properties seem to be affected much also by the substrate used (NdGaO3, SrTiO3, LaAlO3). We will give an overview of this study and suggest possible reasons for this behavior.

[1] D. Di Castro et al., Physical Review B 86, 134524 (2012)

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The long-sought yet elusive Majorana fermion is predicted to arise from a combination of a superconductor and a topological insulator. We present direct evidence for a Josephson supercurrent in superconductor (Nb) - topological insulator (Bi2Te3) - superconductor e-beam fabricated junctions by the observation of clear Shapiro steps under microwave irradiation, and a critical current modulation by magnetic field [1].

The dependence of the critical current on temperature and electrode spacing shows that the junctions are in the ballistic limit on a length scale of 100 nm. Shubnikov-de Haas oscillations in magnetic fields up to 30 T reveal a topologically non-trivial two-dimensional surface state. We argue that the ballistic Josephson current is hosted by this surface state despite the fact that the normal state transport is dominated by diffusive bulk conductivity.

Nanostructured SQUIDs containing topological Josephson junctions are realized experimentally [2]. Clear critical current modulation of both the junctions and the SQUID with applied magnetic fields have been observed. We show that the SQUIDs have a periodicity in the voltage-flux characteristic of !0 consistent with numerical expectations [3]. We propose several strategies towards realizing a doubled periodicity, belonging to the presence of Majorana fermions.

References:

[1] M. Veldhorst, M. Snelder, M. Hoek, T. Gang, V. Guduru, X. Wang, U. Zeitler, W.G. van der Wiel, H. Hilgenkamp, A. Brinkman, Nature Materials 11, 417 (2012).

[2] M. Veldhorst, C.G. Molenaar, X.L. Wang, H. Hilgenkamp, and A. Brinkman, Appl. Phys. Lett. 100, 072602 (2012).

[3] M. Veldhorst, C.G. Molenaar, C.J.M. Verwijs, H. Hilgenkamp, A. Brinkman, Phys. Rev. B 86, 024509

(2012).

1.4 Josephson supercurrent through a topological insulator surface state M. Veldhorst, M. Snelder, M. Hoek, D.P. Leusink, C.G. Molenaar, A. A. Golubov, H. Hilgenkamp, A. Brinkman MESA+ Institute for Nanotechnology, University of Twente, the Netherlands

Corresponding author: Alexander Brinkman, Email: [email protected]

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1.5 Thermoelectric effects and nonlocal Onsager relations in a proximity-coupled superconductor-ferromagnet device W. Belzig, P. Machon, M. Eschrig* Department of Physics, University of Konstanz, 78457 Konstanz, Germany *!Royal Holloway University of London, UK

We study thermal and charge transport in a three-terminal setup consisting of a superconducting and two ferromagnetic contacts. We predict that the simultaneous presence of spin-filtering and of spin-dependent scattering phase shifts at each of the two interfaces will lead to very large nonlocal thermoelectric effects both in clean and in disordered systems. The symmetries of thermal and electric transport coefficients are related to fundamental thermodynamic principles by the Onsager reciprocity. Our results show that a nonlocal version of the Onsager relations for thermoelectric currents holds in a three terminal quantum coherent ferromagnet-superconductor heterostructure including spin-dependent crossed Andreev reflection and coherent electron transfer processes. [1] W. Belzig, P. Machon, M. Eschrig, to be published in Phys. Rev. Lett. (2013)

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1.6 Designing phase-sensitive tests for Fe-based superconductors A.A. Golubov1 and I.I. Mazin2

1 Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands 2 Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC 20375, USA

Corresponding author: A.A. Golubov, Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands

Four years after the discovery of the new family of high-Tc Fe-based superconductors (FeBS) [1] their pairing symmetry is still under dispute [2]. While most researchers favor the so-called s+- pairing, whereupon the sign of the order parameters changes between the hole and the electron bands [3], some advocate [4] the more conventional anisotropic s, (the so-called s++ pairing) and for the extreme cases such as KFe2As2 and KxFe2Se2 other alternatives have been suggested (d-wave, or other types of sign-changing s). Despite recent progress in junction fabrication, no phase-sensitive experiments have been performed so far in FeBS-based Josephson junctions. Apart from problems with sample preparations, and other technical obstacles, a serious barrier preventing such experiments in FeBS is the fact that the two main contenders for the pairing state in the ‘mainstream’ FeBS are s+- and s++, two states that have the same orbital symmetry. Therefore one needs to design the experimental geometry in a special way so that the current in one contact would be dominated by the carriers having one sign of the order parameter, and in the other by carriers with the opposite sign. We suggest experimental designs suitable to test pairing symmetry in multiband Fe-based superconductors [5]. These designs involve Josephson two-junction interferometers that are based on combinations of tunnel junctions and point contacts where currents are dominated by different type of carriers, electrons or holes. The suggested designs should be accessible by available fabrication techniques and should allow to probe pairing symmetry in FeBS. 1. Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am. Chem. Soc. 130, 3296 (2008). 2. P.J. Hirschfeld, M.M. Korshunov, and I.I. Mazin, Reports on Progress in Physics 74, 124508 (2011). 3. I.I. Mazin, D.J. Singh, M.D. Johannes, and M.-H. Du, Phys. Rev. Lett. 101, 057003 (2008). 4. H. Kontani and S. Onari, Phys. Rev. Lett. 104, 157001 (2010). 5. A.A. Golubov and I.I. Mazin, accepted to Appl. Phys. Lett.

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1.7 Out of Equilibrium Gap and Pseudo-gap behavior in Copper-Oxide based Superconductors Fulvio Parmigiani Physics Department, University of Trieste and Sincrotrone Trieste (Italy)

In the last years, a significant theoretical effort has been focused to investigate possible effects relating the interband optical properties of the normal and superconducting states in high-TC superconductors (HTSC) [Hirsch2000,Norman2002]. In search of an experimental evidence of such effects, continuous wave (CW) optical spectroscopies have been widely used to measure the high-energy (>1 eV) dielectric function [Molegraaf2002, Basov2004]. Unfortunately, these conventional spectroscopies failed to spot the evolution of the dielectric function, in the interband spectral region, across the superconducting transition. Here, by adopting a non-equilibrium approach to the problem, we show how the non-thermal photoinjection of excitations affects the high-energy optical properties, demonstrating a superconductivity induced modification of distinctive interband transitions.

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1.8 Establishing Theoretically the Capacity of Resonant Inelastic X-ray Scattering to Probe the Phase and Excitations of the Superconducting Order Parameter !

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Pasquale Marra1, Steffen Sykora1, Krzysztof Wohlfeld2, and Jeroen van den Brink1,3 1Institute for Theoretical Solid State Physics, IFW Dresden, D-01069 Dresden, Germany 2Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory and Stanford University, 2575 Sand Hill Road, Menlo Park, CA 94025 3Department of Physics, TU Dresden, D-01062 Dresden, Germany

Corresponding author: Pasquale Marra, IFW Dresden Helmholtzstraße 20, 01069 Dresden Germany, [email protected]

The capability to probe the dispersion of elementary spin, charge, orbital, and lattice excitations has positioned Resonant Inelastic X-ray Scattering (RIXS) at the forefront of photon science. Here we develop the scattering theory for RIXS on superconductors, calculating its momentum-dependent scattering amplitude. Considering unconventional superconductors with different pairing symmetries and, in particular, evaluating the theoretical RIXS spectra for cuprate and pnictide-like systems, we show that the low- energy scattering is strongly affected by the superconducting gap and coherence factors. This establishes RIXS as a tool to disentangle pairing symmetries and to probe the elementary excitations of unconventional superconductors.

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1.9 Spin fluctuations regime extended up to room temperature in electron doped cuprates.! Pasquale Orgiani1*, Alice Galdi1, John W. Harter3, Kyle M. Shen3,4, Darrell G. Schlom2,4 and Luigi Maritato1,2

1CNR-SPIN and Department of Information Engineering, Electrical Engineering and Applied Mathematics DIEM, University of Salerno, 84084 Fisciano (SA), Italy. 2Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA. 3Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, New York 14853, USA. 4Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA.

Corresponding author: Pasquale Orgiani, CNR-SPIN UOS Salerno, c/o University of Salerno, Via Ponte don Melillo, I-84084 Fisciano (SA), Italy; email: [email protected]

In recent years, considerable effort has been spent investigating the transport properties of electron doped cuprates. Different normal-state resistivity behaviors have, however, been reported, some of them in apparent contradiction with each other. The most consolidated behavior, as emphasized in a recent review on electron-doped cuprates, seems to be a quadratic scaling law, indicating a predominant electron-electron scattering mechanism in a normal metal (i.e. Fermi liquid). On the other hand, other reports point to a linear T dependence of resistivity, associated to spin-fluctuation scattering mechanisms, either in proximity of superconducting transition and/or far from it, in analogy with the normal-state of hole-doped cuprates. With the aim of fully understanding the true nature of the normal-state resistivity in electron-doped cuprates, transport properties of a series of infinite layer La1-xSrxCuO2±" thin films have been investigated in detail. Charge carrier concentration were varied by both changing the heavy ion La/Sr chemical ratio (i.e. x) and the oxygen content (i.e. "). In particular, heavily underdoped samples were usually obtained with excess oxygen content, while in proximity of the optimal doping level, the carrier concentration were varied by small variation of the La/Sr content and characterized by negligible oxygen off-stoichiometries (i.e. " # 0). Underdoped samples clearly showing a metal-to-insulator transition at low temperatures, allowed us to determine the fundamental physical mechanism behind the upturn of the resistivity. Once established, such a physical phenomenon was traced back also in optimally doped and overdoped samples. Evidence arises that, even though a metal-to-insulator transition does not occur, an insulating phase co-exists and severely affects the transport properties, thus revealing the true nature of the metallic phase of electron doped cuprates unambiguously dominated by spin-fluctuation phenomena at all temperatures up to 300K. Such a analysis provides a universal phase-diagram for electron doped cuprates in which spin-fluctuation dominates the normal state, thus demonstrating a qualitatively symmetric phase diagram for both hole-doped and electron-doped cuprates as a function of doping. !

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1.10 Magnetoresistance and spin-transfer torque in hybrid superconductor/ferromagnet structures !

R. Citro1,2, F. Romeo1,2 1Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno 2CNR-SPIN, Uo Salerno Via Ponte don Melillo, I-84084 Fisciano (Sa), Italy

Corresponding author: F. Romeo, Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno, Via Ponte don Melillo, I-84084 Fisciano (Sa), Italy; email: [email protected]

We study the magnetoresistance and the spin-transfer torque of a ferromagnet-superconductor-ferromagnet (FSF) spin-valve structure, allowing for an arbitrary magnetization misorientation and treating superconductivity of both s-wave and p-wave type through a generalized scattering field theory [1, 2]. Taking fully into account Andreev reflection processes and spin-triplet correlations, we analyze to what extent the magnetic torsion and the magnetoresistance are modified when topological zero-energy states are present at the interfaces. In this framework, we find that magnetoresistance curves display a peculiar symmetry-dependent behavior as a function of the interlayer width d of the superconducting region, being this a finite size effect associated to the microscopic properties of the scattering region (e.g. coherence length, spin polarized transport through the superconducting spacer, spin-flip at the interface, etc). We show that a careful analysis of this behavior, considering different symmetries of the order parameter, can characterize triplet and singlet superconductivity from the viewpoint of the spin polarized transport through the device. Our analysis also shows that a spin valve with a superconducting interstitial region makes possible to compare by the same device the FSF (T<Tc) and FNF (T>Tc) spin-valve physics by simply tuning the working temperature [3]. [1] F. Romeo and R. Citro, Phys. Rev. B 84, 024531 (2011)

[2] A. Sorgente, F. Romeo and R. Citro, Phys. Rev. B 82, 064413 (2010)

[3] S. Takahashi, H. Imamura, and S. Maekawa, Phys. Rev. Lett. 82, 3911 (1999); H. Yang, S.-H. Yang, S. Takahashi, S. Maekawa, S. S. P. Parkin, Nature Materials 9, 586 (2010)

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Symposium 2

Dielettrics, ferroeletrics, multiferroic

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2.1 Strong coupling between magnetism and polarization in mutiferroics Y. Taguchi Strong Correlation Materials Research Team, RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan

Two multiferroic materials that show strong coupling between magnetism and polarization are discussed in this presentation: One is rare-earth ortho-ferrite RFeO3 and the other is perovskite-type manganite Sr1-xBaxMnO3.

In the rare-earth ferrite, polarization is induced by the symmetric exchange striction working between the rare-earth moment and iron spin. We succeed in reversing the weak-ferromagnetic moment associated with the G-type ordered iron spins only with the electric field for the first time for a bulk single-component multiferroic material [1].

In the manganite, magnetic Mn4+ ions with S=3/2 are found to exhibit off-centering independently from the magnetic ordering [2]. In this sense, this material is quite different from the known multiferroic materials, such as BiFeO3 where non-magnetic Bi ions show off-center displacement while the Fe is responsible for the magnetism, and TbMnO3 where ferroelectricity is induced by magnetic ordering. The ferroelectric transition in Sr1-xBaxMnO3 is governed by a soft phonon mode, and its dynamics is revealed in detail by far-infrared reflectivity and momentum-resolved inelastic x-ray scattering measurements[2, 3]. Strong coupling between spin and polarization is also found in the temperature-dependent crystal structure and soft-phonon frequency.

This work was done in collaboration with Y. Tokunaga, H. Sakai, J. Fujioka, T. Fukuda, D. Okuyama, D. Hashizume, M. S. Bahramy, R. Arita, H. Nakao, Y. Murakami, F. Kagawa, T. Arima, A. Q. R. Baron, and Y. Tokura, and in part supported by JSPS through “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program)”.

[1] Y. Tokunaga, Y. Taguchi, T. Arima, and Y. Tokura, Nature Phys. 8, 838 (2012).

[2] H. Sakai, J. Fujioka, T. Fukuda, D. Okuyama, D. Hashizume, F. Kagawa, H. Nakao, Y. Murakami, T. Arima, A. Q. R. Baron, Y. Taguchi, and Y. Tokura, Phys. Rev. Lett. 107, 137601 (2011).

[3] H. Sakai, J. Fujioka, T. Fukuda, M. S. Bahramy, D. Okuyama, R. Arita, T. Arima, A. Q. R. Baron, Y. Taguchi, and Y. Tokura, Phys. Rev. B 86, 104407 (2012)

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2.2 Ferroelectric and Multiferroic twin walls in CaTiO3 and CaMnO3

Paolo Barone, Domenico Di Sante and Silvia Picozzi CNR-SPIN, 67100 L'Aquila, Italy !


Interfaces and domain boundaries have been considered for a long time mere juxtapositions of materials in the bulk state and only recently it has been realized that they may present novel features which do not emerge in the bulk.Twin walls represent a particularly interesting situation, since they are an almost two-dimensional nanosized feature within a homogeneous solid. They are characterized by a primary order parameter, which dominates in the bulk of the uniform solid and disappears inside twin walls. However, competing secondary order parameters often show the inverse behavior, appearing inside twin walls but being suppressed in the bulk.[1] A concrete example is given by CaTiO3, a ferroelastic perovskite where the primary order parameter Q, describing TiO6 octahedra tilting, is suppressed at the boundary allowing for the secondary order parameter P, accounting for the polar off-centering of Ti ions , to emerge. The predictions based on an empirical atomic-scale description of the wall have been qualitatively confirmed by Electron Microscopy[2,3]. On the basis of these premise, we performed first-principles calculations for twin walls in CaTiO3 and in the ferroelastic antiferromagnet CaMnO3. While for the former our electronic-structure calculations allow for a quantitative estimate of the polarization profile at the twin wall in terms of Berry- phase, our approach can be also usefully applied to the analysis of the interplay between structural distortions and magnetism in CaMnO3. According to common belief, simultaneous magnetism and ferroelectricity are scarcely found in perovskite oxides, magnetism requiring partially filled d-states while ferroelectricity requiring empy d- states[4]; however we found that ferroelectric distortions may develop also at the twin bounday of the antiferromagnetic CaMnO3, where polar distortions are not inhibited by ferroelastic rotations. The interplay between polar distortions and magnetic exchanges have also been analyzed. Our work represents then the first attempt to describe a multiferroic twin wall, suggesting a new route to integrate ferroelectric features in magnetic materials. [1]W. T. Lee, E. K. H. Salje, and U. Bismayer, J. Appl. Phys. 93, 9890 (2003). [2]L. Goncalves-Ferreira et al., Phys. Rev. Lett. 101, 097602 (2008). [3]S. Van Aert et al., Adv.Mater. 24, 523 (2012). [4] N. A. Hill, J. Phys. Chem. B 104, 6694 (2000).

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2.3 Spin and Lattice Dynamics in Multiferroic TbFe3(BO3)4 V. Kocsis,1 D. Szaller,1 S. Bordács,1 U. Nagel, T. Room, K. Ohgushi, I. Kezsmarki1 1Department of Physics, Budapest University of Technology and Economics and Condensed Matter Research Group of the Hungarian Academy of Sciences, 1111 Budapest, Hungary 2National Institute of Chemical Physics and Biophysics, Tallinn 12618, Estonia 3Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan

Corresponding author: Vilmos Kocsis, Budapest University of Technology and Economics, 1111 Budapest, Budafoki ut 8., Hungary, [email protected]

Spin and phonon excitations of multiferroic TbFe3(BO3)4 [1] with rhombohedral structure have been investigated over the frequency range of 0.15-6 THz using polarized optical reflection and transmission spectroscopy. Below TN=40K this compound is an easy-axis antiferromagnet. Based on former high-field magnetization measurements [2], the dominant magnetic interaction in the system is the JFe-Fe antiferromagnetic exchange between neighboring Fe3+ ions with S=5/2. While the exchange coupling between Tb spins is negligible, the JFe-Tb interaction between Fe and Tb spins is also relevant and induces a mutual long-range ordering in the exchange coupled Fe-Tb system. The easy-axis character of the zero-field ground state results from the strong single ion anisotropy of Tb spins, which is transmitted to the Fe system via JFe-Tb. We have studied the temperature and field dependence of the magnon modes and determined the JFe-Fe and JFe-Tb exchange coefficients together with the g-factor of Fe3+ and Tb3+ ions by fitting the resonance frequencies in a mean-field model. We found that the magnon modes exclusively correspond to the dynamics of Fe spins, while Tb S=6 spins with strong Ising-character act as a static effective field and determine e.g. the strength of the magnetic field necessary for the spin-flop transition. The magnon modes show considerable optical magnetoelectric effect !reflected in the different absorption of counter-propagating light beams also termed as directional dichroism [3, 4]! related to the multiferroic nature of the material. Strong magnetoelasticity is also evidenced by splitting observed for certain phonon modes upon the temperature- and field-induced magnetic transitions. We believe that these results form a solid experimental basis to understand the origin of multiferroicity and electromagnon modes in this compound and generally in coupled f-d spin systems. [1] U. Adem et al., Physical Review B 82, 064406 (2010). [2] E. A. Popova et al., Physical Review B 75, 224413 (2007). [3] I. Kezsmarki et al., Physical Review Letters 106, 057403 (2011). [4] S. Bordacs at al., Nature Physics 8, 734 (2012).

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Antiferromagnetic perovskites with cycloidal magnetic order are known to display ferroelectric polarization (P) due to the so-called inverse Dzyaloshinskii-Moriya interaction. P develops in a direction contained within the plane of rotation of the cycloidal order and perpendicular to its propagation vector, therefore the sense of P is dictated by the helicity of the cycloid. Upon cooling the material through its Néel temperature, domains of distinct helicity and ferroelectric polarization, are expected to be formed. It is also known that the cycloidal plane can be flopped from bc(ab) to ab(bc) by an appropriate magnetic field. Upon flopping under a magnetic field and in absence of any electric-field (E-field) poling, it is also expected that domains with opposite helicity bc± (or ab±) should be equally populated resulting in zero net electric polarization. We will show that in a magnetic cycloidal system (a single-crystal of (Y0.5Sm0.5)MnO3 perovskite) the expected degeneracy of chiral states of opposite sign does not occur and chiral domain configuration is dictated by the magnetoelectric history of the sample. This allows us to control the polarization vector direction by a magnetic field, indicative of the existence of bc-cycloidal (nano)regions within an ab-cycloidal matrix that act as seeds for the growth of the polar domains. More striking is the fact that upon continuous magnetic field cycling (if sample is prepared in appropriate chiral multi-domain state) the magnitude of the polarization can be also tuned. We argue that this arises from the fact that magnetic field induce chiral domain-walls annihilation. We also show that selected chiral states persist within the high-temperature collinear magnetic ordering region, thus implying cycloidal/ferroelectric and collinear/paraelectric phase coexistence. In short, we are able to demonstrate that the the coexistence of phases is ubiquitous across the whole phase diagram of these materials, and far from the transitions between the different phases, which allows the complete voltage-less control of the ferroelectric polarization direction and magnitude.

2.4 Phase coexistence and magnetically-tuneable polarization in cycloidal multiferroics I. Fina1*, D.O’Flynn2,**, G. Balakrishnan2, and J. Fontcuberta1 /! Institut de Ciència de Materials de Barcelona (ICMAB-CSIC). Campus UAB. Bellaterra 08193.

Catalonia (Spain). +!!Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom.

Corresponding author: [email protected], Institut de Ciència de Materials de Barcelona (ICMAB-CSIC). Campus UAB. Bellaterra 08193. Catalonia (Spain). ** Present Address: Department of Medical Physics and Bioengineering, Malet Place Engineering Building , University College London, WC1E 6BT

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2.5 Magnetic field enhanced directional dichroism in the spin resonances of the multiferroic Ca2CoSi2O7 D. Szaller1*, I. Kezsmarki1, S. Bordacs1, T. Room2, U. Nagel2, H. Engelkamp3, H. Murakawa4, S. Miyahara4, Y. Tokura4 1Department of Physics, Budapest University of Technology and Economics and Condensed Matter Research Group of the Hungarian Academy of Sciences, Budapest, Hungary 2National Institute of Chemical Physics and Biophysics, Tallinn, Estonia 3High Field Magnet Laboratory, Institute for Molecules and Materials, Radboud University, Nijmegen, The Netherlands 4Multiferroics Project, ERATO, Japan Science and Technology Agency (JST), Japan c/o and Quantum-Phase Electronics Center, Department of Applied Physics, The University of Tokyo, Tokyo, Japan

Corresponding author: David Szaller, Budapest University of Technology and Economics, Department of Physics, Budafoki ut, 8 – H-1111 Budapest, Hungary [email protected]

In materials simultaneously breaking time-reversal and spatial inversion symmetries the strength of absorption for two counter-propagating light beams can be di!erent irrespective of the polarization state of light [1], which is termed as non-reciprocal directional dichroism. One realization of this effect is the magneto-chiral dichroism (MChD), which appears in chiral systems for light beams propagating parallel and antiparallel to an external magnetic field. Until recent experiments on multiferroic materials [2, 3] the effect was generally found to be weak. The non-centrosymmetric Ca2CoSi2O7 possesess an achiral crystal structure, but in the multiferroic phase chirality can be induced by the spin ordering. The handedness of the sample can be switched by rotating the direction of the external magnetic field. Since the chirality of Ca2CoSi2O7 is a consequence of the magnetic ordering, the appearance of MChD is expected in the energy range of spinwave excitations. These spin resonances are both electric and magnetic dipole active (electromagnons) with matrix elements of comparable magnitude leading to strong magnetoelectric cross effects also responsible for the directional dichroism. The far-infrared optical absorption was investigated in high magnetic fields up to 30 Tesla. When increasing the field, a significant enhancement of the MChD effect was observed for all of the spinwave modes, proving their electromagnonic nature. Our results imply that due to the coupled spin-polarization dynamics significant directional dichroism can be present in a wide variety of multiferroic materials. [1] W.F. Brown, Jr. S. Shtrikman, and D. Treves, J. Appl. Phys. 34, 1233 (1963) [2] M. Saito et al., Phys. Rev. Lett. 101, 117402 (2008)

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2.6 Spin flexoelectricity in multiferroic films M. Bibes1, A.P. Pyatakov2, A.K. Zvezdin2 1) Unité Mixte de Physique CNRS / Thales, 1 av. Fresnel, 91767 Palaiseau & Université Paris-Sud, 91405 Orsay (FRANCE 2)A.M. Prokhorov General Physics Institute, Vavailova 38 ,Moscow, 119991

Corresponding author: A. Pyatakov, MSU, Leninskie gory, 119991, Moscow, [email protected]

The electric field induced spin modulation in magnetic media and reciprocal effect of electric polarization induced by spin cycloids is known as spin flexoelectricity. It is analogues to flexoelectric effect in nematic liquid crystals that relates the curvature of the director field with electric polarization. In multiferroics the spin flexoelectricity plays the central role in the genesis of ferroelectric polarization in so-called spiral multiferroics. The reciprocal effect leads to the spin cycloid formation in proper ferroelectric, e.g. perovskite bismuth ferrite BiFeO3.

In this report the spin configurations in BiFeO3 film are theoretically considered. The magnetic state of the film is the result of competition between the factor that induces the spin modulation on one side and the factors that stabilize the homogeneous state on the other side. The former one is flexomagnetoelectric interaction of Lifshitz invariant-type term in the free energy density:

( ) ))(( LLLLp !"#!#"= $MEF , (1)

where p is the unit vector of electric polarization, (p||[111]), L is unit antiferromagnetic vector, $ is the flexomagnetoelectric constant). The later ones, that support the homogeneous state, are inhomogeneous exchange, magnetic anisotropy, and magnetoelastic energy. The magnetoelastic energy term in the free energy density that takes into account the epitaxial strain in the BiFeO3 thin films is:

( )2_ nL !!"= UF ElasticM , (2)

where U is magnetoelastic energy, n is the normal to the film plane that in general case do not coincide with polar direction p.

The minimization of free energy functional gives several solutions for the magnetic state of the film: (i) easy plane state L at critical compressive strain (Uc1<0) (ii) easy axis state at critical tensile strain (Uc2>0), (iii) various spin cycloid states in the strain range Uc1<U<Uc2 that depend on the crystallographic orientation of the substrate on which the film is grown. Only for (111) films the spin cycloidal states are the same as in bulk crystal, while the less symmetrical films support the other types of cycloids running in the plane of the film with enlarged in-plane period with respect to the bulk crystal case.

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2.7 Strain control of domain wall pinning in hybrid piezoelectric-magneto-resistive nanodevices Na Lei1, T. Devolder1, G. Agnus1, P. Aubert1, L. Daniel2, J-V. Kim1, E. Fohtung3, O. Shpyrko3, E. Fullerton4 and D. Ravelosona1 and P. Lecoeur1

1 Institut d’Electronique Fondamentale, CNRS UMR8622 / Univ Paris-Sud, 91405 Orsay, France 2 Laboratoire de Génie Electrique de Paris, CNRS, UMR8507/SUPELEC/UPMC/Univ Paris-Sud, 91192 Gif-sur-Yvette, France 3 Department of Physics, University of California, San Diego, La Jolla, CA 92093-0401, USA 4 Center for Magnetic Recording Research, University of California, San Diego, La Jolla, CA 92093-0401, USA

Corresponding author: Philippe Lecoeur, [email protected], Bat 220, Université Paris-Sud 11,91405 Orsay Cedex

Multifunctional material systems offer a unique way to control physical properties. In recent years, both fundamental and technological interests have triggered research activities in this field. An attractive system based on the coupling between ferromagnetic layers and piezoelectric materials, using magnetoelectric coupling, has opened the route for new devices for magnetic field detection [1] and anisotropy manipulation in spintronic devices [2]. So far, most of the developments were done on hybrid structures using piezoelectric stressors, however full integration in real devices is still lacking and requires the integration of epitaxial piezoelectric materials to ensure nanoscale reliability on the local generated strain. One of the main concerns then relates to the clamping effect of the piezoelectric layer by the substrate that can be reduced by microstructuration [3]. We have developed hybrid nanodevices, which consist of a laterally polarized epitaxial PbZrTiO3 (PZT) bar inducing a giant in-plane strain that acts on a ferromagnetic in-plane magnetized spin valve. We show that electric field can be used to control domain wall (DW) pinning by inverse magnetostriction and thus can act as a single wall gate. By increasing the amplitude of the applied voltage on PZT, we observe a significant increase of the coercive field in the magnetic free layer. Combining strain simulations with micromagnetic simulations, we show that these results are consistent with a much higher energy barrier for domain wall motion formed by the induced strain gradient at the edge of the side electrodes [4]. To confirm the pinning of a single DW at the corner of PZT electrodes, strain gradient was measured under applied voltage using coherent X-ray diffraction (CXD).

[1] Huong Giang D.T. , Duc N.H. , Sensors and Actuators A, 149, 229-232 (2009) [2] Weiler M., Brandlmaier A., Gepraegs S. et al. New. Jour. Physics, 11, 013021 (2009) [3] V. Nagarajan et al., Nature Materials 2, 43 - 47 (2003) [4] Lei N., et al. accepted in Nature Communications (NCOMMS-2012-00145C)

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2.8 Tuning the magnetoelectric properties of heteroepitaxial CFO/SRO/PZT/SRO films grown on piezoelectric PMN-PT substrates via strain Adrian Petraru, Rohit Soni*!Nikolay Pertsev1, Ravi Droopad2 and Hermann Kohlstedt !Nanoelektronik, Technische Fakultät, Christian-Albrechts-Universität zu Kiel, 24143 Kiel, Germany 1A. F. Ioffe Physico-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia 2Texas State Univ, Dept Phys, San Marcos, TX 78666 USA

Corresponding author: Adrian Petraru, Nanoelektronik, Technische Fakultät, Christian-Albrechts-Universität zu Kiel, 24143 Kiel, Germany. Email: [email protected]

Recently, a significant enhancement of the ME voltage coefficient at the strain induced structural phase transitions between different ferroelectric phases was theoretically predicted for epitaxially grown Pb(Zr1-xTix)O3 (PZT) layers on magnetostrictive substrates. Particularly, near the tetragonal to monoclinic phase transition, a significant increase of the ME voltage coefficient !ME is theoretically predicted. The biaxial strain imposed by the substrate and the composition of the PZT films are the tuning parameters to achieve the desired phase transition. In this work we use PZT films of different compositions epitaxially grown on PMN-PT piezoelectric crystalline substrate. This approach allows a further tuning of the strain in the epitaxial PZT films after deposition by applying a bias voltage to the substrates. The PMN-PT substrates are a good choice due to their huge piezoelectric coefficients, and a good match of the pseudo-cubic lattice parameters with PZT films. X-ray analysis showed that additional to the initial misfit strain of the PZT film due to the substrate, a strain tuning in the range of 0.2 percent was observed when a bias voltage of 300 V was applied (for 0.15 mm thick PMN-PT substrates). We deposited magnetostrictive Metglas films on the back side of the PMN-PT substrates and the strain dependent magnetoelectric properties in these heterostructures were investigated for several compositions of the PZT films.

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2.9 PZT thin films capacitors : a way to sense Electric field ? Didier Robbes1, Abiyoki Théogène1, Sylvain Lebargy1, Corentin Jorel1, Laurence Méchin1, Rachid Bouregba 2, Gilles Poullain 2 Christophe Cibert 2 1 GREYC, ENSICAEN, UCBN, CNRS, UMR 6072, 6 bd du Maréchal Juin, 14050 Caen cedex, France 2 CRISMAT/CNRT-Matériaux /ENSICAEN, UMR6508, UMS 3318, 6 bd du Maréchal Juin, 14050 Caen cedex, France

Corresponding author: Robbes Didier, Université de Caen Basse-Normandie, GREYC Ensicaen 6, Boulevard du maréchal juin, 14050 Caen Cedex. Tel : +33 2.31.45.26 97 Fax : +33 [email protected]

We have as a main goal the development of an electric field meter based on the polarization versus electric field properties of PZT thin fims through the classical D ! E" curve. Making good use of the general principle of duality in physics and in analog signal processing circuits, our approach starts from the working principle of the famous flux gate magnetometer to sense magnetic field from dc, allowed when using a clever modulation scheme of the B!H " in ultra soft magnetic cores. Unfortunately, a strict transformation of circuits using duality rules does not properly applies, due to the lack of ultra soft ferro electric materials that would exhibit hysteresis free polarization characteristics. Thanks to our knowledges in signal processing of various sensors, including those having highly hysteretic characteristics but low noise ( namely rf SQUIDS), we propose a way to use non linear thin film capacitors based on PZT materials, to design electric field sensors using the proper modulation scheme of their hysteretic D! E"characteristics. Especially, our contribution will start using the Landau – Kalatchikof model (KL) of ferro electric materials, to obtain models of capacitors that are subjected both to an ac voltage excitation and a dc voltage bias. The non linear differential equation in that model is numerically solved and time variations of the charge on electrodes are restored and compared to experimental ones. Compared to previous works, effects of dc bias upon ac magnitude and shape of the ac output signal are then obtained, including regimes of parametric amplification of the non linear capacitance, and even chaotic regime. In addition, we have design and we will present an instrument to quickly experimentally investigate the various non linear effects induced by slow variations of a quasi dc voltage. Since the harmonic contents of the output signal is linked to the quasi dc applied voltage, the harmonic distortion of the device reflects the effects. Then, the use of a stop band filter at the ac pumping device frequency acting on the output of a standard Sawer Tower circuit is shown dependant of the bias voltage, and will allow discrimination between various regimes of the solutions of the KL equation. Such a discrimination is an important step to decide which transduction mean is to be used to reach our goal. Preliminary measurements have shown the validity of that method. It makes good use of modern electronic based on microcontroler circuit and is easy to implement.

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2.10 Anisotropic domain formation in NaNbO3 thin films under tensile lattice strain J. Schwarzkopf, A. Duk, M. Schmidbauer Leibniz-Institute for Crystal Growth, Max-Born-Str. 2, D-12489 Berlin, Germany

Corresponding author: Tel.: +49 30 6392 3053; fax: +49 30 6392 3003, e-mail address: [email protected] (J. Schwarzkopf)

Lattice strains in ferroelectric materials have a decisive impact on the stability of ferroelectric phases. Incorporation of strain affects the phase transition temperatures as well as the dielectric and electromechanical properties of the films. However, the existing thermodynamic theories assume uniform biaxial strain states, which is reasonable for example for tetragonal films grown on cubic substrates, but not for materials with orthorhombic symmetry. Incorporation of anisotropic lattice strain may provide new properties in films like relaxor behavior in SrTiO3 or the formation of 1D stripe domains. However, less is known about the influence of anisotropic strain in thin films as well as of plastic strain relaxation on domain formation.

NaNbO3 based materials have attracted scientific interest due to their complexity of phase transitions and excellent piezoelectric properties. NaNbO3 as bulk material exhibit orthorhombic symmetry (Pbma) resulting in strong anisotropic lattice strain even when grown on cubic or nearly cubic substrates.

In this study NaNbO3 thin films were epitaxially grown by liquid-delivery spin metal-organic chemical vapor deposition. We have investigated the influence of the lattice mismatch between film and substrate material by the variation of the substrate oxide (ReScO3(110) with Re = Dy, Tb, Gd) and of the film thickness on the relaxation process and the domain formation by grazing incidence x-ray diffraction and piezoresponse force microscopy. The growth of NaNbO3 thin epitaxial films on the ReScO3(110) substrates leads to anisotropic tensile lattice distortions. Films with a film thickness below the critical film thickness exhibit highly regular one-dimensional arrays of 90° domains of the type a1/a2/a1/a2 with coherent domain walls which are aligned along the [001]o substrate direction. Even for the thinnest films the in-plane pseudocubic NaNbO3 unit cells show a monoclinic distortion. With the onset of plastic strain relaxation the domain pattern transforms in a 2D array with domains aligned also in the [1-10]o substrate direction. For all substrates the observed lateral domain widths can be described by a square-root dependence on the film thickness. Even for the thinnest NaNbO3 thin films on a GdScO3(110) substrate we have observed only 2D domain patterns and attribute this to the partial plastic strain relaxation in the films.

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Tuesday, May 21st 2013

Plenary

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Topological Superconductivity Yoishiteru Maeno Department of Physics, Kyoto University, Kyoto 606-8502, Japan

M. S. Anwar: Department of Physics, Kyoto University, Kyoto 606-8502, Japan

Triggered by the recent predictions and subsequent experimental confirmation efforts of “topological insulators”, the topological classification of materials gains new insights into investigating novel “topological quantum phenomena” in superconductors and superfluids.

We will first review topological classification of materials and some key common concepts such as topological edge states. We then focus on the topological quantum phenomena in Sr2RuO4, a leading candidate of the “p+ip” topological superconductor [1]. Next we introduce recent progress in creating topological superconducting states, as well as the physics of Majorana quasiparticles and its possible application to quantum computing. [1] Maeno, Y., Kittaka, S., Nomura, T., Yonezawa, S. & Ishida, K. J. Phys. Soc. Jpn. 81,

011009 (2012).

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Symposium 1


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1.11 Spin-splitting and magnetic switching in hybrid superconductor-ferromagnet structures !

V. V. Ryazanov Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia#

Superconductivity and ferromagnetism, two deeply antagonistic electronic properties, can coexist in form of Josephson SFS junctions (SFS JJ) [1]. The most impressive feature of the SFS JJ is the ability to be in a Josephson state with the phase difference inversion or %-state [2,3]. This feature makes the SFS JJs valuable phase-shifting elements for utilization in superconducting circuits. Recently, a Toggle Flip-Flop (TFF) with the embedded SFS %-junction was successfully demonstrated [4]. The SFS JJs are also suitable for integration with Josephson qubits. A quantum Josephson circuit, a %-biased phase qubit, has been recently demonstrated too [5]. Another application is based on the simplest feature of a SFS JJ that its critical current can be changed significantly by remagnetization of the F-interlayer [1]. A Josephson magnetic memory element proposed in Ref. [6,7] requires ferromagnet layer with an in-plane magnetic anisotropy and small coercive field. Specifically, an application of small external magnetic field changed the magnetization of the ferromagnetic layer that in turn changes the junction critical current Ic, allowing the realization of two distinct states with high and low Ic, corresponding to logical “0” and “1” states, respectively. It was demonstrated too in Ref.[8] that by inserting an additional isolation tunnel layer (I) in the SFS JJ (i.e., fabricating a superconductor-insulator-ferromagnet-superconductor (SIFS) structure), one should be able to increase significantly the junction characteristic voltage Vc to achieve high switching frequency. The superconducting-ferromagnetic Josephson junctions are electrically compatible with traditional superconductor-insulator-superconductor (SIS) Josephson junctions used for digital energy-efficient single flux quantum (eSFQ/ERSFQ) circuits. Weakly ferromagnetic materials suitable for Josephson S-F-S sandwich fabrication have the superconducting order parameter decay length of about several nanometers which is much smaller compared to the decay length in normal metal. To increase the decay length in structures with ferromagnets different types of planar Josephson junctions with complex bilayered (NF) and trilayered (FNF) weak links have been proposed in theoretical work [9]. The nanofabricated S-(NF)-S structures were realized in Ref.[10] recently. The structure consists of a Cu/Fe bilayer forming a bridge between two superconducting Al electrodes. The appreciable critical current was detected up to 120 nm of the bridge length. It was observed a double-peak peculiarity in differential resistance of the S-(N/F)-S structures at a bias voltage corresponding to the superconducting minigap. The splitting of the minigap was explained by the electron spin polarization in the normal metal which is induced by the neighbouring ferromagnet.

1. V.V. Ryazanov, Physics – Uspekhi 42, 825 (1999). 2. V.V. Ryazanov, V.A. Oboznov, A.Yu. Rusanov, A.V. Veretennikov, A.A. Golubov, and J. Aarts, Phys. Rev. Lett 86, 2427

(2001). 3. V. A. Oboznov, V.V. Bol'ginov, A. K. Feofanov, V. V. Ryazanov and A.I. Buzdin, Phys. Rev. Lett. 96, 197003 (2006). 4. M. I. Khabipov, D.V. Balashov, F. Maibaum, A.B. Zorin, V.A. Oboznov, V.V. Bolginov, A.N. Rossolenko and V.V. Ryazanov.

Superconductor Science and Technology, 23, 045032 (2010). 5. A.K. Feofanov, V.A. Oboznov, V.V. Bol’ginov, J. Lisenfeld, S. Poletto, V.V. Ryazanov, A.N. Rossolenko, M. Khabipov, D.

Balashov, A.B. Zorin, P.N. Dmitriev, V.P. Koshelets and A. V. Ustinov. Nature Physics 6, 593 (2010). 6. V.V. Bol’ginov, V.S. Stolyarov, D.S. Sobanin, A.L. Karpovich, and V.V. Ryazanov, JETP Letters 95, 366 (2012). 7. V.V. Ryazanov, V.V. Bol’ginov, D.S. Sobanin, I.V. Vernik, S.K. Tolpygo, A.M. Kadin, and O.A. Mukhanov, Phys. Procedia

36, 35 (2012). 8. T.I. Larkin, V.V. Bol’ginov, V.S. Stolyarov, V.V. Ryazanov, I.V. Vernik, S.K. Tolpygo, O.A. Mukhanov, Appl. Phys. Lett. 100,

222601 (2012). 9. T. Yu. Karminskaya, A. A. Golubov, M. Yu. Kupriyanov, and A. S. Sidorenko, Phys. Rev. B 81, 214518 (2010). 10. T.E. Golikova, F. Hübler, D. Beckmann, I.E. Batov, T.Yu. Karminskaya, M.Yu. Kupriyanov, A.A. Golubov, and V.V.

Ryazanov. Phys. Rev. B 86, 064416 (2012).

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1.12 Triplet superconductivity in heterostructures with oxide ferromagnetic interlayers G.A. Ovsyannikov1,2, A.E. Sheyerman1,2, Y.V. Kislinskii1, A.V. Shadrin1,2,, K.Y. Constantinian1, A. Kalabukhov2,3, Dag Winkler2 1Kotel’nikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Mokhovaya 11, 125009 Moscow, Russia 2Department of Microtechnology and Nanoscience – MC2, Chalmers University of Technology, SE-41296, Gothenburg, Sweden 3Skobeltsyn Institute of Nuclear Physics, Department of Physics, Moscow State University, 119899 Moscow, Russia

Corresponding author: Gennady A. Ovsyannikov Kotel’nikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Mokhovaya 11, 125009 Moscow, Russia. Tel +74956297431.Fax+74956293678. E-mail: [email protected]

We present experimental investigation of hybrid heterostructures made from YBa2Cu3Ox (YBCO) base electrode with thickness of 150-200 nm. YBCO films were grown epitaxially on NdGaO3 substrates by pulsed laser deposition (PLD). SrRuO3 (SRO), La0.7Sr0.3MnO3 (LSMO) or a bilayer of LSMO/SRO films were deposieted in-situ with YBCO by PLD and used as magnetic barriers. Top superconducting Nb/Au bilayer was deposited ex-situ by magnetron sputtering. Critical current was not observed in heterostrutures with both LSMO and SRO interlayers with thickness, dM, larger than 10 nm. In thinner heterostructures, the observed critical current was attributed to the presence of pinholes pinholes [1]. The critical current was observed in heterostructures with LSMO/SRO bilayer with thickness of interlayer up to 60 nm. Obtained results are discussed in terms of generation of long-range triplet superconducting current component at superconductor/ferromagnetic interfaces consisting of ferromagnetic materials with noncollinear magnetization [2-4].

The work was supported by Russian Academy of Sciences, Russian Ministry of Education and Sciences, Scientific School Grant 2456.2012.2, RFBR project 11-02-01234" and 12-07-31207mol_a, and Swedish Institute Visby Program of Russian-Swedish collaboration.

1. A.M. Petrzhik, G.A. Ovsyannikov, A.V. Shadrin, K.Y. Constantinian, A.V. Zatsev, V.V. Demidov, Yu.V. Kislinskii, JEPT, 112, 1043 (2011).

2. A.V. Zaitsev, JEPT Lett, 83, 277 (2006). 3. F. S. Bergeret, A. F. Volkov and K. B. Efetov, Rev. Mod.Phys., 77, 1321 (2005). 4. G.A. Ovsyannikov, A.E. Sheyerman, A.V. Shadrin, Yu.V. Kislinskii, K.Y. Constantinian, A. Kalabukhov,

JEPT Lett, 97, N3 (2013).

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1.13 Detection of the Triplet Spin-Valve-Effect in Superconductor/Ferromagnet Proximity Coupled Layered Heterostructures !

Anatolie Sidorenko1*, Vladimir Zdravkov1, 2, Roman Morari1, Jan Kehrle2, Hans-Albrecht Krug Von Nidda2, Reinhard Tidecks2, Siegfried Horn2, Lenar Tagirov3 1 Institute of Electronic Engineering and Nanotechnologies ASM, Chisinau MD2028 Moldova 2University of Augsburg, D-86159 Augsburg, Germany 3 Kazan Federal University,420008 Kazan, Russia

Corresponding author: Institute of Electronic Engineering and Nanotechnologies ASM, Chisinau MD2028 Moldova , E-mail: [email protected]

The theory of superconductor-ferromagnet (S-F) heterostructures predicts the generation of a long-range, odd-in-frequency triplet pairing in samples with two ferromagnetic layers at non-collinear alignment (NCA) of the magnetizations of the F-layers. This triplet pairing we have detected experimentally in a Nb/Cu41Ni59/NL/Co/CoOx spin-valve type proximity effect coupled heterostructure (whith a very thin Nb film between the F-layers served as a spacer of normal conducting metal, NL-layer). The resistance of the sample as a function of an external magnetic field shows that the system is superconducting at a collinear alignment of the Cu41Ni59 and Co layers magnetic moments, but switches to the normal conducting state at a NCA configuration. The last is the evidence that the superconducting transition temperature Tc for NCA is lower than the fixed measuring temperature. The existence of a minimum Tc, at the NCA regime below that one for parallel or antiparallel alignments of the F-layer magnetic moments, is consistent with the theoretical prediction of the appearance of the long-range triplet pairing.

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1.14 Strong Contribution of surface Andreev Bound States to the Nonlinear Meissner Effect in d-wave superconductors !

Thomas Dahm, Aida Zare, and Nils Schopohl* Physics Department, University of Bielefeld, D-33501 Bielefeld, Germany *Institute for Theoretical Physics, University of Tübingen, D-72076 Tübingen, Germany

Corresponding author: Thomas Dahm, Fakultät für Physik, Universität Bielefeld, Postfach 100131, D-33501 Bielefeld, Germany

In an unconventional superconductor Andreev bound states appear at the surface under certain circumstances. In the presence of a screening current these states carry an anomalous counterflowing paramagnetic current. At low temperatures this anomalous surface current leads to a strong modification of both linear and nonlinear electromagnetic properties. Here, we will review our recent theoretical work on this subject [1-4]. We have shown that the anomalous surface current leads to a significant increase of the Bean- Livingston barrier for vortex entry and thus may be used to hinder entrance of vortices into an unconventional superconductor [1]. The anomalous surface current also has a strong nonlinear electromagnetic response that grows proportional to 1/T3 at low temperatures [3]. It dominates the bulk nonlinear Meissner effect [5] at temperatures below T/Tc ~ #-1/2 already. This effect can be probed by intermodulation distortion or harmonic generation experiments [5,6]. Relation to recent experimental studies is discussed [7]. If time permits, we will discuss our recent work on the pairing mechanism in YBCO6.6 [8]. It recently became possible to correlate structures observed in angular resolved photoemission spectroscopy (ARPES) with structures observed in inelastic neutron scattering (INS) experiments on the same YBCO6.6 single crystals using a theoretical calculation. The results suggest that the pairing mechanism in YBCO6.6 is mostly of magnetic origin and its strength is sufficient to explain the high Tc values [8].

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[1] C. Iniotakis, T. Dahm, and N. Schopohl, Phys. Rev. Lett. 100, 037002 (2008). [2] S. Graser, C. Iniotakis, T. Dahm, and N. Schopohl, Phys. Rev. Lett. 93, 247001 (2004). [3] A. Zare, T. Dahm, and N. Schopohl, Phys. Rev. Lett. 104, 237001 (2010). [4] A. Zare, A. Markowsky, T. Dahm, and N. Schopohl, Phys. Rev. B 78, 104524 (2008). [5] T. Dahm and D. J. Scalapino, J. Appl. Phys. 81, 2002 (1997). [6] T. Dahm and D. J. Scalapino, Phys. Rev. B 60, 13125 (1999). [7] A. P. Zhuravel et al, arXiv:1208.1511 (2012). [8] T. Dahm et al, Nature Physics 5, 217 (2009).

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1.15 Theory for Collective Modes and Interface Effects of non-centrosymmetric Superconductors !

N. Bittner*, L. Klam**, D. Einzel***, and D. Manske* *Max Planck Institute for Solid State Research, Stuttgart, Germany **Institute for Theoretical Physics, ETH Zürich, Switzerland ***Walther-Meissner-Institut, Garching, Germany

Corresponding author: Prof. Dr. Dirk Manske, Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany. E-mail: [email protected]. Tel/FAX +49-711-689-1552/1702

We present a systematic study of the response and pairing properties of non- centrosymmetric superconductors (NCS). Starting from a matrix kinetic theory, we formulate a general response theory that allows to study various optical experiments and gauge properties [1]. We provide one particular example of electronic Raman scattering in which we predict to determine the unknown singlet-triplet ratio [2]. In a next step, we solve the matrix kinetic equations in the presence of strong asymmetric Rashba-type spin- orbit coupling (ASOC). Particular emphasis is on the existence, the dispersion and the general role of the so-called Leggett mode, which arises as a consequence of interband pairing correlations. The occurrence of this massive collective mode of the order parameter is analyzed in view of its experimental observability in all physically relevant spin- independent collisionless response functions like the Lindhard density response, the dielectric function, the current response (dynamic conductivity) and the electronic Raman response [3,4]. Finally, we study the consequences of pairing mixing between singlet and triplet correlations for interface effects. Particular characteristic features are observed in the vicinity of a diffuse ferromagnet [5].

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[1] L. Klam, D. Manske, and D. Einzel, Kinetic Theory for Response and Transport in Non-Centrosymmetric Superconductors (32 pages) in 'Non-centrosymmetric superconductors', Lecture Notes in Physics 847 (2012), Spinger, Berlin, Heidelberg; Eds.: E. Bauer and M. Sigrist. [2] L. Klam, D. Einzel, and D. Manske, Phys. Rev. Lett. 102, 027004 (2009). [3] N. Bittner, L. Klam, D. Einzel, and D. Manske, preprint. [4] N. Bittner, D. Einzel, and D. Manske, unpublished. [5] G. Annunziata, D. Manske, and J. Linder, Phys. Rev. B 86, 174514 (2012).

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1.16 Magnetic edge states in spin triplet superconductors !

Alfonso Romanoa, Paola Gentilea, Canio Nocea, Ilya Vekhterb and Mario Cuocoa

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a CNR-SPIN and Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno, I-84084 Fisciano (Salerno), Italy bDepartment of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana, 70803, USA

Corresponding author: Dr. Paola Gentile, CNR-SPIN, U.O.S Salerno, Dipartimento di Fisica "E. R. Caianiello", Universita degli Studi di Salerno,Via Ponte don Melillo, I-84084 Fisciano (SA), Italy. Phone: +39 089 969149 Fax: +39 089 969658. E-mail : [email protected]

Recognition that the surface states in correlated materials reflect the nature of the interactions and orders existing in the bulk has led to a very significant research effort aimed at the understanding, and potential control, of these electronic states [1]. Systems where the bulk of the material is gapped but the boundary supports gapless modes are especially interesting since under these conditions the surface states are robust, and may be topologically protected. The bulk gap may be due to the band structure, or, in a metal, may arise at low temperatures from electron-electron interaction, as in superconductors. Simple band insulators or conventional superconductors do not support robust low-energy states at the boundary. It is the study of their counterparts, where the bulk that is topologically nontrivial, and hence the bulk-boundary correspondence theorem dictates the existence of the surface states, that has been a focus of much recent attention. One of the best candidates for the topological superconductivity is Sr2RuO4, where the emergent consensus indicates triplet chiral pairing. In this material the topologically protected edge states have been predicted, and their signatures were recently found in tunnelling spectroscopy experiments. Andreev bound states (ABS) near the surface are also expected in Sr2RuO4 due to the sign change of the superconducting order parameter which is experienced by the quasiparticles reflecting off the sample boundary. We investigate the nature of the ABS at the surface of spin triplet superconductors, combining numerical solution of the Bogoliubov-De Gennes equations for a tight-binding model with nearest-neighbor attraction [2], and the symmetry based Ginzburg-Landau approach. We show that a spontaneous magnetic moment may appear at the edge of a spin- triplet superconductor if the system allows for pairing in a subdominant cannel. We find that a potential barrier modulating the electronic density near the edge of the system leads to a non-unitary superconducting state close to the boundary where spin-singlet pairing coexists with the dominant triplet superconducting order. We demonstrate that the spin polarization at the edge appears due to the inhomogeneity of the non-unitary state and originates in the lifting of the spin-degeneracy of the Andreev bound-states [3]. These results strongly suggest that triplet superconductors can be used in spin-active heterostructures.

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[1] M. Eschrig, Physics Today 64, 43 (2011); M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010); X.L. Qi and S. C. Zhang, Rev. Mod. Phys. 83 1057 (2011) [2] M. Cuoco, A. Romano, C. Noce, P. Gentile, Phys. Rev. B 78, 054503 (2008). [3] A. Romano, P. Gentile, C. Noce, I. Vekhter, M. Cuoco, arXiv:1210.8282

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1.17 Strongly correlated material UPt3 as a topological superconductor

Kazushige Machida and Yasumasa Tsutsumi* Department of Physics, Okayama University, Okayama 700-8530, Japan *Condensed Matter Theory Laboratory, RIKEN, Wako, Saitama, 351-0198, Japan

Corresponding author: Kazushige Machida, Department of Physics, Okayama University, Okayama 700-8530, Japan

Motivating by our own recent angle-resolved thermal conductivity measurement [1], that shows a remarkable two-fold symmetric oscillation in a heavy Fermion superconductor UPt3 with hexagonal crystal, we have theoretically investigated the pairing symmetry of this material and identified it to belong to a two dimensional E1u representation with f-wave orbital character and spin triplet symmetry [2]. The identified pairing class explains not only the observed two-fold oscillation in high field low temperature C phase, but also a variety of experimental data accumulated over two decades. We first classify possible pairing symmetries allowed group-theoretically and examine each state in view of the existing data and established phase diagram in field versus temperature, consisting of three phases. With the identified state with E1u representation we microscopically calculate the vortices for the three A, B, and C phases by using Eilenberger formulation. Then we obtain spatial quasi-particle structures around a vortex core, the core bound states, and complicated phase relation between the multi-components. The gap structure consists of two horizontal nodes and two point nodes in the B phase of low temperature and low field while in A and C phases a vertical line node in addition to the two horizontal nodes. The latter naturally explains the above mentioned two-fold thermal conductivity oscillation in the C phase [1]. Finally, it now becomes clear that UPt3 can be a good candidate for the so-called topological superconductor because the vortex core and surface contain the Majorana modes. We propose several experiments to detect the Majorana quasi-particles in this material. [1] Y. Machida, A. Itoh, Y. So, K. Izawa, Y. Haga, E. Yamamoto, N. Kimura, Y. Onuki, Y. Tsutsumi, and K. Machida, Phys. Rev. Lett., 108, 157002 (2012). [2] Y. Tsutsumi, K. Machida, T. Ohmi, and M. Ozaki, J. Phys. Soc. Jpn., 81, 074717 (2012). !

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1.18 Majorana Fermions in hign Tc superconducting hybrid devices Procolo Lucignano !ISC-CNR, UoS di Tor Vergata e Dipartimento di Scienze Fisiche, Universita` di Napoli “Federico II”CNR


Majorana Fermions have been predicted in a wide class of low-dimensional solid-state devices. Many of the proposals make use of superconductors in contact with topological insulators or quasi one-dimensional materials with strong spin-orbit interaction. We propose an alternative platform based on high-critical-temperature cuprate superconductors. They can induce superconductivity, by proximity effect, in nanowires with strong spin orbit coupling, giving rise to a wider and more robust range of conditions to stabilize Majorana Fermions due to the large gap values. They can also offer novel functionalities in the design of the experiments thanks to their order parameter anisotropy. P. Lucignano, A. Mezzacapo, F. Tafuri, A. Tagliacozzo Phys. Rev. B 86, 144513 (2012) P. Lucignano et al. in preparation!

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1.19 Superconducting Proximity Effect in the Topological Insulator Bi2Te3 S. Charpentier1*, L. Galletti2, M. Ekström1, Y. Song3, S. Wang3, F. Tafuri2, T. Bauch1, F. Lombardi1

1Quantum Device Laboratory, Chalmers University of Technology, Sweden

2Photonic Labororatory, Chalmers University of Technology, Sweden

3Physics departement,University of Napoli, Italy!

Bismuth based compounds Bi2Se3 and Bi2Te3 have been recently identified as Topological Insulators (TIs) materials (1-3). These compounds are insulating in the bulk, but possess a conducting surface. The topological insulators, in combination with the possibility to form Majorana fermions, when they are contacted with a superconductor (4-6), can open new prospective for both fundamental studies and applications. Unfortunately, the signature of the 2D conducting surface layer is obscured by parasitic bulk conduction, coming from vacancies and defects. Here we present a study of induced superconductivity in thin films of Bi2Te3 by Al/Ti electrodes. The films were grown by molecular beam epitaxy using various deposition conditions. This technique is known to be optimal to grow highly pure crystal, making it the natural choice to reduce bulk conductivity hindering the properties of the 2D conducting surface. The films have been exfoliated to obtain flakes which are then transferred on a SiOx substrate. The flakes are contacted with Al/Ti electrodes by using e-beam lithography. The presence of proximity effect through Bi2Te3 has been confirmed by the observation of a Josephson current showing a clear Fraunhofer-like dependence as a function of the external magnetic field and several high order Shapiro steps under microwave irradiation. Measurement at mK temperature to reveal the presence of Majorana fermions at the TI/superconductor interface are in progress. References:

1. Chen, Y. L. et al. Science 325, 179 (2009) 2. Hsieh, D. et al. Nature 460, 1101 (2009) 3. Zhang et al. Nat. Phys. 5, 438 (2009) 4. Fu, L. et al. Phys. Rev. Lett. 100, 096407 (2008) 5. Nilsson et al. Phys. Rev. Lett. 101, 120403 (2008) D1 Tanaka, Y. et al. Phys. Rev. Lett. 103 107002 (2009)!

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1.20 The onset of magnetism around x=1/4 in Lnfe1-xRuxAsOl-yfy (Ln=Sm, Nd or La) optimally electron-doped superconductors !

De Renzi R. 1, Allodi G. 1, Mazzani M. 1, Bonfa P. 1 , Sanna S. 2, Carretta P. 2, Prando G. 2,3, Lamura G. 4, Shiroka, Kobayashi Y.6, Sato M. 6 1 Dipartimento di Fisica e Scienze della Terra, e CNISM, Universita di Parma, I 2 Dipartimento di Fisica e CNISM, Universita di Pavia, I 3 Leibnitz Institute IFW Dresden, DE 4 Istituto Spin CNR, Genova, I 5Laboratorium Festkoerperphysik, ETH Honggenberg, Zuerich, CH 6 Department of Physics , Nagoya University, Japan

We have exploited the substitution of Fe with Ru (and Mn) to gain insight on the superconducting pairing of the quaternary iron oxypnictides LnFe1-xRuxAsO1-yFy with Ln a lanthanide ion.

We have shown that the nominally isovalent Ru:Fe substitution in the parent y=0 composition suppresses magnetic order only at the percolation threshold x00.6. The effect of this substitution in the optimally electron-doped superconductors (0.11<y<0.15) is more intriguing. Muon spectroscopy results indicate that the appearance of static magnetism, nanoscopically coexisting with superconductivity, is a generic feature of this substitution. The magnitude of the internal field at the muon, the signature of this static magnetism, is peaked arounjd x=1/4 and it is more enhanced for those materials characterized by a larger value of the z cell lattice parameter of As. Remarkable, those are also the ones with the highest superconducting transition temperature for x00. The magnetic phase disappears at a temperature TN(x) which shows a dome-like x-dependance.

The reduction of the superconducting transition temperature Tc(x) is found to be significant in the x region of the phase diagram where the static magnetism develops. Superconductivity eventually disappears with increasing x at x=0.6. the role of impurities will be discussed both in terms of pair breaking and order parameter symmetry, as well as in connection with nematic electron phases and pinning of static local configurations.

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1.21 Coexistence of different electronic phases in the K0.8Fe1.6Se2 Fe-based superconductor: a bulk-sensitive hard x-rays spectroscopy study. !

L. Simonelli1, N.L. Saini2, M. Moretti Sala1, Y. Mizuguchi3, Y. Takano3, H. Takeya3, T. Mizokawa4, and G. Monaco1

1European Synchrotron Radiation Facility, BP220, F-38043 Grenoble Cedex, France 2Dip. di Fisica, Università di Roma “La Sapienza”, Roma, ItalyCenter for Biomolecular

The discovery of superconductivity in FeSe (11)-type chalcogenides has been an important finding to progress in the understanding of Fe-based superconductors (pnictides and chalcogenides) since it has emphasized the central role played by the ubiquitous Fe-Fe planes and interacting anion (pnictogen or chalcogen) atoms. Although FeSe (11)-type chalcogenides have been widely regarded as model systems to explore the electronic structure [1], they lack the spacer layers present in the most common RFeAsO (1111) and AFe2As2 (122)-type of pnictides. Very recently, FeSe layers have been successfully intercalated by alkaline atoms (K, Rb, Cs), with intercalated chalcogenides showing superconductivity at 30 K [2-6], unlike the (11)-type of chalcogenides (maximum Tc: 15 K). Several experiments indicate the importance of Fe vacancies in AxFe2-ySe2 (11)-type chalcogenides to play a crucial role in determining their electronic and magnetic properties. Superconductivity appears to occur only in Fe-deficient samples, where the alkali metals are intercalated between the FeSe layers [7-9]. Here we report on the electronic and magnetic structure of the new KxFe2Se2 superconductor by x-ray emission (XES) and high resolution absorption spectroscopy (XAS). We report a temperature dependent study where the electronic and magnetic properties are investigated at the same time as a function of temperature in several consistent thermal cycles in the temperature range 10-600 K. In particular, we discuss the effect of ordered and disordered Fe vacancies [10-12] on the electronic and magnetic structure, the existence of memory effects on thermal cycles, and the relation between electronic and magnetic properties and superconductivity. The results on the KxFe2Se2 are compared with the electronic and magnetic properties of (11)-type chalcogenides [1] to distinctly identify the effect of the intercalation on the electronic properties of Fe-based superconductors.

Reference:

[1] L.Simonelli et al., Phys. Rev. B 85, 224510 (2012) ;[2] J. Guo, et al, Phys. Rev. B 82, 180520 (2010). ;[3] Y. Mizuguchi, et al, Appl. Phys. Lett. 98, 042511 (2011); [4] J. J. Ying et al , Phys. Rev. B 83, 212502 (2011); [5] A. Krzton-Maziopa, et al, J. Phys.: Condens. Matter 23 (2011) 052203; [6] Ming-Hu Fang et al 2011 EPL 94 27009; [7] Jing Guo, et al, arXiv: 1101.0092; Y. Kawasaki, et al, arXiv: 1101.0896; J J Ying et al 2011 New J. Phys. 13 033008; [8] D. M.Wang, et al, arXiv: 1101.0789 ; [9] A. M. Zhang, et al, arXiv: 1101.2168v1; [10] Ricci et al. Supercond. Sci. Technol. 24 No 8, 082002 (2011); [11] Bao Wei et al., Chin. Phys. Lett. 28, 086104 (2011) ; [12] F. Han et al., arXiv :11031347 !

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1.22 Perspectives for electronic applications of iron-based superconductors Sergio Pagano Dipartimento di Fisica “E.R. Caianiello” and CNR-SPIN , University of Salerno, Italy

Corresponding author: Sergio Pagano, Dipartimento di Fisica Università di Salerno, via Ponte Don Melillo, 84084 Fisciano(SA),Italy

Recent investigations on iron-based superconductors have revealed a lot of similarities to MgB2 and the cuprates, for instance, a multiband nature, high upper critical fields and a short coherence length. Beside large current and magnetic field applications, it is important to explore potential electronics applications such as Josephson devices and SQUIDs. The feasibility of electronics applications will be discussed in details by establishing the fundamentals of the iron-based superconductors. Additionally, the exploitation of the Josephson effect and SQUIDs, by the so-called phase-sensitive experiments, paves the way to understanding fundamental properties such as order parameters symmetry and energy gap. These possibilities will also be discussed.

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1.23 Tuning superconductivity and pinning mechanisms of Fe(Se,Te) thin films through different substrates Valeria Braccini, Emilio Bellingeri, Shrikant Kawale, Andrea Gerbi, Luca Pellegrino, Alberto Sala, Marina Putti*, Carlo Ferdeghini CNR – SPIN, Corso Perrone 24, I-16152 Genova, Italy *Also at Department of Physics, University of Genova, Via Dodecaneso 33, I-16146 Genova, Italy

Corresponding author: Valeria Braccini, CNR-SPIN, Corso Perrone 24, I.16152 Genova, Italy. Ph. +39-010-6598722, e-mail: [email protected]

Pnictide thin films of the family FeSe0.5Te0.5 were deposited by Pulsed Laser Ablation on oxide and fluoride substrates: we will try here to present an extensive correlation between their structural, electrical, transport and superconducting properties. Through the growth of thin films it is possible to tune the superconducting properties of the phase depending on the growth conditions. In particular, the in-plane lattice constant of the substrate influences the crystallographic lattice parameters of the films, therefore affecting the growth induced planar strain. This strain is responsible for the deformation of the Iron-calchogen tetrahedron that induces strong enhancements of the critical temperature Tc and deformation of different pockets of the Fermi surface thus increasing the upper critical field. The lattice mismatch also induces the formation of defects that further improve the superconducting properties by enhancing the vortex pinning. Among the oxide substrates, LaAlO3 resulted to be the best one to obtain samples with significant compressive strain that can boost the critical temperature up to 21 K. CaF2 allowed us to grow films with high crystalline quality, excellent epitaxy, clean interface and introducing strain with related Tc enhancement up to similar values as those obtained on LaAlO3 substrates. Furthermore, critical current density shows, at liquid helium temperature, very high values with a very good field dependence (6 105 A/cm2 at 9 T) and completely isotropic at low temperatures. On the other side, if films grown on SrTiO3 showed a lower value of Tc, the critical current and the pinning was somehow enhanced. The presence of strong correlated pinning for field perpendicular to the surface in films grown on SrTiO3 was confirmed by the analysis of the activation energy for vortex motion U0 and by the direct observation through both Scanning Tunnel Microscopy and Transmission Electron Microscopy of nanoscale threading dislocations which are not present in films gown on LaAlO3, and probably also by small angle grain boundaries present in the film, compatible with the lower in plane epitaxy evidenced by the XRD & scans.

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Symposium 2

Dielettrics, ferroeletrics, multiferroic

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2.11 Large tunnel electroresistance controlled by ferroelectric domain switching in BaTiO3 and BiFeO3- based ferroelectric tunnel junctions V. Garcia1, A. Chanthbouala1, H. Yamada1, S. Boyn1, R.O. Chérifi1, F. Bruno1, A. Crassous1, S. Fusil1, X. Moya2, H. Yamada1, S. Xavier3, E. Jacquet1, C. Carretero1, C. Deranlot1, K. Bouzehouane1, N.D. Mathur2, J. Grollier1, M. Bibes1, A. Barthélémy1

Unité Mixte de Physique CNRS/Thales, 1 Av. A. Fresnel, 91767 Palaiseau, France 2Department of Materials Science, University of Cambridge, Pembroke Street, CB23QZ, Cambridge, United Kingdom 3Thales Research and Technology, 1 Av. A. Fresnel, 91767 Palaiseau, France

Corresponding author: Vincent Garcia – Unité Mixte de Physique CNRS/Thales – Thales Research and Technology – 1 Av. A. Fresnel – 91767 Palaiseau, France – [email protected]

After being conceptualized in the early 1970's, ferroelectric tunnel junctions (FTJs), where an ultrathin ferroelectric film is sandwiched between two electrodes, have remained elusive for more than 30 years. At room temperature, we use piezoresponse force microscopy (PFM) to show robust ferroelectricity in BaTiO3 ultrathin films, and conductive-tip atomic force microscopy to demonstrate the non-destructive resistive readout of the polarization state via its influence on the tunnel current [1]. Solid-state devices were fabricated out of these ferroelectric tunnel barriers. These FTJs show OFF/ON ratio as large as 100 with fast and low energy resistive switching correlated to ferroelectric switching [2]. By varying the voltage pulse amplitude, intermediate resistance states are reachable and PFM imaging reveals a direct correlation between resistance and ferroelectric domains configuration. The FTJs behave as memristors where quasi-continuous resistance variations with cumulated voltage pulses in the ns range can be interpreted with standard models of ferroelectric domains dynamics [3]. More recently, we observed giant tunnel electroresistance (OFF/ON ratio >10,000) in solid-state ferroelectric tunnel junctions based on ultrathin films of BiFeO3. Using PFM imaging, we demonstrated that the changes in resistance scale with the nucleation and growth of ferroelectric domains in the ultrathin BiFeO3. These devices emerge as an alternative to other resistive binary or analog memories with the advantage of not being based on voltage-induced migration of matter at the nanoscale, but on a purely electronic mechanism. (We acknowledge financial support from the European Research Council (ERC Advanced Grant FEMMES, No. 267579)) [1] V. Garcia, S. Fusil, K. Bouzehouane, S. Enouz-Vedrenne, N. D. Mathur, A. Barthélémy, M. Bibes, Nature 460 (2009) 81-84. [2] A. Chanthbouala, A. Crassous, V. Garcia, K. Bouzehouane, S. Fusil, X. Moya, J. Allibe, B. Dlubak, J. Grollier, S. Xavier, C. Deranlot, A. Moshar, R. Proksch, N.D. Mathur, M. Bibes, A. Barthélémy, Nature Nanotechnology 7 (2012) 101-104. [3] A. Chanthbouala, V. Garcia, R.O. Cherifi, K. Bouzehouane, S. Fusil, X. Moya, S. Xavier, H. Yamada, C. Deranlot, N.D. Mathur, M. Bibes, A. Barthélémy, J. Grollier, Nature Materials 11, 860-864 (2012). !

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2.12 Magneto-electric effect in multiferroic tunnel junctions from first principals Thomas Archer and Stefano Sanvito. School of Physics, Trinity College Dublin, Ireland.

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Multiferroic compounds represent a class of multifunctional materials that simultaneously exhibit several ferroic orders. The coexistence of different order parameters brings about novel physical phenomena and offers possibilities for new device functionality. Single phase multiferroics are not currently suitable for device applications. Multiferroic tunneling junctions may however be the holy grail, drawing together several ferroic attributes in a single device. The Fe/MgO/Fe magnetic tunneling junction has already revolutionized the hard disk industry by reducing the size of the read head into the nanoscale. By replacing the inert MgO with an active layer, for example a ferroelectric oxide, it may be possible to manipulate the interplay between the ferroic state of the different layers in the device.

The coexistence of tunneling magneto-resistance (TMR) and tunneling electro-resistance (TER) has been experimentally and theoretically demonstrated. However our understanding of how these two processes can couple is somewhat lacking. Several mechanisms have been proposed in the literature for the magneto-electric (ME) coupling, however it is unclear how each mechanism will affect the tunneling current. In this work we calculate the tunneling current for the Fe/BaT iO3 /MgO/Fe and SrRuO3/BaT iO3 /SrTiO3/SrRuO3 tunnel junctions, in various geometries, using the non-equilibrium Green’s function formalism (NEGF) to demonstrate how each mechanism influences the tunneling current.

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2.13 Co/Fe/BaTiO3 junctions – a new type of multiferroic interface Evgeny Plekhanova, Silvia Picozzia, Greta Radaellib, Riccardo Bertaccob

a CNR-SPIN, L’Aquila, Italy b LNESS center, Polo regionale di Como – Politecnico di Milano, Como, Italy

Corresponding author: Evgeny Plekhanov, CNR-SPIN, UOS L'Aquila Via Vetoio, 10 67100 Coppito, L'Aquila, Italy, [email protected]

Ferromagnetic-ferroelectric interfaces represent an important class of multiferroic materials owing to the possible applications in the field of non volatile magnetic RAMs (NVRAMs) and four state memory devices. For the successful realization of such devices, a high magneto-electric coupling (MEC) at the interface between the ferromagnetic and ferroelectric order parameters is necessary. Several types of interfaces have been proposed in the literature, whose two main ingredients being iron and barium titanate BaTiO3 (BTO) [1-4].

Here we propose a new type of MEC interface, containing a FeO layer, in which the transition occurs between the ferromagnetic metallic state and the antiferromagnetic insulating one upon switching the BTO polarization. By using density functional theory calculations within GGA+U approximation, we show how the polarization switch corrugates the ferromagnetic FeO layer, turning it into an antiferromagnet. Upon such a transition, a small gap opens in the local density of states, corresponding to the Fe atoms at the interface. Recently, these theoretical results have been confirmed experimentally by measuring the magnetic properties of interfacial Fe layers at room temperature with the aid of X-ray Magnetic Circular Dichroism (XMCD) technique on fully epitaxial Co/Fe/BTO/La0.7Sr0.3MnO3//SrTiO3(001) heterostructures. The presence of the Co layer (1nm thick) is important in order to stabilize ferromagnetism in the ultrathin (between 1 and 3 MLs thick) Fe film [5]. For a series of Fe layer thicknesses (1, 2, 3 MLs) it is possible to distinguish the two absorption peaks corresponding to the metallic Fe and the oxidized one, belonging the latter to the FeO layer in contact with BTO. When the BTO polarization is switched, no variation is seen in the metallic Fe signal, while a new feature appears in the oxidized Fe dichroic signal, indicating a change in magnetic order at the interface. The sign of this effect (the correspondence between the direction of BTO polarization and FeO magnetic order) predicted theoretically matches the experimental one. Finally, we discuss the strength of MEC in case of the interfaces with terminations other than the hypothesized one (TiO).

[1] Chun-Gang Duan, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett 97, 047201 (2006)

[2] J. D. Burton and E. Y. Tsymbal, Phys. Rev. Lett 106, 157203 (2011)

[3] S. Valencia et al., Nat. Mater. 10, 753–758 (2011)

[4] L. Bocher, et al., Nano Lett. 12, 376-382, (2012)

[5] R. Bertacco et al., Surf. Sci. 454–456 671–675, (2000)

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2.14 Electro-Optical Response and Strain-Dependent Optical Properties of Multiferroic BiFeO3 Thin Films D. Sando1, P. Hermet2, J. Allibe1, C. Carrétéro1, E. Jacquet1, J. Bourderionnet3, J.-C. Mage1, Ph. Ghosez2, A. Barthélémy1 and M. Bibes1* 1Unité Mixte de Physique CNRS/Thales, 1 Av. A. Fresnel, 91767 Palaiseau, France 2Département de Physique, Université de Liège, B-5, B-4000 Sart-Tilman, Belgium 3Thales Research & Technology, 1 Av. A. Fresnel, 91767 Palaiseau, France


The room-temperature multiferroic bismuth ferrite (BiFeO3 – BFO) is proving to be an extremely versatile multifunctional oxide [1]. It is an outstanding ferroelectric with a record polarization of 100 $C/cm2, and it displays G-type antiferromagnetic ordering with a cycloidal modulation of the Fe spins. In addition to its strong promise in spintronics, it exhibits potentially valuable optical properties. Characteristics of interest include a visible range band gap, low absorption in the infra-red range, and a large birefringence, as well as attractive photovoltaic properties. These optical properties make BFO an interesting candidate as the active medium in thin film integrated electro-optic modulators [2]. For this application, it is important to have a detailed understanding of the electro-optical response of the medium, that is, the change in the refractive index when an electric field is applied – the so-called electro-optic (EO) coefficients. To date, the EO coefficients of BFO have not been reported.

Another aspect of particular interest is the dependence of the BFO material properties on misfit strain. Strain can drastically modify many physical properties of epitaxial thin films; a striking example being the strong reduction of the Curie temperature TC of BFO under strong compressive strain [3]. It has also been shown that strain, through its effect of distorting the lattice structure, can cause a rotation in the direction of polarization, and thus the z-component of the polarization can be controlled by strain. Experimentally, strain engineering can be implemented by growing thin films of BFO on various substrates that impose a wide range (4%) of strains ranging from compressive to tensile.

In this presentation we will first describe our efforts in experimentally determining the electro-optic coefficients of BFO using two different sample orientations and symmetries, and compare our results to those of first-principles calculations. In addition to the electro-optical properties, we will discuss the potential of strain engineering for optimizing the optical properties of BFO thin films. Using ellipsometry and transmission spectroscopy measurements we characterize our samples and observe a systematic trend of the optical band gap with strain. Lastly, we will discuss the potential of making use of the presented results to incorporate this multifunctional material into thin film integrated optical devices. References [1] G. Catalan and J.F. Scott, Adv. Mater. 21, 1 (2009). [2] J. Allibe et al., Appl. Phys. Lett. 96, 182902 (2010). [3] I.C. Infante et al., Phys. Rev. Lett. 105 (2010), 057601. !

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2.15 Low Energy Electron Microscopy for the study of oxides surfaces Jan Aarts, Marcel Hesselberth, Alexander v.d. Torren and Sense Jan v.d. Molen Kamerlingh Onnes Laboratory, University of Leiden PO Box 9504, 2300 RA Leiden, The Netherlands

Corresponding author: Prof. Dr. J. Aarts, Kamerlingh Onnes Laboratory, University Leiden, PO Box 9504, 2300 RA Leiden, The Netherlands. [email protected]

Given the current interest in the growth and properties of oxides and oxide interfaces, knowledge of morphology ot other characteristics of a substrate or film surface at various temperatures and pressures, is of obvious importance. We bring a new experimental method to bear on such questions, by using Low Energy Electron Microscopy (LEEM). The method allows dynamic observation of surfaces in a large regime of temperatures up to 1200 °C, both in ultrahigh vacuum and in low oxygen pressures, and with high lateral resolution. Moreover, not only electron diffraction and microscopy, but also photoemission microscopy can be locally performed. We discuss the potential of LEEM with several examples. The first is a study of the SrTiO3 (001) surface in the temperature window between room temperature and 900 0C, and for oxygen pressures between 10-9 mbar and 10-5 mbar. We focus on the occurrence of the (2x1) reconstruction, since it was shown earlier that reconstruction-selective growth can be an issue when growing SrTiO3 or SrVO3 on SrTiO3 [1]. We start from a TiO2-terminated but as yet unreconstructed surface, which is for instance the starting surface for the growth for LaAlO3 and the preparation of a conducting interface. By heating in an oxygen pressure of 5x10-6 mbar (p1) to 550 0C, followed by lowering the pressure to 10-9 mbar (p2), we prepare the (2x1) reconstruction which persists when cycling pressures between p1 and p2 at 600 0C and 700 0C. We then show how the reconstruction is lost and the surface becomes oxygen defective at higher temperatures, and proceed to construct a temperature-pressure phase diagram. As a second example, we show that the local polarization of a ferroelectric furnishes a contrast mechanism for the microscope, by presenting a series of data on ferroelectric BiFeO3.

[1] S.-H. Phark et al. Appl. Phys. Lett. 98, 161908 (2011).

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2.16 Dimensional crossover and thermopower burst in Nb:SrTiO3 superlattices !

P. Delugas, A. FIlippetti, and V. Fiorentini CNR-IOM e Dipartimento di Fisica, Università di Cagliari, Italy

Corresponding author: V.Fiorentini, Dip. Fisica, Uni Cagliari, Cittadella Universitaria, 09042 Monserrato (CA), Italy, [email protected]

Among the virtues attributed to confined electrons in oxide heterostructures, the potentially large thermoelectric power is most appealing. For example, Nb-doped SrTiO3 (STO) superlattices (SLs), alternating n layers of insulating STO with m layers of Nb-doped STO (STOm/Nb- STOm), shows a multifold enhancement of in-plane thermoelectric Seebeck coefficient S over STO bulk at the same doping. The standard interpretation (S increases due to a localization-driven density-of-states boost) remains to be proved, because the degree of confinement of the 2DEG is not quite assessed, and more importantly because the multi-band nature of transport in oxide heterostructures gives rise to complicated thermoelectric behavior (see STO/LAO [1]) requiring a detailed microscopic description.

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Our direct ab initio calculations based on self-interaction corrected DFT for a 1+9 Nb-STO oxide superlattices at different in-plane doping, and transport Bloch-Boltzmann-theory calculations and models based thereupon, indicate indeed that, contrary to intuition, the difference between bulk and superlattice S is mainly determined by the charge dilution through the SL, rather than by the confinement-induced charge localization. This has a simple rationale: for a single-band system, enhancing the effective mass is tantamount to reducing the Fermi energy, in turn increasing the thermopower; but for a multi-band system a very tight 2D confinement can cause EF to rise, and be detrimental for thermopower compared to a milder confinement allowing 2DEG dilution over a larger thickness.

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The electronic properties (effective mass and spatial extension) of the mobile charge in the SL can be tuned via a diagnostic choice of the doping, with a dimensional crossover taking place at 70%, above which a fully-2D gas appears. In agreement with experiment, we find a remarkable increase in thermopower in the SL at equivalent doping, and show it to be due to the delocalization of carriers into a multitude of barely occupied bands. As a general rule, our analysis shows that in a multi-band system, a weak 2D confinement favors large thermopower more than a strong confinement which tightly traps all the charge in one or a few doped layers.

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[1] P. Delugas et al., PRL 106, 166807 (2011); A. Filippetti et al., PRB 86, 195301 (2012).

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2.17 EuTiO3 magnetoelectric properties investigated by neutrons and X-rays scattering experiments. C. Mazzoli1,2, V. Scagnoli3, M. Scavini4, M. Allieta4, H. Walker5 1Politecnico di Milano e unit`a CNISM, p.zza L. Da Vinci 32, I-20133, Milano, Italy 2European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex 9, France 3Swiss Light Source, Paul Scherrer Institut, CH 5232 Villigen PSI, Switzerland 4Dipartimento di Chimica, Universit`a degli Studi di Milano, ISTM-CNR and INSTM Unit, Via Golgi 19, 20133 Milano, Italy 5Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany

Magnetoelectric materials (MEs) are compounds showing multifunctional properties, suitable for exploitation in futuristic compact applications. For the moment, they represent a hard challenge to unravel their complexity, a necessary battle to be fought in the advancement of our understanding and in the rush for room temperature devices, still far to come. A natural battlefield for good candidate materials is the subset of multiferroic materials (MFs) characterized by both electric and magnetic order. Unfortunately, two opposite trends seem to exist in nature: either high transition temperatures (type-I) with practically no ME effect or sizable ME properties but confined at very low temperatures (type-II) are allowed. Useless to say, the two would be ideally needed for mass applications based on easily achievable phases. In particular, type-I MF perovskites of general formula ABO3 (like BiFeO3), have limited ME effects due to the intrinsic way in which the long range orders are achieved:

one of the cations brings in the magnetism (for example Fe3+, 3d5) while the other is

the source of the electric properties (for example Bi3+, via the 6s2 lone pair). By exchanging the role of the two cations in the structure, sometimes funny and unpredictable effects are found: this is the case of the magnetoelectric compound

EuTiO3, where the magnetic ions (Eu2+, S=7/2) order below TN = 5.5K. The electric response never diverges, on the contrary the dielectric constant saturates at the magnetic transition, where the ME effect is maximum [1]. We present an X-rays and neutrons scattering investigation of this peculiar material, reporting its crystallographic and magnetic structures fully refined at low temperature for the first time. Moreover, we show the interplay between the two in the ME phase, highlighting the role of structural transition [2, 3] and commenting on the presence of intrinsic nanoscopic scales in the compound.

References:

[1] T. Katsufuji and H. Takagi, Phys. Rev. B 64 (2001) 054415 [2] M. Allieta et al., Phys. Rev. B 85 (2012) 184107 [3] V. Scagnoli et al., Phys. Rev. B 86 (2012) 094432

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2.18 Assigment of polar modes of BiFeO3 resolved J. Hlinka Institute of Physics AS CR, Na Slovance 2, 18221 Prague, Czech Republic


Studies of lattice vibrations can often help in understanding of ferroelectric materials. The experimental information, however, is really useful only in in the cases where the phonon modes are well identified. Recently, several papers [1-3 and refs. therein] have discussed the assignment of the polar phonon modes of an important model multiferroic crystal BiFeO3. Although the phonon modes in this material are well defined resonances, several different possibilities for their assignment were proposed. To resolve this issue, rather detailed complementary experiments were carried out recently. For example, one recent work analyzed angular dependence of oblique modes[2], other recent work based the assignment on the analysis of the angular polarization dependence of Raman scattering intensities.[3] Still, the conclusions about the mode assignment are not the same. In this contribution, I thus propose to revisit the results of both previous studies and to explain in a as possible simple way the source of the reported disagreements. [1] R. Palai, H. Schmid, J. F. Scott, and R. S. Katiyar, Phys. Rev. B 81, 064110 (2010); 81, 139903(E) (2010). [2] J. Hlinka, J. Pokorny, S. Karimi, and I. M. Reaney, Phys. Rev. B 83, 020101(R) (2011). [3] C. Beekman, A. A. Reijnders, Y. S. Oh, S. W. Cheong, and K. S. Burch, Phys. Rev. B 86, 020403 (2012)

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2.19 Dynamic energy flow in a hybrid organic-inorganic crystal !

A. Caretta, A. O. Polyakov, R.W.A. Havenith, T. T. M. Palstra and Paul H. M. van Loosdrecht Zernike Institute for Advanced materials,University of Groningen, Nijenborg 4,9747 AG Groningen, The Netherlands

Corresponding author: [email protected] Optical Condensed Matter Physics, Nijenborgh 49747 AG Groningen, The Netherlands

Organic-Inorganic Hybrid (OIH) materials represent a new class of multiferroics, with ferroelectric order predominantly arising from the organic constituents, and magnetic order from the inorganic counterpart. A Raman spectroscopic study, combined with DFT calculations shows that the polar nature of the OIH di-phenylethylaminecoppertetrachloride [(C6H5CH2CH2NH3)2CuCl4] originates from a tilting of the dipolar organic cations and that the phase transition has a weakly first order character. In order to investigate the interplay between the organic and the inorganic moieties we performed ultrafast spectroscopy experiments aimed at unraveling the thermal coupling between both constituents. Apart from giving an estimate for the coupling constant, the data shows that the critical phenomena near the phase transition are primarily occurring in the organic part as evidenced by a diverging term of the heat capacity. We interpret these effects in terms of rotational and/or librational motions of the organic molecules. Our investigation brings microscopic insight in the origin of polar moment in this unusual ferroelectric material, and presents a novel method to measure the heat capacity of parts of a compound system as well as the coupling between these parts.

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2.20 Ferroelectric domain walls and other complex structures in BiFeO3 and related materials !

Jorge Íñiguez Institut de Ciència de Materials de Barcelona, ICMAB-CSIC

Corresponding author: Jorge Iñiguez Institut de Ciència de Materials de Barcelona, ICMAB-CSIC Campus UAB, 08193 Bellaterra, Spain. E-mail: [email protected], web: http://www.icmab.es/leem/jorge

Ferroelectric domain walls (FE-DWs) are becoming the focus of renewed attention and excitement. Modern experimental techniques permit an unprecedented control on nano- domain structures, and it is now possible to produce materials with a large volume fraction occupied by the walls themselves. Also, recent experiments suggest that the walls present distinct properties, absent in the domains, especially in what regards electronic phenomena. For example, measures of increased conductivity and photovoltaic activity indicate that FE DWs could serve as active elements for applications in oxide electronics. As a result, a new field has emerged in which the materials properties are determined by the behavior of the walls, opening the door to a wealth of possibilities for materials (i.e., domain-wall) engineering and optimization. Room-temperature multiferroic bismuth ferrite (BiFeO3 or BFO) is the perovskite oxide for which most of the recent discoveries on DW-related phenomena have been made. Previous experimental and theoretical studies have provided a basic characterization of the FE DWs in BFO, but we are still far from understanding in detail the origin of the novel phenomena being found. I will describe our first steps towards a first-principles investigation of FE DWs in BFO. We started by considering ideal walls, as they would occur in a defect-free crystal or film. Interestingly, already at that level we faced unexpected difficulties emerging from what we may call BFO’s structural complexity. In addition to the polar distortions that give raise to the spontaneous polarization, BFO’s FE phase presents a second structural instability of the ideal perovskite structure, i.e., the concerted rotations of the O6-octahedral groups usually called anti-ferrodistortive or AFD modes. I will argue that these two instabilities must be treated as independent primary order parameters of the material. Further, because they are coupled, a FE DW typically implies the occurrence of an AFD DW. In fact, I will show that the DW energy is decided by the AFD discontinuity at the wall, and not so much by the polarization step. Hence, in contrast with the usual assumption that the AFD modes play a secondary (i.e., less important) role in the FE phase of BFO, I will show that they are responsible for determining which DWs are most stable, i.e., they determine the DW structure and properties we are most likely to observe experimentally. I will present a simple rationalization of the computed DW energies, and link our results with recent findings on the occurrence of complex structural phases, involving anomalous O6-rotational and anti-ferroelectric patterns, in BFO and related materials. Time allowing, the possibility of strain-engineering the properties of BFO’s FE DWs, and preliminary results on the role of defects, will also be presented. Main collaborators: O. Diéguez (ICMAB-CSIC) and L. Bellaiche’s group (U. Arkansas).

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2.21 Competing Ferroelectric Phases and Incommensurate Magnetic Orders in Multiferroic Mn1-xCoxWO4 I. Urcelay-Olabarria1, E. Ressouche1, A. A. Mukhin2, V.Yu. Ivanov2, J. Padilla3, A.M. Balbashov4, V. Skumryev5 and J. L. García-Muñoz3* 1Institut Laue-Langevin, 6 rue Jules Horowitz BP 156, F-38042 Grenoble Cedex 9, France. 2Prokhorov General Physics Institute of the Russian Academy of Science, Moscow, Russia 3Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, E-08193, Bellaterra, Catalunya, Spain 4Moscow Power Engineering Institute, 105835 Moscow, Russia 5 Institut Català de Recerca i Estudis Avançats (ICREA), E-08010 Barcelona, Spain; and Dep. de Física, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain


MnWO4 exhibits gigantic magnetoelectric effects and belongs to the new class of frustrated multiferroics in which magnetic and ferroelectric order coexist and mutually interact [1-2]. It undergoes three successive magnetic phase transitions. Below TN = 13.5 K the spins are collinear and sinusoidally modulated (AF3, paraelectric). In the AF2 phase (7.5 K < T < 12.5 K) a spontaneous polarization along b axis coexists with a cycloidal spin structure (k = (-0.214, 1/2, 0.457)). Below 7.5 K the system is collinear commensurate (k = (±1/4, 1/2,1/2), AF1)). Addition of Co+2 ions (S=3/2) modifies the overall magnetic interactions, produces changes in the magnetic anisotropy at M+2 site and stabilizes the multiferroic behaviour [2-5]. We have studied a variety of ferroelectric and magnetic phase transitions induced by temperature or magnetic field up to 14 T in Mn1-xCoxWO4 multiferroics by single crystal and powder neutron diffraction experiments, magnetic and polarization measurements. The competition between the magnetic anisotropy and the isotropic exchange interactions give rise to a rich variety of competing multiferroic magnetoelectric phases, as predicted by the inverse Dzyaloshinskii-Moriya model, with complex magnetic orders. They have been determined (modulated, cycloidal, transverse conical incommensurate orders, ..) and the compatibility of the structure, magnetic symmetry and polarization anisotropy have been analyzed. [1] M. Kenzelmann, A.B. Harris, S. Jonas, C. Broholm, J. Schefer, S.B. Kim, C.L. Zhang, S.-W. Cheong,

O.P. Vajk and J.W. Lynn, Phys. Rev. Lett. 95, 087206 (2005). [2] Y.-S. Song, L. Q. Yan, B. Lee et al, Phys. Rev. B 82, 214418 (2010). [3] I. Urcelay-Olabarria et al, Phys. Rev. B 85, 094436 (2012); also Phys. Rev. B 85, 224419 (2012). [4] I. Urcelay-Olabarria, J. L. García-Muñoz, E. Ressouche, V. Skumryev, V.Yu. Ivanov, A. A. Mukhin,

A.M. Balbashov, Phys. Rev. B 86, 184412 (2012) [5] I. Urcelay-Olabarria, J. M. Perez-Mato, J. L. Ribeiro, J.L. Garcia-Muñoz, E. Ressouche, V. Skumryev, A.A. Muhkin, Phys. Rev. B (2013) in press.

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2.22 Electromagnon in the pyroelectric ferrimagnet !-Fe2O3 Filip Kadlec1, Christelle Kadlec1, Veronica Goian1, Mart´ı Gich2, Martin Kempa1, Ste´phane Rols4, Maxim Savinov1, Jan Proklesˇka3, Milan Orlita3 and Stanislav Kamba1 1Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic 2Institut de Cie`ncia de Materials de Barcelona-Consejo Superior de Investigaciones Cient´ıficas, Campus UAB, 08193, Bellaterra, Catalunya, Spain 3Grenoble High Magnetic Field Lab, CNRS - 25, avenue des Martyrs, Grenoble Cedex 9, France 4Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France

Corresponding author: Phone: (+420) 266 052 176, e-mail: [email protected]

Electromagnons are known as spin waves excited by the electric component of electromagnetic radiation, in contrast to classical magnons, which are excited by the magnetic component. The electromagnons can be detected in the dielectric permittivity spectra; typically, a transfer of dielectric strength from phonons to electromagnons upon varying temperature or magnetic field is observed. Such excitations are known to occur in many multiferroics, where the ferroelectric polarization is induced by a spin order. Here we report a discovery of an electromagnon in the far-infrared spectra of %-Fe2O3 nanograin ceramics appearing below 100 K. %-Fe2O3 is a less known phase of iron oxide, which is stable only in the form of nanoparticles with a diameter of about 30 nm. It is a charge transfer insulator, exhibiting a superexchange interaction in the Fe–O–Fe chains and a record value of the room-temperature coercive field of Hc & 20 kOe. Below 110 K, its ferrimagnetic structure becomes incommensurately modulated. We have performed temperature-dependent infrared reflectivity measurements, which revealed a transfer of phonon strength to the electromagnon peak appearing near 10 meV. Terahertz time-domain spectroscopy shows a temperature- and magnetic-field-dependent ferromagnetic resonance near 0.5 meV and reveals that the electromagnon peak is sensitive to magnetic field. Inelastic neutron scattering data show that the electromagnon frequency corresponds to a magnon from the Brillouin zone boundary. Based on dielectric impedance measurements, we show that the spontaneous polarization of %-Fe2O3, which has a non-centrosymmetric orthorhombic structure (space group P na21), cannot be switched using electric field. Therefore, the compound is pyroelectric and it does not belong to classical multiferroics. This leads us to the conclusion that the existence of electromagnons is not limited to spin-induced ferroelectrics. Moreover, the used combination of techniques enabled us to identify the electromagnon, for the first time, without a single- crystalline sample. Owing to its high coercive field and magnetoelectric coupling, the % phase of Fe2O3 appears to be a promising material for applications, where dynamical magnetoelectric interactions are required.

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2.23 Crystal Engineering of Ferroelectric 214-Ruddlesden-Popper Phases James M Rondinelli and Prasanna Balachandran Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104 USA

Corresponding author: 3141 Chestnut Street, Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104; [email protected]; http://mtdg.materials.drexel.edu

Polar oxides which lack inversion symmetry are of particular technological interest, owing to their functional piezo-, pyro- and ferroelectric responses and their non-linear optical behavior. Yet, structurally simple and polar oxide materials are relatively rare—they comprise less than 2% of the over 150,000 known inorganic compounds. Here, we show how the confluence of state-of-the-art electronic-structure computations with group theoretical methods and materials informatics techniques are proving indispensable in the search for and discovery of new ferroelectrics in oxide thin film and heterostructures.

We explain how these theoretical tools are used to design a new class of A-site cation-ordered perovskite ferroelectrics from combinations of the centric, non-polar, transition metal–oxygen octahedra that form the perovskite framework [1,2]. First, we describe the microscopic origin for the loss of inversion symmetry and the electric polarization in layered (A,A’)BO3 perovskite oxides. We then show that the mechanism can be translated to AnBnO3n+1 (n=1,2) Ruddlesden-Popper (RP) oxides with disconnected layers of corner-sharing octahedra. We use the group theoretical methods to enumerate the structural criteria required to remove the mirror symmetry elements, and then evaluate the stability of the possible ground state structures suggested by that analysis with density functional theory (DFT) calculations. We predict that ordering of divalent and trivalent cations in a layered RP manganite leads to a polar phase, space group Pca21, with a sizeable polarization. We anticipate that these findings will contribute to our understanding of not only new forms of ferroelectricity, but also the behavior of most ferroics materials in artificial geometries.

[1] J.M. Rondinelli, and C.J. Fennie, Adv. Mater., 24, 1961 (2012). [2] N.A. Benedek, A.T. Mulder, and C.J. Fennie, J. Sol. Stat. Chem., 195, 11 (2012).

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Wednesday, May 22nd 2013

Plenary

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Exploiting Dimensionality and Defect Mitigation to Create Tunable Microwave Dielectrics with Record Performance Che-Hui Lee,1,2 Nathan D. Orloff,3,4 Turan Birol,5 Ye Zhu,5 Veronica Goian,6 Ryan Haislmaier,2 Eftihia Vlahos,2 Julia A. Mundy,5 Yuefeng Nie,5 Michael D. Biegalski,7 Jingshu Zhang,1 Margitta Bernhagen,8 Nicole A. Benedek,9 Yongsam Kim,5 Joel D. Brock,5 Reinhard Uecker,8 Xiaoxing Xi,10 Venkatraman Gopalan,2 Stanislav Kamba,6 David A. Muller,5,11 Ichiro Takeuchi,12 James C. Booth,3 Craig J. Fennie,5 and Darrell G. Schlom1,11 1 Department of Materials Science and Engineering, Cornell University, U.S.A. 2 Department of Materials Science and Engineering, Pennsylvania State University, U.S.A. 3 National Institute of Standards and Technology, U.S.A. 4 Department of Physics, University of Maryland, U.S.A. 5 School of Applied and Engineering Physics, Cornell University, U.S.A. 6 Institute of Physics ASCR, Czech Republic 7 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, U.S.A. 8 Leibniz Institute for Crystal Growth, Germany 9 Mechanical Engineering Department, The University of Texas at Austin, U.S.A. 10 Department of Physics, Temple University, U.S.A. 11 Kavli Institute at Cornell for Nanoscale Science, U.S.A. 12 Department of Materials Science and Engineering, University of Maryland, U.S.A.

* Darrell G. Schlom Department of Materials Science and Engineering 230 Bard Hall, Cornell University, Ithaca NY 14853-1501 U.S.A. [email protected] Tel: +1-607-255-6504

The miniaturization and integration of frequency-agile microwave circuits—tunable filters, resonators, phase shifters and more—with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at GHz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems, e.g., BaxSr1–xTiO3, have a paraelectric-to-ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss—Srn+1TinO3n+1 phases—where in-plane crystallographic shear (SrO)2 faults provide an alternative to point defects for accommodating non-stoichiometry. In biaxially strained Srn+1TinO3n+1 phases with n ' 3 we find the emergence of a ferroelectric and highly tunable ground state at frequencies up to 120 GHz. With increasing n the (SrO)2 faults are separated further than the ferroelectric coherence length perpendicular to the in-plane polarization, enabling tunability with a figure of merit at room temperature that surpasses all known tunable microwave dielectrics.

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Symposium 3

Magnetic oxides and strongly correlated systems

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3.1 Crystal Chemistry and Structures of Topological Insulators R.J. Cava Department of Chemistry, Princeton University Princeton new Jersey 08542 USA

Corresponding author: Robert J. Cava, department of Chemistry, Princeton University, Princeton NJ 08542 USA Telephone: (USA) 609 683 1778 Email: [email protected]

The prediction and observation of topological surface states on the cleaved surfaces of bulk crystals of a handful of small band gap semiconductors is well known in the materials physics community and is the subject of a substantial amount of theoretical and experimental effort. Although the materials themselves have been critical to the development of the field from its beginning, relatively little work has been done by materials chemists on the chemistry and structure of the compounds of interest. Nonetheless, continuing discovery of new materials and refinement of the properties of old materials is critical to the advancement of the field. In this talk I will present the materials chemistry of the known topological insulators, and our results on the crystal growth, structures, and defect chemistry of the materials that we have studied with our physicist collaborators at Princeton and elsewhere. !!

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3.2 Plasmonic Excitations in a Topological Insulator S. Lupi Department of Physics and IOM-CNR, University of Rome La Sapienza, P.le A. Moro 2, 00185 Rome, Italy

Corresponding author: Stefano Lupi. E-mail:[email protected]

Plasmons are the quantized collective oscillations of electrons in metals and doped semiconductors. The plasmons of ordinary, massive electrons are since a long time basic ingredients of research in plasmonics and in optical metamaterials. Plasmons of massless Dirac electrons were instead recently observed in a purely two-dimensional system like graphene and their properties are promising for new tunable plasmonic metamaterials in the terahertz and the mid-infrared frequency range.Dirac quasi-particles are known to exist also in the two-dimensional electron gas which forms at the surface of topological insulators due to a strong spin-orbit interaction. Therefore, one may look for their collective excitations by using infrared spectroscopy. Here we first report evidence of plasmonic excitations in a topological insulator (Bi2Se3), that was engineered in thin micro-ribbon arrays of different width W and period 2W to select suitable values of the plasmon wavevector k. Their lineshape was found to be extremely robust vs. temperature between 6 and 300 K, as one may expect for the excitations of topological carriers. Moreover, by changing W and measuring in the terahertz range the plasmonic frequency 'P vs. k we could show, without using any fitting parameter, that the dispersion curve is in quantitative agreement with that predicted for Dirac plasmons.

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3.3 Role and electronic character of magnetic impurities in topological insulators G. Panaccione(, I.Vobornik(, J. Fujii(, P. Torelli(, Francesco Borgatti", M. Marangolo#, M. Eddrief#, and R.J. Cava* (IOM—CNR, Lab. TASC, in Area Science Park, S.S.14, Km 163.5, I-34149 Trieste, Italy "ISMN—CNR, via Gobetti 101, I-40129 Bologna, Italy #Institut des NanoSciences de Paris, UPMC, CNRS UMR 7588, 4 Pl. Jussieu, 75005 Paris, France *Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States

Corresponding author: G. Panaccione, Istituto Officina dei Materiali CNR Laboratorio TASC S.S. 14 Km 163.5 in AREA Science Park 34149 Basovizza (Trieste) Tel. +39.040.3758409 email: [email protected]

Topological Insulators (TIs) are promising materials for a new class of spin-based nanoelectronics enabled by the long spin coherence and fault-tolerant information storage, due to the intrinsically 2-D character coupled to the topologically protected spin environment [1]. A promising direction that goes beyond conventional methodologies is to tailor novel properties by exploiting interface effects in heterostructures, the so called 'spinterface science', as extensively demonstrated in oxide-based materials and diluted magnetic semiconductors. Controlling the magnetic properties of TIs has a complex phenomenology: although the ferromagnetic state has been obtained by bulk doping, i.e. via the inclusion of 3d-metal impurities (Mn, Fe) in the TIs [2], a clear understanding of the chemical/electronic/magnetic properties in TIs after the modification induced by the dopant elements and/or by the ferromagnetic overlayer needs to be reached. We report clear evidence that the deposition of ferromagnetic atoms onto the surface of a TI modifies the surface spin structure and results in a robust long range ferromagnetism located close to the interface, as induced by magnetic proximity effect [3]. Polarization dependent and chemical sensitive electron spectroscopies, with variable depth information (ARPES,XPS,HAXPES, XMCD) , and the comparison between single crystal and thin film results, reveal for Mn-doped Bi2Te3 and Fe-doped Bi2Se3: a) clear changes in the electronic structure near the Dirac cone, b) different chemical environment of the magnetic impurity as a function of the doping percentage.

References

[1] M. Z Hasan. C.L. Kane, Rev. Mod. Phys. 82, 3045 (2010).

[2] Y.S. Hor, et al. Phys. Rev. B 81, 195203 (2010).

[3] I. Vobornik et al., Nano Lett., 11 (10), 4079 (2011).

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Electron distribution among 3d-orbitals of transition metal oxides is key for determining their magnetic and electric properties. Electron occupancy is determined by the number of electrons in the metal and by the relative energies of the five-3d orbitals and spin state. It is known that at interfaces between dissimilar oxides in epitaxial heterostructures, charge redistribution, strain effects and symmetry breaking may produce dramatic effects which may lead to emerging new properties [1]. It has been shown, for instance that interface-mediated tensile strain acting on the 3d-orbitals of regular octahedrally-coordinated metal ions, such as 3d4-Mn3+, breaks the degeneracy of x2-y2 and 3z2-r2 states favoring electron filling of in-plane x2-y2 orbitals subsequently determining the orbital and magnetic orderings [2].

On the other hand, the free-surface of transition metal oxides, is a particular interface of major interest in areas like green-energy and catalysis. From this point of view, bulky transition metal oxides are receiving much attention.

Here, we will make use of surface sensitive X-ray absorption linear dichroism (XLD) at Mn L-edge in La2/3Sr1/3MnO3 (LSMO) to disentangle the free surface contribution from the strain-induced to the orbital x2-y2 and 3z2-r2 occupancy. LSMO films of various thicknesses (from 4 unit cells up to 150 uc) where grown by pulsed laser deposition on single crystalline (001)ABO3 perovskites substrates. The substrate selection: SrTiO3, LSAT, NdGaO3 and LaAlO3, having distinct mismatch with LSMO, allows to explore distinct strain states of the films. Moreover, LSMO on (001)SrTiO3 substrates, either TiO2 or SrO terminated are also grown and used to explore surface termination effects. LSMO films grown on (110)SrTiO3 are characterized.

The systematic study of the XLD spectra of all films as a function of substrate, chemical termination, thickness and orientation conclusively shows that there is a constant 3z2-r2contribution in all XLD spectra that adds to the strain-induced orbital occupation. This net 3z2-r2 contribution arises due to the symmetry-breaking at the free surface which stabilizes the 3z2-r2 orbitals with respect to the x2-y2. This finding shows that electronic occupation at the free-surface orbitals in transition metal oxides, can be tailored at wish by balancing strain and surface contributions, and thus opening the way to active control of surface properties. [1] H. Y. Hwang et al. Nature Materials 11, 103 (2012), E. Benckiser et al, Nature Mat. 10, 189 (2011) [2] C. Aruta, et al, Phys Rev. B 73, 235121 (2006); A.Tebano, et al, Phys Rev. Lett 100, 137401 (2008); M.

Huijben et al, Phys. Rev. B 78, 094413 (2008); A. Tebano, e al, Phys. Rev. B 82, 214407 (2010). !

3.4 Electron distribution among 3d-eg orbitals in manganite thin films D. Pesquera1, G. Herranz1, F. Sánchez1 and J. Fontcuberta1,*, A. Barla2 and E. Pellegrin2, F. Bondino3 and E. Magnano3, 1 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC). Campus UAB. Bellaterra 08193. Catalonia (Spain). 2 ALBA Synchrotron Light Source, Carretera BP-1413 de Cerdanyola a Sant Cugat, Km 3. 08290 Cerdanyola del Vallès, Catalonia, (Spain) 2 Laboratorio TASC, IOM CNR, S.S. 14 km 163.5, Area Science Park Basovizza (Ts), I-34149, Italy

Corresponding author: [email protected], Institut de Ciència de Materials de Barcelona (ICMAB-CSIC). Campus UAB. Bellaterra 08193. Catalonia (Spain).

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3.5 Orbital polarization at complex oxide hetero-interfaces, an angular magnetoresistance study. Norbert M Nemes, Flavio Y. Bruno, Javier Tornos, Mar Garcia Hernandez, Jacobo Santamaria GFMC, Departamento de Fisica Aplicada III Universidad Complutense de Madrid

Corresponding author: Norbert M. Nemes E-mail: [email protected]

Electronic reconstruction at the interface between complex oxides is known to give rise to new electronic ground-states with emergent responses. Charge transfer, modified orbital structure, changes (rotations and tilts) of the oxygen octahedra all strongly couple to the magnetic structure and may cause profound spin rearrangements. At the interface between a manganite and a titanate there is a new Mn-O-Ti superexchange path through the interfacial oxygen that results in both charge transfer from the Mn4+ to the Ti2+ and an induced magnetic moment on the otherwise non-magnetic Ti ion. Furthermore, x-ray linear dichroism, reveal a substantial orbital polarization indicating that the relevant d-orbitals in this exchange are the d3z2-r2. This interfacial interaction can drive multilayers of LaMnO3/SrTiO3, made of non-magnetic and insulating constituents into a magnetic and metallic system. In multilayers of La0.7Sr0.3MnO3/SrTiO3 and La0.7Sr0.3MnO3/BaTiO3 we found an unexpected and highly anomalous form of symmetry breaking as identified from low temperature magnetoresistance sweeps as the applied magnetic field is rotated away from the surface-normal, precisely around the direction parallel to the film. This effect disappears rapidly with temperature, above 20 K, well below the Curie temperature, and with decreasing magnetic field, below 3 T, well above the saturation field. This indicates that its origin is related to the Lorentz-magnetoresistance, sensitive to the band structure and not AMR, sensitive to conventional magnetic anisotropy. The effect also saturates with increasing magnetic field, around 10 T at the lowest temperatures. We argue that this anomaly reflects the orbital polarization at the Mn/Ti interface. I will discuss the possible competition between various energy scales (thermal, Zeeman field, and eg orbital separation) hinting at interesting physics. Were this idea to find theoretical support, low temperature angular magnetoresistance could become a quick screening tool of orbital polarization at a wide variety of complex oxide hetero-interfaces: a rapidly increasing family of materials attracting great interest for both basic physics and technological applications. !

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3.6 Destabilizing an ordered manganite: optical properties of self-doped and Ga substituted thin films A. Nucara1, P. Maselli1, L. Baldassarre2 W.S. Mohamed1, M. Ortolani3, P. Orgiani4, G. Maritato4 F. Miletto Granozio5 and P. Calvani1 1 CNR-SPIN and Dipartimento di Fisica, Università La Sapienza, P.le Aldo Moro, 2 00185 Roma, Italy 2 Center for Life NanoScience@LaSapienza, Istituto Italiano di Tecnologia, Viale Regina Elena 295, Roma 3 CNR-IFN and Dipartimento di Fisica, Università La Sapienza, P.le Aldo Moro, 2 00185 Roma, Italy 4 CNR-SPIN and Department of Mathematics and Informatics, University of Salerno, I-84084 Fisciano, (SA) Italy 5 CNR-SPIN and Dipartimento di Scienze Fisiche, Università Federico II Complesso Universitario di Monte Sant’Angelo via Cinthia 80126, (NA) Italy

Corresponding author: Alessandro Nucara, University Sapienza of Rome, 00185, Rome. Phone:+39 0649913496. e-mail: [email protected]

Functional oxides that exhibit exciting and potentially useful properties including superconductivity, ferroelectricity, piezoelectricity, and magnetism are being intensively studied. These properties, together with the possibility of tuning them through strain, chemical doping or the application of external fields, make such functional oxides suitable for use in microelectromechanical systems (MEMS), transistors, and field effect devices. Moreover interfaces and superlattices of correlated oxides present new opportunities for controlling and optimizing the magnetic and electric properties. Significant progress in the growth of atomic-scale multilayers opens exciting opportunities in the design of materials with novel properties. Recent examples include the new two-dimensional metallic state at the interface between a band insulator as SrTiO3 and a Mott insulator like LaTiO3 (1-3) and the emergence of improper ferroelectricity in PbTiO3/SrTiO3 superlattices (4). In this talk I will present how to engineering thin films and superlattices with abrupt and coherent interfaces by a reactive molecular-beam epitaxy. In particular, the effect of dimensional confinement on manganites has been also investigated in thin films by synthesizing a superlattice of two formula-unit-thick layers of CaMnO3 separated by CaO double layers, i.e., the n=2 Ruddlesden-Popper phase Ca3Mn2O7. First-principles calculations predicted that hybrid improper ferroelectricity, magnetoelectricity, andweak-ferromagnetism are induced simultaneously by octahedron rotation modes in these layered perovskites (5). Preliminary results related to superlattices for photocatalytic applications will be also presented. 1. A. Ohtomo, D. A. Muller, J. L. Grazul, and H. Y. Hwang, Nature 419, 378 (2002) 2. S. Okamoto and A. J. Millis, Nature 428, 630 (2004) 3. S. S. A. Seo, W. S. Choi, H. N. Lee, L. Yu, K. W. Kim, C. Bernhard, and T.W.Noh, Phys. Rev. Lett. 99, 266801 (2007) 4. E. Bousquet, M. Dawber, N. Stucki, C. Lichtensteiger, P. Hermet, S. Gariglio, J.-M. Triscone, and P. Ghosez, Nature 452, 732 (2008) 5. N. Benedek and C. Fennie, Phys. Rev. Lett. 106, 107204 (2011)

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3.7 Magnetic and orbital properties of interfaces and superlattices probed by resonant soft x-ray spectroscopies Giacomo Ghiringhelli CNR/SPIN, CNISM and Dipartimento di Fisica - Politecnico di Milano - Italy

Corresponding author: Dipartimento di Fisica – Politecnico di Milano – Piazza Leonardo da Vinci 32 – 20133 Milano – Italy – ph +39 02 2399 6067 – [email protected]

The large resonant enhancement at the 2p absorption edges of 3d transition metals amplifies enormously the signal of soft x-ray spectroscopy. The resonance can be exploited in absorption spectroscopy to obtain insight on the ground state properties: magnetic moments and occupied 3d orbital symmetry can be studied in thin films and at the interfaces. In scattering experiments, the elastic component carries information on the charge, orbital and spin ordering, and the inelastic signal is provided by the spectrum of local and collective excitations (magnons, phonons, orbital or crystal field excitations, electron-hole pair creation). I will present recent results on two current hot topics. The electronic reconstruction and the much debated magnetism of the 2D electron gas at the SrTiO3/LaAlO3 interface was studied with linear and circular dichroism in absorption spectroscopy: the charge carriers at the interface have a very delocalized nature and their elusive magnetism is mainly ascribed to defects acting as magnetic impurities [1,2,3]. On the other hand resonant inelastic x-ray scattering (RIXS) was used to study how the magnetic excitations evolve upon doping in the artificial cuprate superconductors obtained by overoxygenating the (CaCuO2)n/(SrTiO3)m superlattices [4,5,]. These findings hold several analogies with those obtained recently on YBa2Cu3O6+" with the same technique [6], ie the antiferromagnetic ordering short range order is robust also in the superconducting state. [1] M. Salluzzo et al, PRL 104 166804 (2009) [2] M. Salluzzo et al, Adv Mat. On line adv. Publ., DOI: 10.1002/adma.201204555 (2013) [3] M. Salluzzo et al , submitted to Nat. Mater. [4] D. Di Castro et al, PRB 86, 134524 (2012) [5] M. Minola et al, PRB 85, 235138 (2012) [6] M. Le Tacon et al, Nat. Phys. 7, 725 (2011)

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3.8 Optical and structural evidences of exotic spin-orbital order in tetragonal Sr2VO4 Jérémie Teyssier, Enrico Giannini, Radovan Cerny, Dirk van der Marel DPMC, 24 quai E. Enermet, 1211 Geneva, Switzerland


Sr2VO4 has the same crystal structure as the parent compound of the high Tc superconductors La2CuO4, and was thought to be its electron counterpart. From specific heat and magnetic susceptibility Sr2VO4 has been found to pass, as a function of temperature, through a number of different electronic/structural phases. Below 10 K the material exhibits a tetragonal structure and a weak ferromagnetism. Between 10 K and 97 K the system enters a different magnetic and orbital state predicted as an octupolar state. Our optical spectroscopy measurements on in house synthesized ceramics samples, has evidenced a combined effect of the tetragonal crystal field, spin-orbit interaction and competing ferromagnetic and antiferromagnetic exchanges that tend to align spin and orbital angular momentum opposite to each other, causing an overall “mute” magnetic moment. In a region between 97 K and 127 K, temperature above which the material is again tetragonal and paramagnetic, neither the structure nor the magnetic state is known. To elucidate the correlations between the structure and the orbital degree of freedom, high-resolution and high-throughput X-ray powder diffraction using synchrotron light was used. A full-pattern refinement technique based on the Rietveld method has provided the thermal evolution of the structural parameters and evidenced new details in the structural and orbital origin of all the peculiarities observed in the magnetic susceptibility, specific heat, as well as in optical spectra.

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3.9 Study of the Verwey transition in magnetite thin films M. Baghaie Yazdi1, K.-Y. Choi2, D. Wulferding2, P. Lemmens2, M. Major1, W. Donner1, and L. Alff1 1Institute of Materials Science, TechnischeUniversität Darmstadt, 64287 Darmstadt, Germany 2Institute for Condensed Matter Physics, Technische Universität Braunschweig, 38106 Braunschweig, Germany

Corresponding author: Mehrdad Baghaie Yazdi, Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany. e-mail: [email protected].

We have grown epitaxial thin films of magnetite on MgO and Al2O3 substrates with sharp and distinct signatures of the Verwey transition in resistivity and magnetization at 119 K and 128 K, respectively. A detailed Raman study reveals that for both thin films Raman modes related to the structural phase transition occur at higher temperatures. In contrast, newly emerging modes indicating additional charge and orbital order follow at about 10 K lower temperatures. Finally, the metal-insulator transition appears as a global sample property indicated by characteristic changes of transport and magnetization data. These results suggest that the structural phase transition in magnetite is a necessary precursor triggering a transition into a complex charge and orbitally ordered state.!

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3.10 Quantum Oscillations and high carrier mobility in the delafossites PdCoO2 and PdCrO2 Clifford W. Hicks1, Alexandra S. Gibbs1,2, Andrew P. Mackenzie1, Hiroshi Takatsu3, Yoshiteru Maeno4, Edward A. Yelland1,5 1Scottish Universities Physics Alliance (SUPA), School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS, United Kingdom 2School of Chemistry and EaStCHEM, University of St. Andrews, St. Andrews KY16 9ST, United Kingdom 3Department of Physics, Tokyo Metropolitan University, Tokyo 192-0397, Japan 4Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan 5SUPA, School of Physics and Astronomy, and Centre for Science at Extreme Conditions, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, United Kingdom


We present de Haas-van Alphen and resistivity data on single crystals of the delafossites PdCoO2 and PdCrO2. At 295 K we measure an in-plane resistivity of 2.6 µ)-cm in PdCoO2, making it the most conductive oxide known. The low-temperature in-plane resistivity has an activated rather than the usual T5

temperature dependence, suggesting a gapping of effective scattering that is consistent with phonon drag. Below 10 K, the transport mean free path is ~20 µm, approximately 105

lattice spacings and an astoundingly high value for flux-grown crystals. PdCrO2 has the same crystal structure as PdCoO2, but is a triangular antiferromagnet. The electronic structure of PdCrO2 is strikingly similar to that of PdCoO2, and comparison of these two materials provides an unusual opportunity to isolate the effects of the magnetism.!!

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3.11 Determining the Short-Range Spin Correlations in Cuprate Chain Materials with Resonant Inelastic X-ray Scattering J. Geck1, C. Monney2, V. Bisogni1,2, K. Zhou2, R. Kraus1, V. N. Strocov2, J. Malek1, R. Kuzian1, S.-L. Drechsler1, S. Johnston1, A. Revcolevschi3, B. Büchner1,4, H. M. Rønnow5, J. van den Brink1 and T. Schmitt2 1Leibniz Institute for Solid State and Materials Research IFW-Dresden, Helmholtzstrasse 20, D-01171 Dresden, Germany2Research Department Synchrotron Radiation and 2Nanotechnology, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 3Laboratoire de Physico-Chimie de l’Etat Solide, ICMMO, Université Paris-Sud, 91405 Orsay Cedex, France 4 Department of Physics, TU-Dresden, D-01062 Dresden, Germany 5Laboratory for Quantum Magnetism, ICMP, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland

Corresponding author: Dr. Paola Gentile, CNR-SPIN Salerno and Dipartimento di Fisica "E. R. Caianiello", Università degli Studi di Salerno, Via Ponte don Melillo, I-84084 Fisciano (SA), Italy. E-mail: [email protected]

In strongly frustrated magnetic systems, the melting of long-range spin order by quantum fluctuations can clear the area for novel types of magnetic states to flourish. Disordered spin-liquids or spin nematics may for instance emerge. In these fluid-like states of matter, the constituent spins are highly correlated on very short length scales but still fluctuate strongly, even down to a temperature of absolute zero. These local spin-correlations in the absence of long-range magnetic order are thus a hallmark of these new magnetic states. Their detection, however, constitutes a great challenge for experiment. Here we demonstrate for the two prototypical quantum spin-chain materials Li2CuO2 and CuGeO3 that exactly this can be achieved by means of resonant inelastic x-ray scattering (RIXS) at the O K-edge. Using a charge transfer exciton as a probe, we detect local spin correlations with great sensitivity. While we confirm the FM and AFM intra-chain spin correlations in Li2CuO2 and CuGeO3, respectively, the AFM spin correlations in Li2CuO2 remain surprisingly strong at low temperatures. This observation indicates the proximity of Li2CuO2 to a quantum critical point. The presented method provides an excellent tool to characterize new one-dimensional cuprate materials and to search for novel magnetic ground states. !

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3.12 Structural and electronic properties of Sr2RuO4-Sr3Ru2O7 heterostructures Carmine Autieri1,2, Mario Cuoco1 and Canio Noce1 1SPIN-CNR, I-84084 Fisciano (Salerno), Italy and Dipartimento di Fisica “E. R. Caianiello", Università_a di Salerno, I-84084 Fisciano (Salerno), Italy 2IAS, Forschungszentrum J¨ulich, 52425 Julich, Germany

EF! GHIJK! LM! MNOKP8QONJRNQSHK! RISRTSIPNLJK! UH! KPTVF! PWH! KPOTRPTOIS! IJV! HSHRPOLJNR!

QOLQHOPNHK!LM!I superlattice made of Sr2RuO4 and Sr3Ru2O7 ruthenate oxides. Due to the symmetry mismatch of the two systems a significant structural rearrangement occurs within the superlattice. We find that at the interface the RuO6 octahedra get elongated for the Sr2RuO4 while tend to be compressed for the Sr3Ru2O7 as compared to inner layers and the bulk phases. The positions of the Sr-atoms in the Sr-O layers at the interface are strongly modified and influence the alignment of the Ru atoms with respect to the planar oxygens as well as the Ru-O-Ru in-plane bond angles. Such structural rearrangement leads to a modification of the electronic structure close to the Fermi level. The main changes occurring at the interface and in the inner layers of the heterostructure are analyzed and compared with the bulk phases of the Sr2RuO4 and Sr3Ru2O7 compounds. We show that the positions of the peaks in the density of states close to the Fermi levels get shifted and renormalized in the spectral weight. The balance between the renormalization of the bandwidth of the dxy band and that of the crystal field splitting results into a minor change of the Van Hove singularities position within the superlattice. The effective tight-binding parameters for the 4d Ru bands are determined by means of a maximally localized Wannier functions approach and used to discuss the modification of the electronic structure of the superlattice with respect to the bulk phases. Consequences on the modification of the superconducting and metamagnetic behaviour of the superlattice with respect to the bulk phases are discussed.

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3.13 Rashba spin-orbit driven intrinsic charge instability in oxide heterostructures S. Caprara1, D. Bucheli1, M. Grilli1, F. Peronaci2, G. Seibold3 1Physics Department, University of Rome “Sapienza”,Rome, Italy 2SISSA, Trieste, Italy 3BTU University, Cottbus, Germany

Corresponding author: : Marco Grilli, Dipartimento di Fisica UNiversita di Roma “Sapienza”, 00185 Roma, Italy e-mail: [email protected], web: http://server2.phys.uniroma1.it/gr/clc/marco_grilli.html

The peculiar “tailish” resistance of the LaAlO3/SrTiO3 or LaTiO3/SrTiO3 (LXO/STO) heterostructures is due to the occurrence of a low-dimensional (e.g., filamentary or fractal) structure of the superconducting cluster with small long-distance connectivity embedded in the two-dimensional system [1,2]. To explain the systematic occurrence of such mesoscopically disordered regions, we model the electron gas at the interface of superconducting oxide heterostructures considering a two-dimensional electron gas in the presence of a sizable Rashba spin-orbit coupling (RSOC). Under simple general assumptions, we show that an electronic phase separation occurs for realistic values of the RSOC and of the band parameters [3]. This could provide an intrinsic mechanism for the recently observed inhomogeneous phases at the LAO/STO or LTO/STO interfaces and open the way to new interpretations of the quantum critical behavior of LTO/STO [4]. We investigate the effects of temperature and magnetic field on the charge instability finding a novel type of quantum critical point related to the vanishing of the critical temperature of the electronic phase separation [5].

[1] D. Bucheli, S. Caprara, C. Castellani, and M. Grilli, New Journal of Physics 15, 023014 (2013).

[2] S. Caprara, N. Bergeal, J. Biscaras, D. Bucheli, J. Lesueur, and M. Grilli, in preparation

[3] S. Caprara, F. Peronaci, and M. Grilli, Physical Review Letters 109, 196401 (2012) [4] J. Biscaras, N. Bergeal, S. Hurand, C. Feuillet-Palma, A.Rastogi, R. C. Budhani, M.

Grilli, S.Caprara, J. Lesueur, arXiv:1209.6464, and Nature Materials (2013) [5] D. Bucheli, S. Caprara, M. Grilli, F. Peronaci, in preparation.

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3.14 Magnetism in layered ruthenates: LDA+DMFT study E. Gorelov IAS-3, Forschungszentrum Jülich, 52425 Jülich, Germany

Corresponding author: Evgeny Gorelov, IAS-3, Forschungszentrum Jülich, 52425 Jülich, Germany. [email protected]

The layered ruthenates of the Ruddlesden-Popper family Srn+1RunO3n+1 are interesting examples of strongly correlated transition metal compounds. Due to competing kinetic and Coulomb energies, that are of the same order for Ru 4d electrons, these compounds have very rich phase diagram, including Mott-insulator, ferro- and meta-magnetic phases. In the present study we focus on local electronic structure and magnetic properties of double- and triple-layered compounds, namely Sr3Ru2O7 and Sr4Ru3O10. We use the LDA+DMFT (local-density approximation + dynamical mean-field theory) approach based on maximally-localized Wannier orbitals obtained from Linearized Augmented Plane Wave (LAPW) LDA calculation and the weak-coupling CT-quantum Monte Carlo impurity solver. The low-energy effective model we use contains three Ru t2g orbitals for Sr3Ru2O7 and six t2g orbitals for two non-equivalent Ru atoms in Sr4Ru3O10. We calculate spectral functions and magnetic moments of Ru atoms retaining full on-site Coulomb interaction and self-energy matrix [1].

[1] E. Gorelov, G. Zhang, and E. Pavarini, Correlation effects and spin-orbit coupling in Srn+1RunO3n+1 compounds (in preparation)

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Symposium 4


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4.1 Enhanced conductivity due to tetragonal domain structure in LaAlO3/SrTiO3 heterointerfaces Beena Kalisky1,2, Eric M. Spanton2, Hilary Noad2, John R. Kirtley2, Katja C. Nowack2, Christopher Bell2, Yasayuki Hikita2, Hiroki K. Sato2, Yanwu Xie2, Carsten Woltmann3, Georg Pfanzelt3, Rainer Jany3, Harold Y. Hwang2, Jochen Mannhart3, and Kathryn A. Moler2 1Bar-Ilan University, Israel 2Stanford University, California 3Max-Planck-Institute, Germany

Corresponding author: Beena Kalisky, [email protected], Physics department, Bar-Ilan University, Ramat-Gan, Israel 59200.

The interface of LaAlO3 (LAO) and SrTiO3 (STO) exhibits both conductivity and magnetism. The conductivity is attributed to an electronic reconstruction that demonstrates the ability to control materials properties through interface engineering. Transport and other measurements display many interesting features with properties averaged over the entire sample or device. To better understand the conductivity, we use scanning SQUID microscopy to image the magnetic field locally generated by current in an interface, as well as the static magnetic landscape. We find that at low temperatures the current flows largely in narrow paths oriented along the crystallographic axes. The configuration of these paths changes upon thermal cycling above the STO cubic to tetragonal structural transition temperature, implying that conductivity is strongly modified by STO tetragonal domains. I will describe the details of this exciting new observation as well as our studies of the nanoscale ferromagnetism that appears in interfaces with LAO thickness larger than the critical thickness for the electronic reconstruction.!

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4.2 Conductance and magnetism of LaAlO3/SrTiO3 grown in high oxygen pressure; the role of stoichiometry and oxygen vacancies I. M. Dildar1, K. Uhlirova1, and J. Aarts1, S. Harkema2, M. Neklyudova3, and H. Zandbergen3 1Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, the Netherlands 2Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, the Netherlands. 3National Centre for High Resolution Microscopy, Kavli Institute forNanoscience, Delft Technical University, Lorentzweg 1, 2628 CJ Delft, the Netherlands.

Corresponding author: Ishrat Mubeen Dildar, Neils Bohrweg 2, 2333CA, Leiden, the Netherlands. [email protected], [email protected]

The conducting and magnetic properties of interfaces between LaAlO3 grown on singly terminated SrTiO3 (001) by TiO2 using RF sputtering in a high-pressure oxygen atmosphere have been investigated. The grown LAO films are flat and epitaxial. TEM indicates that the interfaces are atomically sharp. EELS shows some slight intermixing. However, we find these interfaces to be non-conducting. Moreover, the ratio between La and Al in the LaAlO3 films is found to be 1.07. We also measured the elemental ratio for some PLD grown interfaces [1]. Also in PLD, a sample with a badly conducting interface grown at high oxygen pressure shows a La/Al ratio above 1 while growth at lower pressures leads to conducting interfaces and a ratio less than 1, similar to what is found in MBE-growth [2]. Also stoichiometry therefore seems to play a role in the mechanism which leads to conducting interfaces. The results are found to be in good agreement with Ref. 3 where the stoichiometry of LaAlO3 films and their lattice constants are related. We also measured the magnetic properties of the sputter-grown non-conducting interfaces and found them to be non-magnetic, taking into account that the substrates themselves can show a magnetic signal. [1] LaAlO3/SrTiO3 samples using PLD were grown by Dr. J. Kleibeuker in group of Prof. dr. ing. A.J.H.M. Rijnders, Twente University, the Netherlands.

[2] M.P. Warusawithana, A.A. Pawlicki, T. Heeg, D.G. Schlom, C. Richter, S. Paetel, J. Mannhart, M. Zheng, B. Mulcahy, J.N. Eckstein, W. Zander, and J. Schubert, Bulletin of the APS 55,nr. 2 (2010), abstract ID BAPS.2010.MAR.B37.1. [3] L. Qiao, T. C. Droubay, T. Varga, M. E. Bowden, V. Shutthanandan, Z. Zhu, T. C. Kaspar, and S. A. Chambers, PRB 83, 085408 (2011).

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4.3 High mobility conduction and superconductivity at (110) and (111) LaAlO3/SrTiO3 interfaces G. Herranz1, M. Scigaj1, N. Dix1, J. Gázquez1, M. Varela2,3, N. Bergeal4, J. Lesueur4, F. Sánchez1, J. Fontcuberta1 1Institut de Ciència de Materials de Barcelona, ICMAB-CSIC Campus de la UAB, Bellaterra 08193, Catalonia, Spain 2Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 3Dpt. Física Aplicada III, Universidad Complutense de Madrid, Madrid, 28040 Spain 4LPEM-UMR8213/CNRS-ESPCI ParisTech-UPMC, 10 rue Vauquelin-75005 Paris, France 2Physikalisches Institut, Karlsruher Institut für Technologie (KIT), 76131 Karlsruhe, Germany

Corresponding author: [email protected], Institut de Ciència de Materials de Barcelona ICMAB-CSIC Campus de la UAB, 08193 Bellaterra, Catalonia, Spain

A high-mobility two-dimensional electron liquid (2DEL) was uncovered a few years ago at the LaAlO3/SrTiO3 (LAO/STO) interface [1]. This unexpected interface conductivity between two wide-bandgap insulators has been widely accepted to be originated by electronic charge transfer driven by the polarity discontinuity across the (001)-oriented LAO/STO interface. The beauty and simplicity of such a model has probably stimulated the deep investigation mostly in (001)-oriented LAO/STO, with exceptionally very few other interfaces, such as (001) LaGaO3/STO and LnTiO3/STO (Ln rare earth) [2, 3]. We demonstrate here that the family of high-mobility 2DELs in oxide interfaces can be expanded to other growth orientations. Specifically, we demonstrate that, above a critical thickness, 2DELs are also generated in epitaxial (111)- and (110)- oriented LAO/STO interfaces, with sheet carrier density and mobility values very similar to (001) interfaces [4]. We also report on superconductivity at (110)-interfaces below # 200 mK, in which the 2D nature of the electronic state is unambiguously ascertained by the Landau-Ginsburg analysis of the superconductive state under the application of magnetic fields. Our discovery of 2DEL at different crystalline orientations opens up the door to study new physics and new phenomena at interfaces, where the effects of crystal orientation on the electronic band structure and properties may be of relevance. At the same time, these findings open a new perspective for elucidating the ultimate microscopic mechanism of carrier doping. Electron microscopy work at ORNL was supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division. [1] Ohtomo, A. & Hwang. Nature 427, 423–426 (2004) [2] Perna, P. et al., Appl. Phys. Lett. 97, 152111 (2010). [3] Jang, H. W. et al., Science 331, 886–889 (2011). [4] G. Herranz et al., Scientific Reports 2 758 (2012) !

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4.4 Investigation of Current Channels at the Interface between Complex Oxide Heterostructures Aaron Rosenberg1, John Kirtley1, Eric Spanton1, Emiliano Di Gennaro2, Umberto Scotti Di Uccio2, Carmela Aruta2, Francesco Tafuri3, Fabio Miletto Granozio2, Kathryn Moler1

1Stanford University – Department of Physics Stanford, United States 2CNR-SPIN and Dipartimento Scienze Fisiche dell'Università di Napoli "Federico II" Naples, Italy 3CNR-SPIN and Dipartimento Ingegneria Informazione della Seconda Università di Napoli Naples, Italy

Corresponding author: Aaron Rosenberg, 137 Running Farm Lane Apt 109, Stanford, CA United States 94305, Email: [email protected], Cell: 1-908-361-0350

The interface between SrTiO3 and LaAlO3, both perovskite oxide insulators, supports metallic and superconducting states under certain conditions. Previous unpublished data by Kalisky et al. shows spatial variation in the current flow in these interfaces, including enhanced conductivity associated with structural domains. The microscopic origin of this enhanced conductivity is unknown. We extend the previous work to LaGaO3/SrTiO3, and NdGaO3/SrTiO3 interfaces, observe similar stripe-like modulations in the current flow, and study their temperature and frequency dependence. Additionally, we plan to study how the current channels in LaAlO3/SrTiO3 change under a uniaxial strain. Investigation of these spatial variations may improve our understanding of the relationship between structure and conductivity in complex oxide interfaces.

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4.5 Two-dimensional electron gases at a spinel/perovskite complex oxide heterointerface with electron mobilities exceeding 100,000 cm2V-1s-1

Y. Z. Chen, F. Trier, D. V. Christensen, S. Linderoth, and N. Pryds Department of Energy Conversion and Storage, Technical University of Denmark, Risø Campus, 4000 Roskilde, Denmark

Corresponding author: Yunzhong Chen, Email: [email protected], Frederiksborgvej 399, DK 4000, Roskilde, Denmark, Tel: +45 4677 5614; Fax: +45 4677 5858

The high-mobility two-dimensional electron gas (2DEG) confined at the interface of two insulating complex oxides provides opportunities for a new generation of electronic devices. So far, such oxide 2DEG is nearly exclusively created within the frame of interface polarity, such as the case of the intensively explored LaAlO3/SrTiO3 (LAO/STO) heterointerface. Alternatively, when building heterostructures on STO, the basis material for oxide electronics, the conductance can also originate from tunable redox reactions at the interface, i.e. the oxygen-vacancies dominated conductivity in reduced STO substrates [1]. In this presentation, the mechanism of the interface conductance in STO-based oxide heterostructures will be discussed. Moreover, relying on redox reactions, we created a new type of 2DEG at the heterointerface between SrTiO3 and a spinel (-Al2O3 epitaxial film with compatible oxygen ions sublattices [2]. Electron mobilities more than one order of magnitude higher than those of hitherto investigated perovskite-type interfaces were obtained. The spinel/perovskite 2DEG, where the two-dimensional conduction character is revealed by quantum magnetoresistance oscillations, is found to result from interface-stabilized oxygen vacancies confined within a layer of 0.9 nm in proximity to the interface. Our findings pave the way for studies of mesoscopic physics with complex oxides and design of high-mobility all-oxide electronic devices

1. Y. Z. Chen et al., Nano Lett. 11, 3774 (2011).

2. Y. Z. Chen et al., Nature Communications. doi:10.1038/ncomms2394 (2012) (in progress).

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4.6 Phase coherent transport in the 2DEG at the LaAlO3/SrTiO3 interface S. Gariglio1, D. Stornaiuolo1, A. Fête1, D. Li1, M. Gabay2, J.-M. Triscone1

1DPMC, University of Geneva, 24 Quai E.-Ansermtet 1211 Genève, Switzerland

2Laboratoire de Physique des Solides, Bâtiment 510, Université Paris-Sud 11, Centre d’Orsay, 91405 Orsay Cedex, France

Corresponding author: Stefano Gariglio, DPMC, University of Geneva, 24 Quai E.-Ansermet CH-1211 Geneva, [email protected]

The 2D electron gas (2DEG) present at the LaAlO3/SrTiO3 interface provides a valuable platform for the study of phase coherent transport phenomena in low dimensional oxide systems [1]. In a set of nano-channels realized in field effect devices, we have investigated the temperature and electric field evolution of the inelastic and spin-orbit scattering lengths by analyzing magneto-conductance traces. Lowering the temperature, the inelastic scattering length increases until it reaches the value of the spin-orbit scattering length: at this point a transition from weak-localisation to weak anti-localisation is observed. Using the field effect, we could modify the strength of the Rashba spin-orbit interaction [2]. In these nano-devices, at the lowest temperatures, we observe universal conductance fluctuations in the magnetoresistance, confirming the phase coherent origin of the weak-localization regime [3].

[1] A. D. Caviglia, M. Gabay, S. Gariglio, N. Reyren, C. Cancellieri, and J.-M. Triscone, Phys. Rev. Lett. 104, 126803 (2010).

[2] A. Fête, S. Gariglio, A. D. Caviglia, J.-M. Triscone, M. Gabay, Phys. Rev. B 86, 201105 (2012).

[3] D. Stornaiuolo, S. Gariglio, N. J. G. Couto, A. Fete, A. D. Caviglia, G. Seyfarth, D. Jaccard, A. F. Morpurgo, and J.-M. Triscone, Appl. Phys. Lett. 101, 222601 (2012).

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4.7 Novel 2D electron gases at the surface of transition-metal oxides A. F. Santander-Syro1, C. Bareille1, F. Fortuna1, T. Rödel1, M. Gabay2, F. Bertran3, G. Herranz4, M. Bibes5, A. Barthélémy5, T. Maroutian6, P. Lecoeur6, R. Weht7, J. Guevara7, and M. J. Rozenberg2 1CSNSM, Université Paris-Sud and CNRS/IN2P3,Bât. 104 et 108, 91405 Orsay, France. 2Laboratoire de Physique des Solides, Université Paris-Sud, Bât. 510, 91405 Orsay, France. 3SOLEIL, L’Orme des Merisiers, Saint-Aubin-BP48, 91192 Gif-sur-Yvette, France. 4ICMB/ CSIC, Campus de la UAB, 08193 Bellaterra, Spain. 5UMPhy-CNRS/Thales, 1 Av. A. Fresnel, 91767 Palaiseau, France. 6Institut d’Electronique Fondamentale, Université Paris-Sud, Bât.220, 91405 Orsay, France. 7Gerencia de Investigacion y Aplicaciones,CNEA, 1650 San Martin, Argentina.

Corresponding author: Andrés Felipe Santander-Syro, CSNSM – Université Paris-Sud, Bât.104 et 108 91405 Orsay cedex, FRANCE [email protected]

Electronic states at surfaces or interfaces can lead to groundbreaking phenomena and applications, like the quantum-Hall effect and the field-effect semiconductor transistors. At present, the two-dimensional electron gases (2DEGs) at interfaces between SrTiO3 (STO) and other insulating transition-metal oxides TMOs are the subject of intense research [1]. These interfaces can display stunning properties, such as superconductivity or magnetoresistance. However, the physical origin of these interfacial 2DEGs, and their generalization to other multifunctional oxides, remain open issues.

In this talk, we will present our recent discovery that a 2DEG can be simply realized at the vacuum-cleaved surface of SrTiO3 [2]. Using angle-resolved photoemission spectroscopy, we directly observed multiple subbands of heavy and light electrons confined within ~5 unit cells beneath the surface, and ordered by their orbital symmetries. We will then show that such a procedure can be readily extended to obtain 2DEGs in other TMOs, opening a wide realm of possibilities in the field of oxide electronics. In particular, we created a 2DEG at the (001) surface of KTaO3, a wide-gap insulator with a spin-orbit coupling more than one order of magnitude larger than in STO. As it turns out, the Fermi energy and subband splittings of this 2DEG are comparable to the strong spin-orbit coupling, resulting in a new physical system with respect to the bulk: the orbital symmetries of the 2DEG’s subbands are entirely reconstructed and their masses are renormalized [3]. These results demonstrate that in TMOs the strong couplings between the active electronic degrees of freedom, combined with the electron confinement, can lead to novel electronic states at the surface that are not simple extensions of the bulk bands.

[1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004); C. H. Ahn, J.-M. Triscone and J. Mannhart, .Nature 424, 1015 (2003); N. Reyren et al., Science 317, 1196-1199 (2006); A. Brinkman et al., Nature Mater. 6, 493-496 (2007). [2] A. F. Santander-Syro et al., Nature 469, 189 (2011). [3] A. F. Santander-Syro et al., Phys. Rev. B 86, 121107(R) (2012).

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4.8 A two-dimensional electron gas with honeycomb electronic structure at the (111) surface of KTaO3 C. Bareille,) F. Fortuna,) T. C. Rödel,) * F. Bertran,+ M. Gabay, A. Taleb-Ibrahimi,+ P. Le Fèvre,+ M. Bibes, A. Barthélémy, T. Maroutian, P. Lecoeur, M. J. Rozenberg, and A. F. Santander-Syro) 1CSNSM, Université Paris-Sud and CNRS/IN2P3, Bâtiments 104 et 108, 91405 Orsay cedex, France 2Universität Würzburg, Experimentelle Physik VII, Am Hubland, 97074 W ürzburg, Germany 3Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin-BP48, 91192 Gif-sur-Yvette, France 4Laboratoire de Physique des Solides, Université Paris-Sud, Bâtiment 510, 91405 Orsay, France 5Unité Mixte de Physique CNRS/Thales, Campus de l’Ecole Polytechnique, 1 Av. A. Fresnel, 91767 Palaiseau, France and Université Paris-Sud, 91405 Orsay, France 6Institut d’Electronique Fondamentale, Université Paris-Sud, Bâtiment 220, 91405 Orsay, France

Corresponding author: C. Bareille - CSNSM, Université Paris-Sud and CNRS/IN2P3, Bâtiments 104 et 108, 91405 Orsay cedex, France - email: [email protected]

Transition-metal oxides (TMOs) are correlated-electron systems presenting remarkable properties, such as high-temperature superconductivity or multi-ferroic behaviour. The realization of two-dimensional electron gases (2DEGs) at surfaces or interfaces of TMOs, a field of current active research, is crucial for harnessing the functionalities of these materials in view of future applications. From a fundamental point of view, these 2DEGs offer the possibility to explore new physics emerging from the combined effects of electron correlations and low dimensional confinement. In this framework, the theoretical study of correlated electrons confined in exotic lattice symmetries, like the graphene-like 2D honeycomb lattice, has been recently receiving a lot of attention. Here we create a 2DEG at the (111) surface of KTaO3, an insulator with a strong spin-orbit coupling. Using angle-resolved photoemission, we directly observe that the confined states form a honeycomb network of quasi-1D “electron gutters”, an electronic structure wholly novel with respect to all known oxide-based 2DEGs. These findings demonstrate that the Fermi Sea of the oxide-based 2DEGs can be dramatically modified by properly choosing the cleaving plane of the perovskite crystal, suggesting new routes to craft exotic low-dimensional states in correlated oxides.

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4.9 Mixed Dimensionality of Confined Conducting State on SrTiO3 Tied to Ferroelectric Surface Distortion N. C. Plumb1, M. Salluzzo2, E. Razzoli1, M. Månsson3,4, M. Falub1, J. Krempasky1, C. E. Matt1, J. Chang1,5, M. Schulte6, J. Minár6, J. Braun6, H. Ebert6, B. Delley7, K.-J. Zhou1, T. Schmitt1, M. Shi1, J. Mesot1,4,5, L. Patthey1,8, M. Radovi,1,5,8 1Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 2CNR-SPIN, Complesso Universitario Monte S. Angelo, Via Cinthia I-80126, Napoli, Italy 3Laboratory for Neutron Scattering, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 4Laboratory for Solid State Physics, ETH Zürich, CH-8093 Zürich, Switzerland 5Institut de la Materie Complexe, EPF Lausanne, CH-1015 Lausanne, Switzerland 6Department Chemie und Biochemie, Ludwig-Maximilians-Universität Mu !nchen, 81377 Mu !nchen, Germany 8Condensed Matter Theory Group, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland SwissFEL, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

Corresponding author: Nicholas Plumb, Paul Scherrer Institut, WSLA/206, CH-5232 Villigen PSI, Switzerland, +41 (0)56 310 3087.

Conducting oxide interfaces have been the subject of intense research due to the technological promise and theoretical challenges they present. The recent discovery of a metallic surface state on SrTiO3 may open a route to simplified low-dimensional oxide-based conductors, as well as give new insights into interfacial phenomena, but its origin -- particularly as it might relate to oxygen vacancies -- has been unclear. We will demonstrate that not only quasi-2D but also non-bulklike 3D Fermi surface components make up the surface state. Like their more 2D counterparts, the size and character of the 3D components are fixed with respect to the preparation of the sample. Similar to reports from other groups, the surface state can be “prepared” by photon irradiation under UHV conditions. While this had previously suggested a role for photo-induced oxygen vacancies, we find that the high fraction of near-surface valence and conduction band electrons involved in the formation of the metallic state is incongruous with any small changes in stoichiometry. The results instead suggest that the distribution and electronic structure of carriers in the surface region, including the mixed dimensionalities the states exhibit, are directly linked to the electrostatic conditions that support a well-known ferroelectric surface distortion on SrTiO3. We argue that the concept of such a “displacement state” is potentially relevant for a broad class of materials and heterostructures that are close to ferroelectric instabilities at their surfaces or interfaces. !

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4.10 Origin of the 2DEG at the LAO/STO Interface U. Scotti di Uccio, 1 S. Amoruso, 1 C. Aruta, 1 C. Cantoni,2 R. Bruzzese, 1 E. Di Gennaro, 1 M. Lanzano, 1 A. Sambri, 1 X. Wang, 1 and F. Miletto Granozio1 1CNR-SPIN and Dipartimento di Scienze Fisiche, Complesso Universitario di Monte Sant’Angelo, Via Cintia,I-80125 Napoli, Italy 2Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6116, USA

Corresponding author: U. Scotti di Uccio, [email protected]

The interface between the insulating oxides LaAlO3 (LAO) and SrTiO3 (STO) hosts a high-mobility 2D electron gas (2DEG). After seven years of sustained theoretical and experimental work, there is no consensus regarding the dominant mechanism responsible for electrical conductivity. First principles calculations predict an electronic reconstruction in response to the diverging electrostatic energy generated when depositing the polar material (LAO) on the non-polar substrate (STO). However, theoretical studies presuppose an atomically abrupt interface with negligible defects and/or disorder. Oxygen vacancies and cation substitution are instead known to easily form in perovskites, acting as electron donors and giving rise to electrical conduction. In this talk I will present an experimental work aimed to answer the crucial question: can defect-generated electrons explain all the observed properties of LAO/STO? The proposed contribution rely upon both considerations regarding the fabrication process,1 and upon the microstructural/spectroscopic characterization of conducting STO/LAO interfaces.2

1. C. Aruta, S. Amoruso, G. Ausanio, R. Bruzzese, E. Di Gennaro, M. Lanzano, F. Miletto Granozio, Muhammad Riaz, A. Sambri, U. Scotti di Uccio, X. Wang, Appl. Phys. Lett. 101, 031602 (2012)

2. C. Cantoni, J. Gazquez, F. Miletto Granozio, M. P. Oxley, M. Varela, A. R. Lupini, S. J. Pennycook, C. Aruta, U. Scotti di Uccio, P. Perna, D. Maccariello, Adv. Mater. 3952, 24 (2012)

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4.11 Spin injection at LaAlO3/SrTiO3 interfaces E. Lesne1, N. Reyren1, R. Mattana1, F. Choueikani2, V. Cros1, J.-M. George1, F. Petroff1, C. Deranlot1, P. Ohresser2, A. Barthélémy1, H. Jaffrès1 and M. Bibes 1Unité Mixte de Physique CNRS/Thales, 1 Av. A. Fresnel, Palaiseau, France 2Synchrotron SOLEIL, Gif-sur-Yvette, France

Manuel Bibes – Unité Mixte de Physique CNRS/Thales – Thales Research and Technology – 1 Av. A. Fresnel – 91767 Palaiseau, France – [email protected] !

Future spintronics devices will be built from elemental blocks allowing the electrical injection, propagation, manipulation and detection of spin-based information. Owing to their remarkable multifunctional and strongly correlated character, oxide materials already provide such building blocks for charge-based devices such as ferroelectric field-effect transistors as well as for spin-based two-terminal devices like magnetic tunnel junctions, with giant responses in both cases. Until now, the lack of suitable channel materials and the uncertainty of spin injection conditions in these compounds had however prevented the exploration of similar giant responses in oxide-based lateral spin transport structures. In this talk, we will report magnetotransport measurements and their dependence on bias and back-gate voltages that demonstrate electrical spin injection into LaAlO3/SrTiO3 interfaces. We will give perspectives on how to achieve spin injection with increased efficiency, as well on the implementation of efficient top gating schemes for spin manipulation. Refs. N. Reyren et al, Phys. Rev. Lett. 108, 186802 (2012) M. Bibes et al, Phil. Trans. R. Soc. A 370, 4958 (2012)

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4.12 Dimensionality control of spin state in LaCoO3/LaAlO3 superlattices D. W. Jeong,1 W. S. Choi,2 S. Okamoto,2 C. H. Sohn,1 H. J. Park,1 J. -Y. Kim,3 H. N. Lee,2 K. W. Kim,4 S. J. Moon,5 T. W. Noh1 1 Center for Functional Interfaces of Correlated Electrons Systems, Institute for Basic Science, and Department of Physics & Astronomy, Seoul National University, Seoul 151-747, Korea 2Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States 3Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 790-784, Korea, 4Department of Physics, Chungbuk National University, Cheongju 361-763, Korea. . 5Department of Physics, Hanyang University, Seoul 133-791, Korea,


We investigated the electronic structure of [(LaCoO3)n(LaAlO3)n]8 (n = 2, 6, and 10) superlattices using x-ray absorption spectroscopy, and optical conductivity. High quality superlattices samples were fabricated by pulsed laser deposition. X-ray diffraction and X-ray absorption spectroscopy experiments indicated stoichiometric and coherent growth of high quality superlattices. Spectroscopic investigation revealed the critical role of dimensionality on the electronic structure of ultra-thin LaCoO3 layers. The system with low dimensional LaCoO3 exhibits a low spin state with larger optical gap due to the quantum confinement. Dynamic mean field calculation clearly simulated the critical role of dimensionality on the spin state and electronic structure of LaCoO3.

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4.13 2-dimensional electron gas at the interface of oxide semiconductors: (Mg, Zn)O/ZnO heterostructures A. Leveratto, 1 E. Bellingeri, 1 G. Gadaleta, 1 G. Scicolone, 1 I. Pallecchi, 1 L. Pellegrino, 1 D. Marré, 1 A. Jost, 2 U. Zeitler2 1CNR-SPIN corso Perrone 24, 16152 Genova, Italy and Dipartimento di Fisica, Via Dodecaneso 33, 16146 Genova, Italy 2High Field Magnet Laboratory, Institute for Molecules and Materials, Radboud University Nijmegen, 6525 ED Nijmegen, The Netherlands

Corresponding author: Alessandro Leveratto – PhD student / email: [email protected], University of Genoa, Physics Depertment, Via Dodecaneso 33, 16146, tel: +39 010 353 6323 CNR-SPIN, Corso Perrone 24, 16152, tel: +39 010 6598 780

In recent years, two-dimensional electron gases (2DEG) in oxide heterostructures have attracted a lot of attention. Among the heterostructures investigated only ZnO based multilayers unambiguously showed the presence of a 2DEG at the interface [1, 2]. Such heterostructures resemble to GaAs or GaN based systems; at the heterointerfaces Zn1-xMgxO/ZnO a strong built-in potential arises from macroscopic polarization mismatch and bands align because of different bandgaps in the two layers (*0.6 eV), causing the formation of a potential well at the interface where a 2DEGs is confined. With respect to GaAs or GaN heterostructures, being ZnO a multifunctional materials with interesting opto-magneto-electronic properties, the realization of ZnO based quantum devices could lead to significant technological developments. Here we present the realization of ZnO and MgxZn1-xO (x=0.15) heterostructures deposited by pulsed laser deposition on ZnO single crystal on Zn polar side. In detail we will discuss the optimization of substrate treatments and the optimization of deposition parameters necessary to obtain high quality films growth. The films were in-situ monitored by RHEED and routinely checked ex-situ by AFM, and XRD measures. The introduction of a low temperature buffer layer before the heterostructure effective layers resulted a mandatory step in order to produce samples with a smooth interface allowing the formation of a macroscopically connected high mobility 2DEG. A patterning process was optimized and 50x10µm Hall bars were obtained by a phosphoric acid (H3PO4) ‘wet etching’ process. Our samples show a metallic behaviour and thanks to electric transport measures in magnetic field up to 9T down to 1.9K we observed slight Shubnikov-de Haas Oscillations and Quantum Hall Effect and obtained respectively as number of carriers and mobility, n2D*1012 [cm-2] and µ*10000 [cm2V-1s-1]. We’ll also show measures in field up to 32 T and temperature down to 350mK of electric and thermal transport performed at HFML (High Field Magnet Laboratory) in Nijmegen (Holland). Certain Quantum phenomena, IQHE and SdHO, definitely confirmed the 2DEG presence and farther they were surprisingly observed up to 20K. Moreover thermoelectric power experiments showed for the first time in an oxide based 2DEG quantum oscillations in the Seebeck and Nerst thermoelectric coefficients. [1] A. Tsukazaki et al, Science 315, 1388 (2007) [2] A. Tsukazaki et al.Physical Review B 78, 233308 (2008).

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4.14 Geometrically confined doping in LaVO3/SrVO3 superlattices Ulrike Lüders CRISMAT, UMR 6508 CNRS-ENSICAEN, Caen, France

Corresponding author: Ulrike Lüders, CRISMAT, 6 Bvd Maréchal Juin, 14050 Caen cedex 04 033231452607, [email protected]

The combination of low dimensionality of the charge carriers and strong electron correlations in complex oxides has been very fruitful for the development of materials with interesting functional properties, as for example high TC superconductivity, transparent conductors or large thermopower. Recently, the creation of a low dimensional electron gas with highly mobile charge carriers has been demonstrated in SrTiO3-based systems. However, the electron correlations seem to be rather weak compared to other complex oxide systems. We have been able to demonstrate that charge carriers with a reduced dimensionality can be created also in strongly correlated systems: films of some unit cells of SrVO3 were introduced between films of the Mott insulator LaVO3 in order to constitute geometrically confined doping areas. This contribution will be centered on the physical properties of the LaVO3/SrVO3 superlattices. In the two dimensional limit of the charge carriers, emergent phases have been observed, which are not present neither in the parent materials, nor in the solid solution. Especially two phases are interesting: a room-temperature magnetic phase, and a low temperature metallic phase with high mobility of the charge carriers. With the help of band structure calculations, we will illustrate the role of the strain and the structural distortions due to the thin film form, as well as the role of the electronic degrees of freedom in the emergence of these phases which are accessible only by geometrically confined doping. !

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Thursday, May 23rd 2013

Plenary

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MAGNETIC MATERIALS BASED ON DEFECTS, INTERFACES AND O2p HOLES George A. Sawatzky Physics dept and Max Planck-UBC Centre for Quantum Materials University of British Columbia Vancouver Canada

Ideas based on theory and some experiments will be presented regarding possible new magnetic materials based on extended and point defects (1), interface engineering (2), anion substitution in oxides and hole and electron doping of oxides. The concentration will be on rather ionic oxides mostly not involving conventional magnetic elements. Special attention will also be placed on surface and interface effects involving polar surfaces as well as on the role of doped holes in O 2p in charge transfer gap oxides. O 2p holes play an extremely important role in the magnetism and superconductivity of oxides and new results will be presented regarding the ferromagnetic exchange coupling they introduce in transition metal oxides(3) and the interplay between transport properties, magnetic order and the general phase diagrams of materials involving O 2p holes either in the so called self doped case of stochiometric oxides or in chemically substituted systems and cation or anion vacancies. 1. I. S. Elfimov, S. Yunoki, and G. A. Sawatzky PRL 89, 216403, (2002) 2. N. Pavlenko, T. Kopp, E.Y. Tsymbal, G.A. Sawatzky, J. Mannhart PRB 85, 020407, (2012) 3. Bayo Lau, Mona Berciu and George A. Sawatzky, PRL 106, 036401 (2011) 4. Berciu and G. A. Sawatzky, PRB 79, 195116 (2009) !

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Symposium 3

Magnetic oxides and strongly correlated systems

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3.15 Unravelling emergent order in charge-ordered magnetic oxides A. T. Boothroyd Department of Physics, University of Oxford, U.

Corresponding author: Professor Andrew Boothroyd, Department of Physics, Oxford University, Oxford, OX1 3PU United Kingdom, [email protected]

A major theme in correlated electron physics is the existence of complex forms of nanoscale order involving several different electronic degrees of freedom. Such phenomena are important to many physical properties of materials, most prominently the problem of superconductivity in layered copper oxides. This talk will be concerned with how charge and orbital order influence the spin dynamics in complex antiferromagnetic oxides. Advances in neutron spectroscopy have made it possible to measure the complete spectrum of cooperative spin excitations in magnetically ordered systems in great detail. I will illustrate how studies of the spin dynamics can provide key insights into the nature of complex ground states. I will present recent results on a half-doped layered manganite which conclusively distinguish between different models proposed for its ground state [1], and I show how a characteristic hour-glass magnetic spectrum found in layered cobalt oxides sheds light on the existence of charge stripe correlations in the copper oxide superconductors [2]. [1] G. E. Johnstone, T. G. Perring, O. Sikora, D. Prabhakaran, and A. T. Boothroyd Phys. Rev. Lett. 109, 237202 (2012). [2] A. T. Boothroyd, P. Babkevich, D. Prabhakaran, and P. G. Freeman Nature, 471, 341 (2011).

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3.16 Stripe dynamics in the La2"xSrxCoO4 layered cobaltates by 59Co and 139La NMR and µSR S. Bordignon1, G. Allodi1, M. Mazzani1, R. De Renzi1,T.Lancaster2, S.J. Blundell3, A.T. Boothroyd3 1Dipartimento di Fisica e Scienze della Terra, Università di Parma, I-43124, Parma, Italy. 2Department of Physics, Durham University, Durham DH1 3LE, United Kingdom 3Clarendon Laboratory, University of Oxford, Oxford OX1 3PU,United Kingdom.

Corresponding author: Marcello Mazzani, email: [email protected] Tel: +390521 905245, Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Parma, Parco Area delle Scienze, viale G.P.Usberti 7/A, 43124 Parma, Italia.

Rare-earth-based transition metal oxides are among the traditional interests of the magnetism research community. They display a large variety of different phenomena, including hight-Tc superconductivity[1], magnetic order [2], spin waves [3] and stripes [4,5]. Recently, new members of this family could be synthesized and their characterization could offer a valuable contribution to our understanding of the interplay between magnetic order and superconductivity. In particular here we focus on the hole doped lanthanum cobaltates La2!xSrxCoO4 , layered, magnetic insulators, isostructural with the ‘214’ copper oxide family La2!xSrxCoO4. They exhibit similar magnetic properties, but the superconductive phases present in the cuprates are completely missing in the cobaltates. Holes localizes inducing a spinless Co3+ state and tend to arrange into arrays of charged stripes [6]. As a result, the magnetic Co2+ ions give rise to a striped antiferromagnetic spin structure, similar to the ones observed in cuprates [4], and the non-magnetic stripes behave as antiphase domain walls. A special interest in these compounds developed recently, because inelastic neutron scattering on La5/3Sr1/3CoO4 revealed a peculiar “hour-glass”-shaped dispersion branch in the magnetic excitations of the system [7]. The same hour-glass dispersion was also observed in several cuprates, and was assumed to be closely related to the coexistence between magnetism and superconductivity. The presence of similar dispersion features in both classes of compounds suggests that the underlying excitations arise from magnetic stripes (either static or fluctuating) in all cases and are therefore not related to superconductivity. To characterize the stripe dynamics we have developed a 139La and 59Co NMR investigation on a series of La2!xSrxCoO4 crystals (x = 0.25-0.9) by. In the magnetically ordered phase, below TN & 40K, broad featureless 139La and 59Co spectra are observed. On the opposite well-resolved quadrupolar patterns are detected by both nuclei well above TN. Evidence for slow dynamical excitations, ascribed to the collective motion of the stripes, is provided by the partial or total loss of the NMR signal (wipeout), due to short and inhomogeneous T2 times. Two wipeout regimes are detected over distinct temperature intervals, in the 10-40K range and above 100K. Comparison with µSR experiments allowed to identify the different origin of the fast nuclear relaxations: below 40 K the dynamics is dominated by magnetic interactions, as could be expected, while charge fluctuations, mediated by the quadrupolar interaction absent for muons (Sµ =1/2), are responsible for the strong nuclear relaxation in the paramagnetic region. [1] V.J. Emery, Phys. Rev. Lett. 58, 2794 (1987). [2] G. Matsumoto, J. Phys. Soc. Japan 29, 606 (1970). [3] R. Coldea, Phys. Rev. Lett. 86, 5377 (2001). [4] H.H. Klauss, J.Phys.: Condens. Matter 16, S4457 (2004). [5] J.M. Tranquada et al., Nature 375, 561 (1995). [6] L.M. Helme et al., Physical. Review B 80, 134414 (2009). [7] A.T. Boothroyd et al., Nature 471, 341 (2011). !

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3.17 Exotic spin orders driven by orbital fluctuations in the spin-orbital model for d9 electrons Wojciech Brzezicki Marian Smoluchowski Institute of Physics, Jagellonian University, Reymonta 4, PL-30059 Kraków, Poland

Corresponding author: Wojciech Brzezicki, e-mail: [email protected], address: Reymonta 4, PL-30059 Kraków, Poland

We study zero temperature phase diagrams of the spin-orbital Kugel-Khomskii model [1] for the 3d transition metal oxides in a two- and three-dimensional (2D and 3D) versions realized by K2CuF4 and KCuF3, respectively, and also by a bilayer K3Cu2F7 [2]. The spin-orbital superexchange model contains Heisenberg interactions between spins S=1/2, which depend on the relative orientation of the occupied eg orbitals described by pseudo-spins T=1/2. The orbital interactions have much lower (cubic) symmetry than the SU(2) symmetry of spins S and are intrinsically frustrated even on a square lattice [3]. Therefore the ground state can exhibit different types of spin and orbital orders as the parameters of the model, being crystal-field splitting Ez and Hund’s exchange -, are varied. For instance, when |Ez| is large we expect the orbitals to be strongly polarized in a state where either all orbitals point along the c axis or lie within the ab planes. This situation corresponds to the large uniaxial pressure applied to the system along the c axis. On the other hand - controls the tendency towards ferromagnetism – when - is small then the spins prefer to form antiferromagnetic (AF) order and increasing - drives a transition to the ferromagnetic (FM) state. The model is solved on square lattices using cluster mean field theory and effective spin Hamiltonians together with a spin wave expansion. We find that in addition to the AF, FM and valence-bond phases known before [3], the model exhibits configurations with exotic magnetic order, i.e., with:

(i) nearest-neighbor (NN) spins being perpendicular and second NN being AF in ab plane(s), both in the 2D [4] and 3D cases;

(ii) NN spins having non-trivial angle along the c axis interpolating between AF and FM; (iii) spins forming AF stripes in the ab planes in the 3D case. All these orders go beyond the Heisenberg physics which describe the spin sector alone and emerge from the entangled spin-orbital fluctuations. We prove it by deriving effective spin Hamiltonians in phases realizing these orders, which is achieved by perturbative expansions in orbitals around certain classical orbital orders. In the most puzzling case of the order (i) we show that the virtual orbital flips in the ground state get dressed with singlets whose density is controlled by the crystal-field splitting. We argue that such configurations can be found experimentally in transition metal oxides by applying (chemical) pressure or strong magnetic field to induce the AF-FM transition. [1] K. I. Kugel and D. I. Khomskii, Usp. Fiz. Nauk 136, 621 (1982). [2] W. Brzezicki and A.M. Ole., Phys. Rev. B 83, 214408 (2011). [3] L. F. Feiner, A. M. Ole., and J. Zaanen, Phys. Rev. Lett. 78, 2799 (1997). [4]!W. Brzezicki, J. Dziarmaga, and A.M. Ole., Phys. Rev. Lett. 109, 237201 (2012). !

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3.18 Robustness of basal-plane antiferromagnetic order and the Jeff = 1/2 state in single-layer iridate spin-orbit Mott insulator. S. Boseggia, R. Springell, H. C. Walker, H. M. Rønnow, Ch.Ruegg, M. Isobe, R. S. Perry, S. P. Collins and D. F. McMorrow. University College of London

Corresponding author: Miguel Angel Niño Orti. E-mail:[email protected]

The magnetic structure and electronic groundstate of the layered perovskite Ba2IrO4 have been investigated using X-ray resonant magnetic scattering (XRMS). Our results are compared with those for Sr2IrO4, for which we provide supplementary data on its magnetic structure. In spite of significant structural differences, the dominant, long-range antiferromagnetic order is remarkably similar in the two compounds. Our results thus establish the effective Hamiltonian describing the two systems is essentially identical regarding the leading isotropic interaction. From an investigation of the XRMS L3/L2 intensity ratio in Ba2IrO4, we also conclude that the Jeff = 1/2 description of the electronic ground state is robust to the changes in structure.

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3.19 Defect states and excitations in a Mott insulator with orbital degrees of freedom Adolfo Avella1,2,3,4, Peter Horsch1, Andrzej M. Oles1,5 1Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart, Germany 2Dipartimento di Fisica, Università degli Studi di Salerno, I-84084 Fisciano (SA), Italy 3CNR-SPIN, UoS di Salerno, I-84084 Fisciano (SA), Italy 4Unità CNISM di Salerno, Università degli Studi di Salerno, I-84084 Fisciano (SA), Italy 5Institute of Physics, Jagellonian University, PL-30059 Krakow, Poland

Corresponding author: Dr. Adolfo Avella, Dipartimento di Fisica "E.R. Caianiello", Università degli Studi di Salerno, Via Ponte don Melillo, I-84084 Fisciano (SA), Italy; Tel. +39 089 96 9131; Fax: +39 089 96 9658; E-mail: [email protected].

We address the role played by charged defects in doped Mott insulators with active orbital degrees of freedom. It is observed that defects feature a rather complex and rich physics, which is well captured by a degenerate Hubbard model extended by terms that describe crystal-field splittings and orbital-lattice coupling, as well as by terms generated by defects such as the Coulomb potential terms that act both on doped holes and on electrons within occupied orbitals at undoped sites. We show that the multiplet structure of the excited states generated in such systems by strong electron interactions is well described within the unrestricted Hartree-Fock approximation, once the symmetry breaking caused by the onset of magnetic and orbital order is taken into account. Furthermore, we uncover new spectral features that arise within the Mott-Hubbard gap and in the multiplet spectrum at high energies due to the presence of defect states and strong correlations. These features reflect the action on electrons/holes of the generalized defect potential that affects charge and orbital degrees of freedom, and indirectly also spin ones. The present study elucidates the mechanism behind the Coulomb gap appearing in the band of defect states and investigates the dependence on the electron-electron interactions and the screening by the orbital polarization field. As an illustrative example of our general approach, we present explicit calculations for the model describing three t2g orbital flavors in the perovskite vanadates doped by divalent Sr or Ca ions, such as inLa1-xSrxVO3 and Y1-xCaxVO3 systems. We analyze the orbital densities at vanadium ions in the vicinity of defects, and the excited defect states which determine the optical and transport gaps in doped systems. !

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3.20 Importance of exchange anisotropy and superexchange for the spin-state transitions in RCoO3 (R = rare earth) Guoren Zhang1, Evgeny Gorelov1, Erik Koch 2,3,and Eva Pavarini1,3 1Institute for Advanced Simulation, Forschungszentrum Jülich, D-52425 Jülich, Germany 2 German Research School for Simulation Sciences, 52425 Jülich, Germany 3JARA High-Performance Computing

Corresponding author: Guoren Zhang, Institute for Advanced Simulation, Forschungszentrum Jülich, Wilhelm-Johnen-Straße, 52428 Jülich, Germany Phone: +49 2461 61-4799 E-mail: [email protected]

In this work [1], we study the nature of the spin-state transitions in LaCoO3 (Fig. 1) which has been the subject of controversy for decades. By identifying all parameters which shift the energy balance between spin states in rare-earth cobaltates, we find that the eg-t2g crystal-field splitting increases by ~250 meV when increasing pressure to 8 GPa, by about 150 meV when cooling from 1000 K to 5 K and by less than 100 meV when La is substituted with another rare earth. Hund’s rule coupling is about the same in systems with very different spin-state transition temperature. In addition, the Coulomb-exchange anisotropy and the super-exchange energy-gain play a crucial role. In the LnCoO3 series (Ln = Y or rare earth), super-exchange progressively stabilizes a low-spin ground state as the Ln3+ ionic radius decreases. We use a simple model to describe spin-state transitions and show that, at low temperature, the formation of isolated high-spin/low-spin pairs is favored, while in the high-temperature phase, the most likely homogeneous state is high-spin, rather than intermediate spin. An orbital-selective Mott state could be a fingerprint of such a state. [1] G. Zhang, E. Gorelov, E. Koch and E. Pavarini, Phy. Rev. B 86 184413 (2012). !

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3.21 Magnetic phases and high-Tc superconductivity: A love/hate story José Lorenzana ISC-CNR and Dipartimento di Fisica, Università di Roma La Sapienza, P. A. Moro 2, 00185 Roma, Italy


After introducing the phenomenology of unconventional superconductivity close to magnetic or charge ordered phases I will discuss the case of iron based superconductors and cuprates. In the former a systematic Landau analysis combined with a density functional study allows to identify the presence of a quantum tricritical point in the phase diagram which suggest a regimen of strong quantum fluctuations particularly favorable to high-Tc superconductivity[1,2,3]. In cuprates the phase diagram is less clear due to the presence of a mysterious pseudogap phase for which it is not known which is the order parameter or which are the broken symmetries. I will discuss a recent proposal where a well defined broken symmetry survives despite the strong disorder which has an unexpected link with ferroelectricity[4]. Work done in collaboration with: G. Seibold, M. Capati, C. Di Castro, M. Grilli, S. Caprara, G. Giovannetti,C. Ortix, M. Marsman, M. Capone, J. van den Brink. [1] J. Lorenzana, G. Seibold, C. Ortix, and M. Grilli, Phys. Rev. Lett. 101, 186402 (2008). [2] M. Capati, M. Grilli, J. Lorenzana, Phys. Rev. B 84, 214520 (2011). [3] G. Giovannetti, C. Ortix, M. Marsman, M. Capone, J. van den Brink, and J. Lorenzana. Nature comm. 2, 398 (2011) [4] G. Seibold, M. Capati, C. Di Castro, M. Grilli, and J. Lorenzana. "Hidden Ferronematic Order in Underdoped Cuprates”, Phys. Rev. B in press.

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3.22 Coexistence of superconductivity and magnetism in 2-dimensional electron gases at oxide interfaces Emiliano Di Gennaro, Fabio Miletto Granozio, Davide Massarotti, Carmela Aruta, Francesco Tafuri*, Umberto Scotti di Uccio. CNR-SPIN UOS Napoli e Dipartimento di Fisica, Università di Napoli “Federico II” *Dipartimento Ingegneria Informazione, Seconda Università of Napoli - SUN

Corresponding author: Emiliano Di Gennaro, Dip. Fisica, Via Cinthia Montesantangelo, 80126, Napoli, [email protected]

The nature of the ground state of the two dimensional electron gas (2DEG) at the interface between the LaAlO3 and SrTiO3 wide bandgap insulators has been for years highly controversial [1], due to the contrasting reports of magnetism [2] and superconductivity [3]. More recently, several reports have accredited the idea of a phase-separated ground state, where superconductivity and magnetism can coexist [4,5,6]. An accurate magnetotransport characterization [7] recently demonstrated peculiar hysteretical magnetoresistance curves in the 50mK range, suggesting the switching of the sample magnetization under an applied magnetic field. Such behaviour was presented as evidence for charge–vortex duality effects in this 2D superconducting system. In this work, the low temperature magneto-transport properties of the widely celebrated polar-nonpolar LaAlO3/SrTiO3 interfaces are compared with the properties of two novel oxide-based polar-nonpolar interfaces also hosting a 2DEG, i.e. LaGaO3/SrTiO3 [8,9,10] and NdGaO3/SrTiO3 [11], recently developed in Naples. Our results collected in the 20mK - 1K range confirm the coexistence of superconductivity and magnetism both in LaAlO3/SrTiO3 and in the new interfaces, as well as the magnetization switching under an applied field. References [1] A. J. Millis, Nature Phys. 7, 749 (2011) « News and views » [2] A. Brinkman et al., Nature. Mat. doi:10.1038/nmat1931 (2007) [3] N. Reyren et al, Science 317, 1196 (2007) [4] Ariando et al., Nature Comm. doi:10.1038/ncomms1192 (2010) [5] Lu Li et al., Nature Phys. doi:10.1038/nphys2080 (2011) [6] J. A. Bert et al. Nature Phys. doi: 10.1038/nphys2079 (2011) [7] M. M. Metha et al., Nature Comm. doi:10.1038/ncomms1959 (2012) [8] P. Perna et al., Appl. Phys. Lett.97, 259901, (2010) [9] C. Aruta et al., Appl. Phys. Lett.97, 252105 (2010) [10] C. Aruta et al., Appl. Phys. Lett.101, 031602 (2012) [11] U. Scotti di Uccio et al., arXiv:1206.5083v1 [cond-mat.mtrl-sci]

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3.23 Orbital Selectivity in heavy-fermion d-electron materials Gianluca Giovannetti1, Luca De` Medici2, Massimo Capone31 CNR-IOM-Democritos National Simulation Centre and International school for Advanced Studies (SISSA), Via Bonomea 265, I-34316, Trieste, Italy 2 Laboratoire de Physique et Etude de Materiaux, UMR8213 CNRS/ESPCI/UPMC, Paris, France

In a system with a multi orbital band structure, the various bands can display remarkably different properties which can be traced back to a different degree of correlation triggered by bandwidth differences or crystal-field splitting. Mobile electrons coexist with almost localized partners, leading to peculiar bad metallic behavior.Performining slave-spin mean-field and dynamical mean field theory calculations we show that Iron superconductors (FeSe, LaOFeAs, BaFe2As2) and LiV2O4 display a similar behavior as a consequence of a sizable Hund's exchange Jh.The Hund's rule simultaneously reduces the quasi particle weights and increases the critical U for the Mott transition as the spectral weight is not transferred to the high-energy Hubbard bands, but spreads over a scale controlled by Jh.The evolution of the quasi particle weight Z as a function of the interaction terms reveals a clear crossover from a weakly-correlated phase to a bad bad metallic phase in which they become weakly dependent on U while strongly depend on Jh with localized and delocalized conduction electrons coexisting.In this regime inter-orbital charge correlations are suppressed and the different orbitals decouple with each other behaving as a collection of independent single band models.!

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3.24 Fingerprints of Spin-Orbital Entanglement in Transition Metal Oxides

Andrzej M. Ole. Marian Smoluchowski Institute of Physics, Jagellonian University, Reymonta 4, Krakow, Poland Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, Stuttgart, Germany

Corresponding author: Prof. Andrzej M. Ole., email: [email protected] postal address: Institute of Physics UJ, Reymonta 4, PL-30059 Krakow, Poland

Spin-orbital superexchange models are in general characterized by entangled states [1]. They provide a theoretical framework for describing magnetic and optical properties of Mott insulators [2]. Recent examples of spin-orbital models with highly anisotropic and intrinsically frustrated interactions are discussed. We show that when both spins and orbitals correlation functions change with increasing temperature, the physical properties are determined by spin-orbital entanglement [3]. Entanglement at finite temperature is observed in several properties of the RVO3 perovskites (R=La,...,Lu), including: (i) their phase diagram [4], with two phase transitions, orbital and antiferromagnetic (AF), (ii) temperature dependence of the low-energy optical spectral weight in LaVO3, (iii) dimerization of ferromagnetic (FM) interactions in the C-AF phase, and (iv) hole propagation in the AF phase accompanied by alternating orbital order. Therefore, present experimental challenges concern spin-orbital entanglement in excited states. Their simplest example are the low-energy excitations of one-dimensional spin-orbital model with FM spin and ferro-orbital order, which consist of spin waves, orbital waves, and joint spin-orbital excitations. Among the latter we identify strongly entangled spin-orbital bound state and spin-orbital quasiparticle state which both appear as peaks in the von Neumann entropy spectral function [5]. The scaling of the von Neumann entropy with system size exhibits qualitatively different behavior for the bound state and the quasiparticle state --- the strong entanglement of these states is manifested by a universal logarithmic scaling of the von Neumann entropy with system size, while the entropy saturates for other spin-orbital excitations. We suggest that spin-orbital entanglement can be experimentally explored by the measurement of the dynamical spin-orbital correlations using resonant inelastic x-ray scattering. Recently, it has been shown that entangled spin-orbital fluctuations may trigger novel exotic types of magnetic order, inter alia, the noncollinear AF phase in the two-dimensional Kugel-Khomskii model [6]. This phase consists of four sublattices with mutually orthogonal nearest neighbor spins, and is stabilized by second nearest neighbor AF and third nearest neighbor FM spin interactions that involve spin singlets entangled with orbital excitations.!!

[1] A.M. Ole., Journal of Physics: Condensed Matter 24, 313201 (2012). [2] A.M. Ole., G. Khaliullin, P. Horsch, and L.F. Feiner, Phys. Rev. B 72, 214431 (2005). [3] A.M. Ole., P. Horsch, L.F. Feiner, G. Khaliullin, Phys. Rev. Lett. 96, 147205 (2006). [4] P. Horsch, A.M. Ole., L.F. Feiner, G. Khaliullin, Phys. Rev. Lett. 100, 167205 (2008). [5] W.-L. You, A.M. Ole., and P. Horsch, Phys. Rev. B 86, 094412 (2012). [6] W. Brzezicki, J. Dziarmaga, and A.M. Ole., Phys. Rev. Lett. 109, 237201 (2012).

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3.25 Surface and edge states in topological insulators Carmine Ortix Institute for Theoretical Solid State Physics, IFW Dresden, Germany

The discovery of two- and three-dimensional topological insulators (TI) has brought to light a new state of quantum matter. TIs are insulating in the bulk but have topologically protected edge and surface states. Topology dictates that these metallic surface states are spin-locked: electrons with opposite spin counterpropagate at the sample boundaries. Starting out from the recent synthesis of Bi14Rh3I9 [1] -- a TI consisting of stacks of graphene-like bismuth based layers -- we analyze the electronic characteristics of the topologically protected edge states in graphene-like materials and demonstrate their fundamental differences at their zig-zag and armchair terminations [2]. We will also show how such a strong surface termination dependence of topological surface states naturally arises in the entirely different family of inverted cubic mercury chalcogenide semiconductors with zincblende crystal structure. References:

[1] B. Rasche, A. Isaeva, M. Ruck, S. Borisenko, V. Zabolotnyy, B. Buchner, K. Koepernik, C. Ortix, M. Richer, J. van den Brink, Nature Materials 10.1038/nmat3570

[2] L. Cano-Cortes, C. Ortix, J. van den Brink arXiv:1303.2252

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3.26 Insulator to Metal transition in Cs3C60 under pressure L. Baldassarre1, M. Mitrano2, D. Nicoletti2, A. Perucchi3, C. Marini4, M. Riccò5, D. Pontiroli5 M. Capone6, S. Lupi7 1Center for Life NanoScience@LaSapienza, Istituto Italiano di Tecnologia Roma, Italy 2 Max Planck Department for Structural Dynamics, Universität Hamburg, Germany 3 Sincrotrone Trieste, Area Science Park, Trieste, Italy 4European Synchrotron Radiation Facility, Grenoble Cedex, France 5Dipartimento di Fisica e Scienze della Terra, Università di Parma, Parma, Italy 6CNR-IOM-Democritos National Simulation Centre and International School for Advanced Studies (SISSA) 7CNR-IOM, and Dipartimento di Fisica, Universita` di Roma “La Sapienza”, Rome, Italy

Corresponding author: [email protected]; CLNS IIT@Sapienza, V.le Regina Elena 295, Roma, Italy

Strongly correlated systems are characterized by rich and often complex phase diagrams that depend on temperature, pressure and doping [1] and, by changing lightly one of the aforementioned parameters, the properties of these compounds change significantly. Chemical substitution is, for instance, a convenient method for tuning the properties of a compound. However, it inevitably introduces impurities, affecting many of the physical parameters in an incontrollable way. On the other hand the application of external pressure allows for tuning these systems in a more systematic fashion. In recent years high pressure demonstrated to be a useful technique for investigating the physics of strongly correlated electron systems, where, by acting on the ratio between the electronic Coulomb repulsion and the hopping integral, it drives the system towards an insulator-to-metal transition [2,3]. The expanded fulleride Cs3C60 is an antiferromagnetic insulator at ambient condition and becomes superconducting for T < 38K. There is mounting evidence that this superconducting state is not a conventional BCS-type and that electron – electron interactions have to be taken into account in order to understand its physical properties [4]. We present here infrared measurements on Cs3C60 under pressure at room temperature following its evolution from an insulating state to a metallic one. By comparing the experimental kinetic energy with theoretical calculations we discuss the “degree of correlation” of Cs3C60, showing that electron-electron interactions play a key role in this system. [1] M. Imada, A. Fujimori and Y. Tokura, Rev. Mod. Phys. 70, 1039 (1998). [2] E. Arcangeletti et al., Phys. Rev. Lett. 98, 196406 (2007). [3] S. Lupi et.al., Nature Communications 1:105 (2010). [4] M. Capone, M. Fabrizio, C. Castellani and E. Tosatti, Rev. Mod. Phys. 81, 943 (2009). !

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3.27 Intrinsic robust antiferromagnetism at manganite interfaces S. Valencia1, L. Peña2, Z. Konstantinovic2, Ll. Balcells2, R. Galceran2, D. Schmitz1, F. Sandiumenge2, M. Casanove3, and B. Martínez2 1Helmholtz-Zentrum-Berlin Albert-Einstein-Str. 15, 12489 Berlin (Germany) 2Institut de Ciéncia de Materials de Barcelona-CSIC. Campus de la UAB, 08193 Bellaterra (Spain) 3Centre d´Elaboration de Matériaux et d´Etudes Structurales (CNRS-CEMES) BP 94347, 29 rue Jeanne Maarving, 31055 Toulouse (France)

Corresponding author: Benjamín Martínez Perea. ICMAB-CSIC. Campus de la UAB, 08193 Bellaterra (Spain)

e-mail: [email protected]

In this work we investigate the interface between La2/3Sr1/3MnO3 (LSMO) thin films and different capping layers by means of x-ray linear dichroism and transport measurements. Our data reveal that, irrespective to the capping material, LSMO/capping layer bilayers exhibit an antiferromegnetic/insulating phase at the interface, likely to originate from a preferential occupancy of Mn 3d 3z2-r2 eg orbitals. This phase, which extends 2 to 4 unit cells, is also observed in an uncapped LSMO reference sample thus, pointing to an intrinsic interfacial phase separation phenomenon, likely to be promoted by the structural disruption and symmetry breaking at the LSMO free surface/interface. These experimental observations strongly suggest that the structural disruption at the LSMO interfaces play a major role on the observed depressed magnetotransport properties in manganite-based magnetic tunneling junctions and it is at the origin of the so-called dead layer.

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Symposium 4


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4.15 Functional oxides films: from single crystals to polycrystalline substrates W.Prellier Laboratoire CRISMAT, ENSICAEN, CNRS UMR 6508, 6 Bd Mal Juin, 14050 Caen Cedex, France

Corresponding author:

Complex oxides represent a class of materials with several of exiting properties including magnetism, superconductivity or multiferroics. Thus, there are interesting for both fundamental research and applications. Using epitaxial strain, it is also possible in a thin film to modify the electronic properties as compare to bulk materials. While usually, the material is deposited on a single crystal to achieve the perfect epitaxy, it is also possible to synthesise the film on other type of substrate. In the first case, the film can be also be made artificially using the superlattices approach. In this talk, I will show recent results on superconductor superlattices [1,2] as well as our recent developed approach on thermoelectric films grown on a polycrystalline ceramic sample.[3] At the end, it will provide insight into current perspectives and future trends of functional oxide thin films.

[1] P. Boullay et al., Phys. Rev. B 83, 125403 (2011). [2] D. Di Castro et al, Phys. Rev. B 86, 134524 (2012). [3] D. Pravarthana et al., submitted (2012)

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4.16 Pulsed laser deposition of amorphous-LaAlO3/SrTiO3 interfaces: correlating the LaAlO3 plasma plume and interface conduction. A. Sambri,1 D. V. Cristensen,2 F. Trier,2 Y. Z. Chen,2 S. Amoruso,1 X. Wang1,R. Bruzzese,1 and N. Pryds2 1CNR-SPIN and Dipartimento di Fisica, Complesso Universitario di Monte Sant'Angelo, Via Cintia, I-80125 Napoli, Italy. 2Department of Energy Conversion and Storage, Risø Campus, Technical University of Denmark, DK-4000 Roskilde, Denmark.

Corresponding author: Salvatore Amoruso, CNR-SPIN and Dipartimento di Fisica, Complesso Universitario di Monte Sant'Angelo, Via Cintia, I-80125 Napoli, Italy – E-mail: [email protected]

Amorphous-LaAlO3/SrTiO3 (a-LAO/STO) interfaces fabricated by pulsed laser deposition present metallic conductivity similar to those found for the extensively studied crystalline-LaAlO3/SrTiO3 interfaces. Here, we address how the a-LAO/STO interface conductivity can be controlled by modifying the properties of the ablation plasma plume impinging on the SrTiO3 (STO) substrate. We focus on the effects of (a) the background gas pressure & type and (b) the charged plasma plume components. In case (a), the LAO plasma plume is subjected to different pressures of O2 and Ar and examined by fast photography and space-resolved optical emission spectroscopy. We find that the interface conduction rely on both the gas type (O2 and Ar) as well as pressure, and deduce the role of the kinetic energy and oxidation degree of the plume species. In case (b), the ablation plume is modified by an external electric field and studied using a Langmuir probe. We show that the electric field modulates the charged LAO plume species, which – albeit being a minority – shifts the a-LAO/STO interface from metallic to insulating. All these experimental findings display a critical dependence of the interfacial conductivity in a-LAO/STO interfaces on the plasma plume properties, which allows adding further insights on underlying mechanisms leading to interfacial conductivity in these systems. We have shown that tuning the plasma plume (e.g. internal energy, charge balance, etc.) may provide a helpful, strategic control on the final conduction properties (e.g. carrier density, mobility, etc.) of STO-based heterostructures.

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4.17 Metal-insulator transition in LaAlO3/SrTiO3 interface induced by Ar+ ion-beam irradiation Pier Paolo Aurino,1 Alexey Kalabukhov,1 Nikolina Tuzla,2 Eva Olsson,2 Dag Winkler1 and Tord Claeson1

1Department of Microtechnology and Nanoscience – MC2, Chalmers University of Technology, Gothenburg, Sweden

2Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden Type of Contribution: Oral

Corresponding author: Pier Paolo Aurino. Address: Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296 Gothenburg, Sweden Phone +46 31 7725477, Fax: +46 31 7723471, e-mail: [email protected]

A quasi-two-dimensional electron gas (q2DEG) formed at the interface between two wide band gap insulators, LaAlO3(LAO) and SrTiO3(STO),[1] shows plethora of fascinating phenomena, such as two-dimensional superconductivity, giant electric field effect, and the co-existence of ferromagnetic and superconducting phases. Remarkably, the metallic interface appears only when 4 or more unit cells of the LAO film is deposited on the top of the TiO2-terminated STO substrate.[2] We have recently demonstrated that it is possible to make the interface insulating by low energy Ar+ ion-beam irradiation and used this effect to pattern conducting nanostructures in the q2DEG.[3] The low energy Ar+ ions do neither physically etch the film nor create oxygen vacancies at the interface. In this work we present detailed investigation of possible mechanisms of the metal-insulator transition induced by ion beam irradiation. High-resolution scanning transmission electron microscopy (HR-STEM) of irradiated10 unit cell thick LAO showed that the thickness of the crystalline part is about 5-6 unit cells that is thicker than 4 unit cells required for the formation of q2DEG. The surface structure of the film can be fully restored after annealing in oxygen atmosphere as evidenced by reflection high-energy electron diffraction (RHEED). However, the interface remained insulating after the annealing in oxygen atmosphere. We found that introducing an additional annealing step followed up by annealing in oxygen atmosphere restores the metallic state that is identical to the one before irradiation. This points out to charged defects introduced by ion irradiation that are responsible for the insulating state of the interface. [1] A.Ohtomo and H.Y. Hwang; Nature (London) 427, 423 (2004). [2] S. Thiel, G. Hammerl, A. Schmehl, C.W.Schneider and J. Mannhart; Science 313 (2006) 1935. [3] P. P. Aurino,!A. Kalabukhov, N. Tuzla, E. Olsson, D. Winkler and T. Claeson; (submitted).

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4.18 Interface Fermi states of LaAlO3/SrTiO3 and related heterostructures C. Cancellieri, M. L. Reinle-Schmitt, M. Kobayashi, V. N. Strocov, "#!$%&'()'*+,!-#./#!0&'12*34+,!T. Schmitt, and P. R. Willmott Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland *DPMC, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland

Corresponding author: Claudia Cancellieri, SLS-PSI, WBBA/011, 5232 Villigen, Switzerland; Tel: +41 56 310 5689; [email protected]

At the interface between complex oxides, unexpected electronic properties different from those of the constituent bulk materials can arise. A particularly interesting example is the appearance of 2-dimensional conductivity at the interface of the band insulators LaAlO3 (LAO) and SrTiO3 (STO) [1–3] above a critical LAO thickness of 4 unit cells (u.c.) [4]. The interfaces of LaAlO3/SrTiO3 and (LaAlO3)x(SrTiO3)1!x/SrTiO3 heterostructures have been investigated by soft x-ray photoelectron spectroscopy for different layer thicknesses across the insulator-to-metal interface transition. The valence band and Fermi edge were probed using resonant photoemission across the Ti L2,3 absorption edge. We measured, for the first time to our knowledge, clear spectroscopic signatures of Ti3+ signal at the Fermi level in fully oxygenated samples of LAO/STO and the related system of mixed LASTO:0.5/STO [6]. Our results show that Ti3+-related charge carriers are present in both systems, but only for conducting samples. No Fermi-edge signal could be detected for insulating samples below the critical thickness. Furthermore, the angular dependence of the Fermi intensity allows the determination of the spatial extent perpendicular to the interface of the conducting electron density. [1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004). [2] N. Reyren et al., Science 317, 1196 (2007). [3] A. D. Caviglia et al., Nature 456, 624 (2008). [4] S. Thiel et al., Science 313, 1942 (2006).

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4.19 Soft x-ray photoemission studies of LaAlO3/SrTiO3 and NdGaO3/SrTiO3 interfaces Andreas Koitzsch, Uwe Treske, Umberto Scotti di Uccio*, Emiliano Di Gennaro*, Fabio Miletto Granozio* IFW Dresden, P.O. Box 270016, 01171 Dresden, Germany *!CNR-SPIN and Dipartimento di Scienze Fisiche, Univ. di Napoli “Federico II”, Compl. Univ. di Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy

Corresponding author: A. Koitzsch; IFW Dresden, P.O. Box 270016, 01171 Dresden, Germany; [email protected]

We have systematically investigated PLD (pulsed laser deposition) grown LaAlO3/SrTiO3 and NdGaO3/SrTiO3 samples with varying layer number and grown under different conditions by x-ray photoemission spectroscopy and –microscopy. Using the resonance condition of the Ti L edge the metallic states and their evolution across the metal-insulator transition are directly monitored. The interface sensitivity of the photoemission process can be tuned by the choice of the photon energy. The cation characterisitcs of e.g. Sr/La have been studied in detail and show an unexpected fine structure and depth profile. !

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4.20 Theory of spin-orbit coupling at LaAlO3/SrTiO3 interfaces and SrTiO3 surfaces

Zhicheng Zhong, Anna Toth, and Karsten Held Institute of Solid State Physics, Vienna University of Technology, A-1040 Vienna, Austria

A full theoretical understanding of the spin-orbit coupling (SOC) effects at LaAlO3/SrTiO3 interfaces and SrTiO3 surfaces is still needed. We perform first-principles density-functional-theory calculations and derive from these a simple tight-binding Hamiltonian, through a Wannier function projection and group theoretical analysis. We find striking differences to the standard Rashba theory for spin-orbit coupling in semiconductor heterostructures, because the relevant t2g orbitals are very different from nearly free electrons. The key ingredients to the spin splitting are the atomic SOC and the interface asymmetry, which enters via asymmetric t2g orbital lobes. !

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4.21 Symmetry and magnitude of spin-orbit torques in ferromagnetic heterostructures Kevin Garello1, Ioan Mihai Miron2, Can Onur Avci1, Frank Freimuth3, Yuriy Mokrousov3, Stefan Blügel3, Stéphane Auffret2, Olivier Boulle, Gilles Gaudin2, and Pietro Gambardella1,4,5 1 Catalan Institute of Nanotechnology (ICN), E-08193 Barcelona, Spain

2 SPINTEC, UMR-8191, CEA/CNRS/UJF/GINP, INAC, F-38054 Grenoble, France

3 Institut für Festkörperforschung and Institute for Advanced Simulation, Forschungszentrum Jülich, 52425 Jülich, Germany

4 Departament de Física, Universitat Autonoma de Barcelona (UAB), E-08193 Barcelona, Spain

5 Institució Catalana de Recerca i Estudis Avançats (ICREA), E-08010 Barcelona, Spain

Current-induced spin torques are of great interest to manipulate the orientation of nanomagnets without applying external magnetic fields. They find direct application in non-volatile data storage and logic devices, and provide insight into fundamental processes related to the interdependence between charge and spin transport. Recent demonstrations of magnetization switching induced by in-plane current injection in ferromagnetic heterostructures [1] have drawn attention to a class of spin torques based on orbital-to-spin momentum transfer, which is alternative to pure spin transfer torque (STT) between noncollinear magnetic layers and amenable to more diversified device functions [1,2]. Due to the limited number of studies, however, there is still no consensus on the symmetry, magnitude, and origin of spin-orbit torques (SOTs). Here we will report on the quantitative vector measurement of SOTs in Pt/Co/AlOx trilayers using harmonic analysis of the Hall Voltage as a function of the applied current and magnetization direction. Based on general space and time inversion symmetry arguments, we show that asymmetric heterostructures allow for two different SOTs having odd and even behavior with respect to magnetization reversal. We will present our experimental results, revealing a scenario that goes beyond simple models of the spin Hall and Rashba contributions to SOTs. [1] I. M. Miron, K. Garello, P. Gambardella Et. al., “Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection”, Nature, 476, 189-193 (2011) [2] L. Liu, et al., “Spin torque switching with the giant spin Hall effect of tantalum”, Science, 336, 555-558 (2012)

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4.22 Annealing study of spin-orbit torques in perpendicularly magnetized Ta/CoFeB/MgO layers

C. O. Avci1, K. Garello1, M. Miron2, G. Gaudin2, S. Auffret2, and P. Gambardella3 1 Catalan Institute of Nanotechnology (ICN), E-08193 Barcelona, Spain

2 SPINTEC, UMR-8191, CEA/CNRS/UJF/GINP, INAC, F-38054 Grenoble, France 3 Eidgenössische Technische Hochschule (ETH) Zurich, CH-8093 Zurich, Switzerland

Corresponding author: Can Onur Avci ([email protected])

Catalan Institute of Nanotechnology (ICN) Campus de la UAB, Edifici CM7 08193 Bellaterra(Barcelona)/SPAIN

Current-induced spin-orbit torques (SOT) provide interesting solutions to improve functionalities and efficiency of memory units. Recently, magnetization switching of a single ferromagnetic layer induced by in-plane current injection has been demonstrated in heterostructures possessing strong spin-orbit coupling and perpendicular magnetic anisotropy such as Pt/Co/AlOx(MgO), Ta/CoFeB/MgO [1-3]. These systems constitute the base stack for magnetic tunnel junctions which are promising candidates for future non-volatile memories. The main advantage of in-plane switching is that, by decoupling the writing and reading current paths, it prevents the damaging of the tunnel barrier by the switching (writing) current, in contrast to the conventional spin-transfer torque switching mediated by perpendicular current.

Switching was found to occur due to an effective torque perpendicular to both the current and instantaneous magnetization directions, equivalently to an effective field that rotates with the magnetization. Although the manifestation of SOTs is clear, the exact mechanism behind is still in debate and two mechanisms are proposed to explain the phenomena: one is based on the combination of spin dependent conductivity and the Rashba effect, which is mostly sensitive to the interfacial properties of the heterostructure, while the other mechanism is based on a torque acting on the ferromagnetic layer generated by the spin Hall effect in the heavy metal (e.g. Ta, Pt).

To shed light on the origin of the mechanisms, we will present a detailed study of the current-induced SOTs and the magnetization switching behavior (with ultrashort current pulses) in perpendicularly magnetized Ta/CoFeB/MgO layers as a function of the annealing temperature. With a refined spin-orbit field measurement method in the low current regime, we will demonstrate that, the magnitude of the SOTs strongly depends on annealing temperature. We will also show that the perpendicular magnetic anisotropy dramatically changes with annealing temperature which consequently has an influence on the switching behavior of the system. Finally, with the help of transmission electron microscope images and X-ray magnetic circular dichroism data, we will correlate the physical and chemical changes mediated by annealing with the observed dependences of SOTs.

[1] I. M. Miron, K. Garello, G. Gaudin, P.-J. Zermatten, M. V. Costache, S. Auffret, S. Bandera, B.

Rodmacq, A. Schuhl, & P. Gambardella, Nature. 476, 189-193 (2011) [2] L. Liu, C.-F. Pia, Y. Li, H.W. Tseng, D.C. Ralph, & R.A. Buhrman, Science. 336, 555-558 (2012) [3] C. O. Avci, K. Garello, I. M. Miron, G. Gaudin, S. Auffret, O. Boulle & P. Gambardella,

Appl. Phys. Lett. 100, 212404 (2012)

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4.23 High-Temperature Magnetic Insulating Phase in Ultrathin La0:67Sr0:33MnO3 Films Gertjan Koster, Hans Boschker, Jaap Kautz, Evert P. Houwman, Wolter Siemons, Dave H. A. Blank, Mark Huijben, Arturas Vailionis, and Guus Rijnders

We present a study of the thickness dependence of magnetism and electrical conductivity in ultrathin LSMO films grown on SrTiO3 (110) substrates. We found a critical thickness of 10 unit cells below which the conductivity of the films disappeared and simultaneously the Curie temperature increased, indicating a magnetic insulating phase at room temperature. These samples have a Curie temperature of about 560 K with a significant saturation magnetization of 1.2 mB. The canted antiferromagnetic insulating phase in ultra thin films of n < 10 coincides with the occurrence of a higher symmetry structural phase with a different oxygen octahedra rotation pattern. Such a strain engineered phase is an interesting candidate for an insulating tunneling barrier in room temperature spin polarized tunneling devices. In this presentation, the latest experiments and characterizations will be discussed.

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4.24 Influence of Double-TiO2 Terminated (001)SrTiO3 Surface on the Nucleation and Growth of La(Al,Mn)O3 Thin Films Yu. A. Boikov1*, A. Kalabukhov1, T. Claeson1

, I. T. Serenkov*, V. I. Sakharov*, J. Borjesson1, N. Ljustina1, E. Olsson1, D. Winkler1. 1Chalmers University of Technology, S-41296 Gothenburg, Sweden. *Ioffe Physical-Technical Institute RAS, 194021 St.Petersburg, Russia.

Nucleation and growth of ultra-thin LaAlO3, LaTiO3, or LaMnO3 films are strongly influenced by the surface structure of the substrate. This, in turn, determines structural and electronic properties of the interface between the film and the substrate. (1x1) TiO2 surface termination of most commonly used SrTiO3 substrate is usually obtained using preferential BHF etching of the Sr and annealing at high temperature [Kawasaki Science 1994]. Different atomic reconstructions and relaxations at the substrate surface may occur depending on the treatment and under different deposition conditions, including (2x1) reconstruction [R. Herger et al, PRL 2007] or formation of a double (1x1)TiO2 layer [N. Erdman et al, Nature 2002]. In this work we show that surface reconstructions different from (1x1)TiO2 may affect growth of thin LaAlO3 and LaMnO3 films. Medium Energy Ion Scattering (MEIS) was used to probe structure and composition of the surface layer of a TiO2 terminated (001)SrTiO3 single crystal substrate as well as those of 1-6 unit cell (u.c.) thick LaMnO3 (LaAlO3) epilayers grown by pulsed laser deposition on top of it. Large width and enhanced intensity of the Ti peak in the aligned MEIS spectrum of TiO2 terminated (001)SrTiO3 indicate clearly enrichment of titanium at the substrate surface. This agrees well with the TiO2 double layer (DL) proposed by N. Erdman. From the DL (1+1) configuration follows that La atoms in LaMnO3 layers grown on top reside in rows, which are laterally shifted relative to Sr ones in the substrate. This agrees well with the abnormally large intensity of the La peak in the aligned MEIS spectra obtained for LaMnO3/(001)SrTiO3 heterostructures as well as for LaAlO3/(001)STO ones grown at high oxygen pressure However, shifts in atom columns have not been clear seen in reported electron microscopy images of similar heterostructures. Simulated random MEIS data for (1uc)LaMnO3/(001)SrTiO3 and (1u.c.)LaAlO3/(001)SrTiO3 heterostructures match well experimental ones if a substantial part of the La atoms is assumed to diffuse into the top layers of the substrate. Impact of the DL arrangement on structure of the LaAlO3 and LaMnO3 nanometer thick films grown at low oxygen pressure was hardly observed. We acknowledge partial support the Swedish Research Council, the K. A. Wallenberg foundation and project 11-02-00609a of the Russian Foundation of Basic Research.

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4.25 Interface effects in nanocrystalline electronically and ionically conducting oxides Giuliano Gregori, Piero Lupetin, Marcus C. Göbel and Joachim Maier Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany

Corresponding author: [email protected] / Ph. +40-711-689-1770 / Fax +49-711-689-1722

In many ionically and electronically conducting oxides the concentration of the charge carriers at interfaces can deviates remarkably compared with the bulk situation, which is unaffected by the presence of boundaries. Thus, scale effects (e.g. the reduction of the grain size in polycrystalline materials) can result in the intriguing situation, in which the defect chemistry of a conducting oxide is fully controlled by the boundary properties rather than by the bulk ones. Such a situation can induce the occurrence of a number of fascinating size-induced phenomena that have a remarkable impact not only on the electrical conduction but also on other related electrical properties such as for example thermopower. This obviously discloses new opportunities to adjust the properties of such materials by modifying the grain size. In this contribution, the effects of reducing the grain size on the electrical conductivity of ionic and mixed conductors such as SrTiO3 and CeO2 will be discussed. Particularly relevant is the case of nanocrystalline SrTiO3, whose conductivity was measured – for the first time – over a broad range of oxygen partial pressure (pO2) at temperatures ranging between 450 and 550°C. Compared with the bulk properties, at low pO2 an increase of the n-type conductivity by almost 3 orders of magnitude is observed while in oxidizing conditions (high pO2) the p-type conductivity is depressed by more than 3 orders of magnitude. Remarkably, an even stronger depression affects the oxygen vacancy conductivity when compared with the unaffected bulk properties (6 orders of magnitude). Consistent with such findings, an even more spectacular effect is observed, namely a giant shift of the transition point from n to p-type conductivity by 12 orders of magnitude in terms of oxygen partial pressure.

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4.26 Capacitance Enhancement in LaNiO3-based Strongly Correlated Electrode System Qiran LI1,2, Wolfgang Donner2, Bernard Mercey1, Ulrike Lüders1

1Laboratoire Crismat (UMR6508 CNRS/Ensicaen)6 Bd du Maréchal Juin, 14050Caen, France

2Institut für Materialwissenschaft, Technische Universität Darmastadt Petersenstr. 23, 64287 Darmstadt, Germany

Corresponding author: Qiran LI, email: [email protected], Laboratoire Crismat, 6,Bd du Marechal Juin, 14050 Caen, FR

In order to miniaturize the passive devices such as capacitor, the geometric capacitance has to be enhanced. Nowadays, most of the efforts dedicated to this development for enhancing the capacitance are attached to the manipulation of the dielectric layer and the structure of the device. Nevertheless, another promising solution on the subject is to exploit the electrodes. It was shown theoretically that capacitance can be enhanced above the geometric capacitance by a careful choice of the electrode material and ultra-thin dielectric layer [1]. The most promising candidates are materials with a 2D electron gas or strong electron-electron correlation. A 40% of capacitance enhancement by using the 2D electron gas in the interface of LaAlO3/SrTiO3 has been shown by recent experimental studies [2].

We will present the results on strong correlated electrodes: LaAlO3/LaNiO3 system prepared by Pulsed Laser Deposition (PLD) under optimum deposition conditions. Multilayer samples with various structural designs have been processed and investigated on their structural and electrical properties. Results from the X Ray Diffraction (XRD) and X Ray Reflectivity (XRR) indicate a good thin film growth quality. We observed that the ultrathin films of LaNiO3 as the electrode undergo a transition from metallic to semiconducting behaviour abcribed to weak localization. The strong electron-electron correlation has been proved by temperature dependence measurements with varied magnetic field. These samples yielded interesting results on the capacitance values beyond the geometric capacitance and low leakage current given by the impedance spectroscopy. A careful analysis of this study enables us to explore the further applications of this system with comparaison to other alternative systems.

[1], T.Kopp et al, Journal of Applied Physics 106, 064504 (2009)

[2], L.LI et al, Science 332, 825 (2011)

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4.27 Field-effect modulation of exchange bias and conductivity at ferroelectric/ferromagnetic interfaces L. Bégon-Lours, P. Altuntas, A. Crassous, S. Fusil, K. Bouzehouane, E. Jacquet, M. Bibes, A. Barthélémy and J. E. Villegas Unité Mixte de Physique CNRS/Thales, 1 av Augustin Fresnel, 91767 Palaiseau Cedex

Corresponding author: Laura Bégon-Lours, Unité Mixte de Physique CNRS/Thales, 1 av Augustin Fresnel, 91767 Palaiseau Cedex, [email protected], +33 1 69 41 56 51.

The strongly-correlated nature of d electrons in complex oxides allows the manipulation of physical properties such as magnetism and superconductivity via the field-effect modulation of the charge carrier density [1]. Here we investigate field-effects at the interface between the ferromagnet La0,67Sr0,33MnO3 (LSMO) and the ferroelectric BiFeO3 (BFO) in BFO/LSMO//STO thin film heterostructures [2,3]. We found that this system exhibits magnetic exchange bias [4] in the temperature range T,100 K, whose magnitude is modulated upon switching the BFO’s out of plane polarization. This is controlled by applying voltages with a conductive-tip atomic force microscope [5]. The exchange bias modulation is accompanied by a large modulation of the LSMO conductivity (up to a factor of 2.2 at room temperature). The different scenarios that allow for an understanding of these effects will be discussed.

Work supported by the DIM Oxymore (région Ile-de-France)

[1] C. H. Ahn, M. Di Ventra, J. N. Eckstein, C. D. Frisbie, M. E. Gershenson, a. M. Goldman, I. H. Inoue, J. Mannhart, A. J. Millis, A. F. Morpurgo, D. Natelson, and J.-M. Triscone, Rev. Mod. Phys. 78, 1185–1212 (2006).

[2] S. M. Wu, S. a Cybart, P. Yu, M. D. Rossell, J. X. Zhang, R. Ramesh, and R. C. Dynes, Nature Mater. 9, 756–61 (2010).

[3] J. Heron, M. Trassin, K. Ashraf, M. Gajek, Q. He, S. Yang, D. Nikonov, Y.-H. Chu, S. Salahuddin, and R. Ramesh, Phys. Rev. Lett. 107, 217202 (2011).

[4] H. Béa, M. Bibes, F. Ott, B. Dupé, X.-H. Zhu, S. Petit, S. Fusil, C. Deranlot, K. Bouzehouane, and A. Barthélémy, Phys. Rev. Lett. 100, 017204 (2008).

[5] A. Crassous, R. Bernard, S. Fusil, K. Bouzehouane, D. Le Bourdais, S. Enouz-Vedrenne, J. Briatico, M. Bibes, A. Barthélémy, and J. Villegas, Phys. Rev. Lett. 95, 247002 (2011).

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Poster Session :

Monday, May 20th 16:30 -18:30

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PS1.1 Anomalous behavior of Nb/Ru/Sr2RuO4 topological junctions M. S. Anwar, Taketomo Nakamura*, S. Yonezawa, M. Yakabe**, R. Ishiguro**, H. Takayanagi**,† and Y. Maeno Department of Physics, Kyoto University, Kyoto 606-8502, Japan *Institute for Solid State Physics, the University of Tokyo, Kashiwa 277-8581, Japan **Department of Applied Physics, Faculty of Science, Tokyo University of Science, Tokyo 162-8601, Japan †International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan

Corresponding author: M. S. Anwar: Department of Physics, Kyoto University, Kyoto 606-8502, Japan

Since the discovery of superconductivity in Sr2RuO4, various experiments reveal that its order parameter exhibits the spin-triplet chiral p-wave nature with broken time reversal symmetry [1]. Due to two-fold degeneracy of chirality originating from the direction of the Cooper pair orbital moment, chiral domain structure is expected to emerge in the bulk of such a superconductor. Chiral domains are separated by chiral domain walls anomalous to ferromagnetic domain walls. To investigate its topological aspects, originating from its orbital phase winding and chiral domain structure, we fabricated SNS’ micron sized Josephson junctions consisting of niobium (as an S) eutectic crystals of Ru (as an N) metal inclusions imbedded in Sr2RuO4 (as an S’). Such junctions are termed as the Topological junctions [2,3]. We report observations of unusual temperature dependence of the critical current Ic, anomalous hysteresis (reverse and unstable) with current, and telegraphic noise in voltage. These observations reflect unusual switching of Ic between higher and lower Ic states. These phenomena are well explained by chiral domain wall dynamics [4]. The switching can be partly controlled by external parameters such as temperature, magnetic field and electrical current. These results open up a possibility to utilize the superconducting chiral domain wall motion for future devices. [1] Y. Maeno, S. Kittaka, T. Nomura, S. Yonezawa, and K. Ishida, J. Phys. Soc. Jpn. 81, 011009 (2012). [2] T. Nakamura, R. Nakagawa, T. Yamagishi, T. Terashima, S. Yonezawa, M. Sigrist, and Y. Maeno, Phys. Rev. B 84, 060512 (2011). [3] T. Nakamura, T. Sumi, S. Yonezawa, T., Terashima, M. Sigrist, H. Kaneysasu, and Y. [4] Maeno, J. Phys. Soc. Jpn 81, 064708 (2012). A. Bouhon, and M. Sigrist, New J. Phys. 12, 043031-043056 (2010).

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PS1.2 Structural and transport properties in epitaxial films of Nd2"xCexCuO4±# A.Guarino1,2*, R. Fittipaldi2, A. Romano1,2, A. Vecchione2, A.Leo1,2, G. Grimaldi2, S. Pace1,2, A. Nigro1,2

1 Dipartimento di Fisica “E. R. Caianiello” Università di Salerno, I-84084 Fisciano (Salerno), Italy 2 CNR-SPIN UOS Salerno presso Università di Salerno, I-84084 Fisciano (Salerno), Italy

Corresponding author: Dipartimento di Fisica “E. R. Caianiello” Università di Salerno-stecca 9, I-84084 Fisciano (Salerno), Italy - Tel 089968266

We present here a study on the influence of the oxygen reduction process on the structural and transport properties of epitaxial thin films of the electron-doped cuprate Nd2!xCexCuO4±0. As is well known, the gradual removal from as-grown samples of a tiny percentage of excess oxygen ions leads to a drastic improvement of the metallic character of this system, which eventually becomes superconducting for suitable values of the cerium concentration, with a maximal critical temperature Tc!25 K. We find that the oxygen loss occurring in thermal treatments in the temperature range 500–850 °C leads to a reduction of the disorder hindering conductance processes, but is insufficient to make the system become superconducting. On the other hand, as soon as the annealing temperature is raised above 850 °C, superconductivity appears, and at the same time a systematic variation of the length of the unit cell along the c-axis direction is detected. This is a clear indication that the transition to the superconducting phase is always accompanied by a structural modification. A further salient feature characterizing samples annealed at high temperatures is the emergence of a linear contribution in the normal state resistivity, which superimposes to the quadratic one already present in samples which are oxygen reduced below 850 °C. This contribution is probably associated with the formation of hole-like carriers located at hole pockets developing at the Fermi energy along the nodal direction in the Brillouin zone. We conjecture that the evolution of the electronic states with oxygen removal for a given cerium concentration close to optimal doping, is similar to the one taking place in optimally annealed samples where cerium concentration is raised from the underdoped to the lightly overdoped regime value [1]. ! [1] A.Guarino, R. Fittipaldi, A. Romano, A. Vecchione, A. Nigro, Thin Solid Films 524, 282 (2012).

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PS1.3 Optical Characterisation of High Tc SuperConductors for Plasmonics and MetaMaterials Fabrication !

M.Gombos*,a,b, S.Romanoa, I.Rendinaa, R.Ciancioc, G.Carapellab, and V.Mocellaa

* a CNR-IMM UOS Napoli, Napoli, Italia * b CNR-SPIN UOS Salerno & Dip. di Fisica “E.R.Caianiello”, Università di Salerno, Baronissi (SA) ITALIA (UE) c CNR-IOM TASC Trieste, Basovizza (TS) ITALIA (UE)

Corresponding author is M.Gombos c/o CNR-IMM UOS NA, via P.Castellino 111, Napoli, ITALIA (UE)

e-mail: [email protected]

High frequencies applications of metamaterials and plasmonic structures are strongly limited by dissipative losses in structures, due to poor conductivity of most used metals in this frequency range. The use of high temperature superconductors (HTSC) is a possible approach to this problem, being HTSC plasmonic materials at nonzero temperature. Negative dielectic constant and variety of charge carriers (electrons or holes) are further very attractive features for plasmonic applications. Characterization of the high frequency response of these materials is then necessary in order to correctly understand the optical parameters of HTSC. !

We report on FTIR and ellipsometry measurements on NdBa2Cu3O7-d (Nd123) and the ruthenocuprate superconductor GdSr2RuCu2O8-d (Gd1212) in optical and near infrared regime. !

Among YBCO-like cuprate superconductors, Nd123 presents the highest Tc (96K), and the most interesting magnetic response properties. Even more interesting, in view of use for metamaterial, is Gd1212, whose main characteristic is the coexistence, in the same cell, of superconductivity and magnetic order below Tc: Ru ions intrinsic magnetic moments order themselves below 135K, whereas superconductivity onset is at about 40K, depending on fabrication details. !

We performed measurements on Melt-Textured bulk samples, which present the best superconducting properties. Results confirm the promising feature of the considered materials; further analyses, also on powders and films, are in progress. !

1] A.H.Majedi, IEEE T.Appl.Supercond. 19, 907 (2009) !

2] A.Tsiatmas et Al., Appl.Phys.Lett. 97, 111106 (2010) !

3] P.Tassin et Al., Nature Photon. 6, 259 (2012) !

4] M.Gombos et Al., Physica C 408-410, 189 (2004) !

Keywords: FTIR, Ellipsometry, High Temperature Superconductors, Plasmonics, Melt- Textured Growth, Magnetic Ordering, Cuprates and Ruthenocuprates, Metamaterials !

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We report the dependence on applied strain of the resistivity of Sr2RuO4 around its superconducting transition. To do this we have constructed a piezoelectric-based device that can both compress and tension the sample, and where it is the strain, rather than the stress, that is the controlled parameter. The strain is applied along the long axis of needle-like samples, in order to maximise the homogeneity of the strain field near the centre of the sample, where the measurements are performed.

PS1.4 Strain dependence of the superconducting transition of Sr2RuO4 !

Clifford W. Hicks, Daniel O. Brodsky, Alexandra S. Gibbs, Jan A.N. Bruin, Andrew P. Mackenzie Scottish Universities Physics Alliance (SUPA), School of Physics and Astronomy, University of St.

Andrews, St. Andrews KY16 9SS, United Kingdom !

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PS1.5 A disorder effect on current stability of superconducting materials !

Gaia Grimaldi1, Antonio Leo1,2*, Angela Nigro1,2, Elena Bruno3, Francesco Priolo3, Sandro Pace1,2

1 CNR SPIN Salerno, via Ponte Don Melillo Fisciano (SA), 84084, Italy 2 Department of Physics, Salerno University, Fisciano (SA), 84084, Italy 3Matis IMM-CNR and Physics-Astronomy Department, Catania University, CT 95123 Italy

Corresponding author: Dr. Antonio Leo, mail: [email protected], cel.phone: +393476598218

We study current stability in the flux flow dissipative state by a proper current-voltage measurement mode in several superconducting films exposed to light ion irradiation. The comparison between the details of the current-voltage (I-V) characteristics on the as-grown superconductor and on the irradiated material shows how disorder can play a significant role in the stability of the current carrying state in superconducting materials. Although it results that the light ion irradiation does not affect the overall superconducting properties, such as the critical current Ic , nevertheless it determines an increase of disorder in the pinning landscape which is able to improve the dynamic current stability above Ic. This effect has a direct impact on the potential applicability of superconducting materials, as well as an important prediction on the microscopic physical mechanisms which compete in maintaining stable the superconducting state.

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PS1.6 Ultrafast relaxation dynamics in iron-based superconductors Ba(Fe1-

xCox)2As2 L. Parlato1,2, C. Bonavolontà1,2, C. De Lisio1,2, G.P. Pepe1,2, M. Valentino2, G. A. Ummarino3, F. Kurth4, K. Iida4

1 Dipartimento Scienze Fisiche, Università di Napoli “Federico II”, Napoli, Italy

2 CNR-SPIN, I-80125 Napoli, Italy

3 Dipartimento di Fisica and CNISM, Politecnico di Torino,Torino, Italy

4 IFW Dresden, P. O. Box 270116, 01171 Dresden, Germany


The discovery of the superconductivity at Tc’s up to 55 K in iron–pnictide systems has sparked enormous interest in this class of materials. Investigations on this new class of superconductors have revealed a lot of similarities to MgB2 and the cuprates, for instance, layered structures, a multiband nature, high upper critical fields and a short coherence length. On the other side there are important differences like the quite isotropic behavior in contrast to the cuprates and the peculiar pairing symmetry. A common phase diagram for iron–pnictides suggests that Fe-pnictide has an antiferromagnetic spin density-wave (SDW) ground state that is stabilized by Fermi-surface nesting as well as by strong antiferromagnetic spin interactions along the Fe-square-diagonal. Then, the question of the relative importance of the spin degrees of freedom for the superconducting pairing interaction becomes immediately apparent, and understanding the Fe-pnictide compounds from the point of electron-phonon, the spin-spin and spin-charge interactions is therefore beneficial for understanding the nature of the superconducting coupling in the doped compounds.

Time-resolved spectroscopy is a very useful technique to investigate the nature of the electronic excitation in superconductors, mainly the role of the electron-phonon interaction, and other mechanisms involved into the competition between superconductivity and magnetism. In this work we present the time-resolved femtosecond spectroscopy study of optimally doped Ba-122 thin films growth on Fe buffer layer, with the aim to understand the mechanism of relaxation in the superconducting state. An evaluation of the two gaps has been carried out using some hypothesis based on bottleneck scenario and taking into account the role of the magnetic interactions that hold over the superconducting transition. A simple model based only on electron-phonon interactions yields a value of the electron-phonon constant coupling of -=0.18. Moreover, the experimental results have been validated using a three-band numerical model through solving the Eliashberg equations. Both experimental and theoretical results demonstrate that an exhaustive description of the superconducting state will be obtained if the effect of spin fluctuations is taken into account. The role of the Fe buffer layer in films with different thickness of Ba-122 and in films growth without the Fe buffer layer will be also discussed in terms of the proximity effect.

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We have conducted a detailed thin film growth structure of oxygen engineered monoclinic HfO2±x grown by reactive molecular beam epitaxy (MBE). Depending on the chosen oxidation conditions a (111) and (002) texture of hafnium oxide can be obtained [1]. The band gap of oxygen deficient hafnia decreases with increasing amount of oxygen vacancies by more than 1 eV [2]. For high oxygen vacancy concentrations, a defect band forms inside the band gap that induces a metal insulator transition. The conductivity is of p-type and changes by several orders of magnitude as a function of oxidation conditions. Oxygen vacancies do not give rise to ferromagnetic behavior. [1] E. Hildebrandt et al., J. Appl. Phys. 112, 114112 (2012). [2] E. Hildebrandt et al., Appl. Phys. Lett. 99, 112902 (2011)

PS2.1 Physical properties and band structure of reactive MBE grown oxygen engineered HfO2±x E. Hildebrandt, J. Kurian, and L. Alff Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany

Corresponding author: Lambert Alff, Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany. e-mail: [email protected].

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PS2.2 Magnetism and unusual Cu valency in quadruple perovskites !

P. Alippi and V. Fiorentini* CNR-ISM, Montelibretti (RM), Italy * CNR-IOM e Dipartimento di Fisica, Università di Cagliari, Italy

Corresponding author: V.Fiorentini, Dip. Fisica, Uni Cagliari, Cittadella Universitaria, 09042 Monserrato

(CA), Italy, [email protected]

The quadruple perovskites ACu3X4O12 have attracted interest recently due to several puzzling behaviors such as anomalous dielectric response, isostructural metal-insulator phase transitions, the cation selectivity of metallic or insulating character, and, not least, the unusual valency of Cu, depending on the A and X octahedral cations. We apply ab initio density-functional calculations to a selection of different materials in this class, specifically for the A, X pairs (Ca, Ti), (La, Fe), (Y, Co), i.e. for CaCu3Ti4O12 (CCTO), LaCu3Fe4O12 (LCFO), and YCu3Co4O12 (YCCO). !

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The presence of active 3d shells – especially that of Cu – suggests a possible role of electron correlation in these materials. The use of a beyond-(semi)local density- functional approach may be advisable or outright necessary. The front runners in this field today are hybrid or self-interaction corrected functionals, which both correct the dominant error of local functionals in dealing with localized states - namely, self-interaction. For computational simplicity, here we use GGA+U (generalized gradient approximation plus “Hubbard U”), which in this context can be viewed as an approximate parametrized self- interaction correction for a specific orbital shell. !

We find that Cu is in an effective divalent Cu(II)-like state or a trivalent Cu(III) state depending on the choice of octahedral cation. More generally we show that CCTO is a Cu- site AF-G Mott insulator; LCFO is a Fe-site AF-G band insulator; YCCO is a non- magnetic correlated band insulator. Fe is in a Fe(III) high-spin state; Co is in a Co(III) low- spin state; Cu is Cu(II) and magnetic in CCTO, and Cu(III) and non-magnetic in LCFO and YCCO. This is borne out both by magnetic moment and charge population analyses. We discuss the (unimportant) role of Zhang-Rice physics in CCTO and LCFO, suggesting that they are end points of a potential Zhang-Rice doping series, and proposing routes to produce it. !

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The interest of piezoelectric MEMS and NEMS can be drastically improved if they are compatible with silicon based technology, which means developing deposition processes on silicon. High quality crystalline films are required to enhance the piezoelectric coefficients. We report on the deposition of two different piezoelectric oxide films on YSZ based buffered Si (001) substrates: lead zirconate titanate (PbZr0.54Ti0.46O3, commonly referred to as PZT) and barium titanate (BaTiO3, or BTO).

We developed an all oxide Pulsed Laser Deposition procedure using SrRuO3 (SRO) oxide layers as epitaxial conducting electrodes. This oxide metallic layer has a good lattice and chemistry match with both piezoelectric films and allows avoiding a dielectric constant reduction, serious fatigue degradation and blocking contacts as commonly observed when using noble metal (Pt) electrodes.

We demonstrate a full in plane epitaxy of the YSZ buffer layer with a low RMS rugosity and the SRO shows a high crystallinity and a metallic behaviour.

We will present the effect of oxygen pressure on the selection of the crystallographic orientation of PLD grown SRO, PZT and BTO, which allows maximizing the piezoelectric effect for the targeted application.

Finally polarisation versus electric field characteristics of Metal-Ferroelectric-Metal structures will be presented using an innovative method. This method should open a new way to detect the electric field and is of interest for practical sensor applications.

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PS2.3 Piezoelectric thin films on silicon for sensor application Corentin Jorel1, Héloïse Colder2, Didier Robbes1, Alice Galdi1,3,4, Laurence Méchin1

1 GREYC, ENSICAEN, UCBN, CNRS, UMR 6072, 6 bd du Maréchal Juin, 14050 Caen cedex, France

2 CRISMAT/CNRT-Matériaux /ENSICAEN, UMR6508, UMS 3318, 6 bd du Maréchal Juin, 14050 Caen cedex, France

3 CNR SPIN Superconductors Oxides & Other Innovat M, I-84084 Fisciano, SA, Italy

4 Univ Salerno, I-84084 Fisciano, SA, Italy

Corresponding author: Corentin Jorel, Université de Caen Basse-Normandie, GREYC Ensicaen 6, Boulevard du maréchal juin 14050 Caen Cedex. Tel : +33 2.31.45.27.22 Fax : +33 2.31.45.26.98 [email protected]

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We have recently synthesized BiFe0.5Mn0.5O3, a novel multiferroic compound belonging to the class of bismuth-based double perovskites, in HP/HT conditions. In this system the electric properties are ascribed to the stereochemical effect induced by the 6s2 lone pair of the Bi ion, while magnetism depends on the complex pattern of interactions involving iron and manganese. Single-crystal X-ray diffraction allowed us to determine the crystallographic structure of the system as a B-site disordered perovskite and suggests the presence of antiferroelectricity. The magnetic characterization indicates an unconventional magnetic behavior producing a weak magnetic transition around 420 K followed, below 288 K, by a field-dependent reversal of the magnetization process. We registered the higher negative value of the magnetization - 0.75emu/g, for an applied field of 70 Oe. Larger fields gradually inhibit the magnetization reversal process, which is completely suppressed at 1500 Oe. Despite the actual interpretation of the magnetization reversal process [1], our studies give a different explanation of this complex phenomenon. By performing temperature dependent neutron diffraction experiments, we could determine the spin ordering scheme of the system as a G-type structure, involving the sole presence of AFM interactions. On the other hand, Fe-57 Mossbauer spectroscopy revealed the presence of iron ions with dramatically different behaviour: a little part of them being ordered at RT, due to the presence of iron-rich clusters, and a large population of paramagnetic ones that gradually gets ordered with a continuous mechanism taking place at 288 K, when the long range antiferromagnetism occurs leaded by the manganese ions ordering. The current results suggest that the weak ferromagnetic effects observed can be due to uncompensated Dzyaloshinkii-Moriya (DM) interactions characterized by different D vectors and thermal dependencies as could be for the Fe-O-Fe, Mn-O-Mn and Fe-O-Mn interactions. This mechanism is responsible for the magnetization reversal and allows to justify its strong dependence on the applied magnetic field according also to the low magnitude of the magnetic response. As a consequence, in absence of a sufficiently large field, the magnetization goes down to negative values below 250 K but can be easily reversed up only increasing the external field [2]. Furthermore, by measuring electrical permittivity and resistivity as a function of temperature, we observed indirect effects of these electrical properties with respect to a proper variation in the intensity of the applied magnetic field. Also ZFC magnetic susceptibility showed a notable dependence on a static electric field application suggesting the possible existence of a magnetoelectric coupling between the two ferroic orders. !

[1] P.Mandal et al., Phys. Rev. B 82, 100416(R) (2010) [2] D.Delmonte et al., Phys. Rev. B, submitted (2012)

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PS2.4 Study of complex thermally induced field dependent magnetization reversal in BiFe0.5Mn0.5O3, a potentially RT multiferroic type-I perovskite. !

D. Delmonte1*, F. Mezzadri2, C. Pernechele1, G. Calestani2-3, G. Spina4, M. Lantieri5, M. Solzi1, R. Cabassi2, F. Bolzoni2, A. Migliori6 and E. Gilioli2

1Dipartimento di Fisica, Università di Parma, Parco Area delle Scienze 7/A, 43124 Parma, Italy 2IMEM-CNR, Parco Area delle Scienze 37/A, 43124 Parma, Italy 3Dipartimento di Chimica, GIAP Università di Parma, Parco Area delle Scienze 17/A 43124 Parma, Italy 4Dipartimento di Fisica, Università di Firenze, via Sansone 1, 50019 Sesto Fiorentino (FI), Italy 5Istituto Sistemi Complessi-CNR, via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy 6IMM-CNR, via Gobetti 101, 40129 Bologna, Italy

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PS2.5 Study of the ferroelectric phase transition in GeTe using time-domain THz C. Kadlec*, F. Kadlec, P. Ku1el and J. Petzelt Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Prague 8, Czech Republich.

Corresponding author: [email protected] ! !

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Among various multiferroic materials Bi2FeCrO6 showed the coexistence of ferromagnetism and ferroelectricity at or above room temperature. In this system, the ferroelectricity is due to the 6s2

lone pair on the Bi while the ferromagnetism arises from a ferromagnetic interaction between the Cr3+ and Fe3+ ions, which alternatively occupy the B and B’ sites of the double perovskite structure.. We study the effect of substrate induced strain on the structural and magnetic properties of pulsed laser deposited (Bi0.9La0.1)2FeCrO6 (BLFCO) thin films to probe the effect of epitaxial strain on its structural evolution in relation to magnetic properties. In order to elucidate effect of compressive and tensile strain, 30 nm BLFCO (001) epitaxial films were deposited on various single crystalline substrates, having lattice mismatch with the film in the range of - 4.16% to +7.2%. Magnetic measurements show all the films exhibit a magnetic hysteresis loop at room temperature and the saturation magnetic moments affecting by strain induced by strain. The effects of both the compressive and tensile strains on magnetization are found to be different. It is found that the magnetic properties is more sensitive for compressive strain as compare to tensile strain.

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PS2.6 Effect of substrate induced strain on the structural and magnetic properties of (001) (Bi0.9La0.1)2FeCrO6 thin films !

Amit Khare1, Abhishek Singh2, S. S. Prabhu2 and D. S. Rana1 1Indian Institute of Science Education and Research, Bhopal – 452 023, India 2DCMP&MS, Tata Institute of Fundamental Research, Mumbai – 400 005, India

Corresponding author: Amit Khare ([email protected]), Indian Institute of Science Education and Research, Bhopal – 452 023, India ! !

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In recent years there has been remarkable growth in the synthesis of nanoporous hybrid frameworks, known as metal-organic frameworks (MOFs), largely due to their potential applications in gas storage and separations, catalysis, nonlinear optics, photoluminescence, magnetic and electronic materials. However, there have been very few reports in the literature on ferroic or multiferroic hybrid frameworks. We present Second Harmonic Generation (SHG) on a new emerging class of materials, namely organic-inorganic compounds like [(CH3)2NH2]Mn(HCOO)3. This material already showed interesting dielectric properties, in addition it exhibits paraelectric!antiferroelectric phase transition behavior around 185K [1]. To address these features we applied SHG to single crystals and measured the non-linear tensor components as function of the temperature. As expected for the perovskite cubic symmetry, the SHG signal is zero when the material is at room temperature. However below 200K we find a large SHG signal even in absence of external electrical or magnetic fields. The SHG signal as a function of temperature displays a hysteretic behavior. !

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[1] P. Jain, V. Ramachandran, R.J. Clark, H.D. Zhou, B.H. Toby, N.S. Dalal, H.W. Kroto, A.K. Cheetham, “Multiferroic Behavior Associated with an Order-Disorder Hydrogen Bonding Transition in Metal-Organic Frameworks (MOFs) with the Perovskite ABX(3) Architecture”, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol. 131, Issue: 38, 13625 (2009). !

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PS2.7 Hybrid perovskites investigated by Second Harmonic Generation !

A. Marino, P. Jain1, A. Rubano, A. Stroppa2, M. Masakazu3, M. Fiebig3, D. Paparo CNR-SPIN and Physical Science Department, University of Naples Federico II, via Cinthia, 80126 Naples, Italy 1Los Alamos National Laboratory, Los Alamos, New Mexico 87544, USA 2CNR-SPIN, L’Aquila, Italy 2Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, 8093 Zurich, Switzerland


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PS2.8 Polar-nonpolar oxide interfaces investigated by Surface Second Harmonic Generation Domenico Paparo, Andrea Rubano, Lorenzo Marrucci CNR-SPIN and Dipartimento di Scienze Fisiche, Università di Napoli “Federico II”, Compl. Univ. di Monte S. Angelo, v. Cintia, 80126 Napoli, Italy

The technology for the growth of ultrathin oxide films or heterostructures is nowadays approaching the same level of atomic control as in the case of semiconductors. Yet, in contrast to semiconductors, high electron densities lead to novel and sometimes exotic states at the interface. One of the most relevant examples of this, is the formation of a two-dimensional electron gas (2DEG) at the interface between two textbook band insulators, LaAlO3 (LAO) and SrTiO3 (STO), whose physical origin is still controversially discussed in spite of an intense research effort. In general, there are many open issues regarding the interfacial properties of heterostructures based on transition-metal oxides with a Perovskitic structure.

Up to now, the experimental techniques, suitable for investigating the so-called ‘buried’ interfaces at nanometer scale, are few. One of the most promising technique, specifically sensitive to interfaces, is Optical Surface Second Harmonic Generation (SSHG). The technique is based on the induction of light waves of frequency 2" by incident waves of frequency ". The process couples to the symmetry reduction at the interface, being the generation from the centrosymmetric bulk material suppressed. Therefore it is an ideal tool for investigating the polar asymmetry developed at a polar-nonpolar interfaces.

SSHG is a multifunctional tool. It displays: (i) spectral resolution (Spectroscopic SSHG), (ii) surface lateral resolution (SSHG Imaging), (iii) phase resolution (Phase-Sensitive SSHG) and (iv) time resolution (Pump-Probe SSHG).

Here we present our results, based on the whole set of techniques, upon LAO/STO interfaces, including interfaces where LAO is amorphous, and with different terminations of the STO substrate, and discuss them.

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We showed recently that V-doped La2Ti2O7 is properly multiferroic [1]. Ferromagnetism is driven by the ordering of V into dimerized chains along the a crystal axis, whereas polarization is due to composite, mainly rotational modes (around the same axis) whose net dipoles along the c crystal axis fail to compensate due to the layered structure. In [1] we estimated that the vibrational magnetoelectric response around the equilibrium structure due to the subset of modes inducing ferroelectricity should be marginal. Here instead we directly explore magnetoelectric coupling upon complete polarization reversal between the two equivalent equilibrium states, a reversal which is easily obtained by a DC field. Using ab-initio magnetic-anisotropy calculations, we find that V spins point approximately along the b crystal axis in one polarization state, whereas they reverse to -b in the other state (with a substantial ~1 meV b/-b anisotropy energy). The spins seem to reorient by rotating through the a direction, which is the hard axis at equilibrium. In summary, polarization switching is accompanied by magnetization switching, and therefore the magnetization M can indeed be switched electrically. We note in passing that while LTO has P=(0,0,Pc), upon V doping polarization acquires a component along a, i.e. P=(PV,0,Pc). As the polarization Pc c is reversed (e.g. by an electric field E0c, and M=MVb is reversed with it, as we showed), the component Pv a remains unchanged, i.e. the polarization is not inverted but rather reflected through the a-b plane. We suspect, and are investigating, a relation of this finding with magnetization inversion. [1] M. Scarrozza et al, PRL 109, 217201 (2012); EPJB, to appear (2013) !

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PS2.9 Electrical switching of magnetization in ferromagnetic V-doped La2Ti2O7 M. Scarrozza*, M. B. Maccioni**, and V. Fiorentini** *CNR-Spin, L'Aquila, Italy **CNR-IOM e Dipartimento di Fisica, Università di Cagliari, Italy

Corresponding author: M. B. Maccioni, Dip. Fisica, Uni Cagliari, Cittadella Universitaria, 09042 Monserrato (CA), Italy

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Multiferroic junctions combining metallic ferromagnets and insulating ferroelectrics are a

currently fashionable avenue to obtain electroresistance and magnetoresistance modulation of

tunneling as well as in-plane transport exploiting polarization switching in the ferroelectric

interlayer.

Using first-principles density functional calculations, we investigated a thick asymmetric

SrRuO3/PbTiO3/SrRuO3 multiferroic junction with both PbO/SrO2 and PbO/TiO2 interfaces.

We find a substantial residual depolarization field in the ferroelectric, despite the effective

screening by SrRuO3, wich is both electronic and ionic in nature. In the two states of

polarization of the ferroelectric, the local potential seen by tunneling electrons is different,

leading to resistivity asymmetry under polarization switching. The large ferroelectric

monopoles at the interfaces carry a significant spin polarization, again different in the two

polarization states.

Thus this junction enables a number of control modes. First, polarization switching by e.g. a

large "write" field produces tunneling resistance asymmetry and hence electroresistance.

Second, the two polarization states have different interface spin density, therefore we naturally

get magnetoresistance asymmetry. To assess quantitatively the degree of asymmetry, in the case

of (ballistic) tunneling we simply compare 1D transmittivities through our calculated 1D

averaged local potential; in the case of in-plane (diffusive) trasport we calculate spin-selective

Bloch-Boltzmann resistivity as function of polarization direction. The effect should be

measurable both in tunneling and field-effect-like devices.

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PS2.10 Polarization-driven magneto- and electroresistance in multiferroic junctions Francesco Ricci, Alessio Filippetti, and Vincenzo Fiorentini CNR-IOM e Dipartimento di Fisica, Università di Cagliari, Italy

Corresponding author: V.Fiorentini, Dip. Fisica, Uni Cagliari, Cittadella Universitaria, 09042 Monserrato (CA), Italy, [email protected]!!

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PS2.11 Electric field gradient and magnetic hyperfine field calculations and measurements in multifunctional materials J. N. Gonçalves1*, V. S. Amaral1, J. G. Correia2, A. S. Fenta1, A. Baghizhadeh1, A. Stroppa3, S. Picozzi3 1. Departamento de Física and CICECO, Universidade de Aveiro, 3810 Aveiro Portugal 2. IST/ITN, Instituto Superior Técnico, Universidade Técnica de Lisboa, 2686-953 Sacavém, Portugal 3. CNR-SPIN, 67100 L'Aquila, Italy

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In multifunctional materials, for example magnetoelectric multiferroics, where both electric and magnetic spontaneous orders coexist, the potential for technological devices, and the complex physics, with competition and interplay of different phases, is motivating the use of techniques which can give information of the materials at nano and atomic scales. The simultaneous measurement and calculations of local quantities specific to each atom, and correlation between them in different phases of the materials, can identify where changes are most prevalent in a given phase transition, related to the different magnetic or electric orders. We present an overview showing examples of how the hyperfine quantities probe multifunctional materials at an atomic scale. The scale of the electric field gradient (EFG) is illustrated by using the perturbed angular correlations (PAC) spectroscopy measurements using radioactive isotopes at ISOLDE-CERN and accurate density functional theory (DFT) calculations. The study of layered materials such as hexagonal alkaline and rare-earth manganites illustrates how the combination of measurements and calculations can specify a few crystallographic sites where the measurement is done, clearly distinguishing between environments of different symmetry even when the same chemical element is present. Calculations of composites intercalating ferroelectric phases and layers of magnetic elements also illustrate the locality of the measurements. On the other hand, other calculations show how a relationship can be established between local EFGs and the spontaneous polarization in ferroelectrics [1]. The magnetic hyperfine field [2] can be related to the global magnetic moment in the material and the competition of coexistent phases, in addition to also being a microscopic site specific quantity. Changes of electron charge and spin density around each atom leading to magnetoelectric effects which can be conveniently quantified using hyperfine properties are also illustrated.

We acknowledge support of FCT and FEDER-COMPETE through project CERN/FP/123585/2011 and European Union 7thFP through ENSAR, contract no. 262010. References: [1] J. N. Gonçalves et al. Phys. Rev. B 86, 035145 (2012); [2] J. N. Gonçalves, et al. Phys. Rev. B 83, 104421 (2011)

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In the search for multiferroicity, we have explored via density functional calculations the magnetic doping of the layered perovskite La2Ti2O7 (LTO). This titanate is a high temperature ferroelectric (Tc=1770K), with a sizable polarization along the c axis (Pc=5 $C/cm2), and with an unconventional mechanism of ferrelectricity, as it is produced by antiferrodistortive (AFD) oxygen-octahedra rotations rather than by B-cation displacements. We first performed an extensive study of the effects of magnetic doping by 3d cations at B-site as Ti substitutionals (energetics, magnetic and electronic properties), at varying doping concentrantion and configurational sites. We then focused on two cases, V and Fe-doped LTO, for their peculiar and appealing properties: a strong structural and magnetic anisotropy, with a marked tendency to cluster into chain, hence forming magnetic chains (ferromagnetic (FM) and antiferromagnetic (AF) for V [1] and Fe, respectively) along the a axis, embedded in the ferroelectric host. Moreover, the doping maintains the insulating nature and hence the ferroelectricity of the host, forming a small gap (0.2 and 0.5 eV for LTO:V and Fe, respectively). V-chains may form at the inner perovskite or external layers, in proximity of the extra-O layer (similar energies), with a pretty robust FM order (estimated intrachain couplings of Ja~ 80 and 20 meV along the a direction, for the external and internal chains). At lower doping, V clusters along the chain, forming dimers or chain fragments, and maintaining the insulating nature and the FM coupling. Regarding the magnetic anisotropy properties, from noncollinear calculations we found an easy-plane (bc) and a as hard-axis. We attempted to estimate the magnetoelectricity (ME) term mediated by lattice using Iniguez formalism [2], i.e. by evaluating the polarization (P) and magnetization (M) responses to the AFD-rotations (we considered the multimode displacement connecting the ferro (FE) to the paraelectric (PE) structure). Unfortunately, the lattice mediated P-M coupling seems to be negligible in this case, and this can be explained by the fact that the FE-to-PE mode rotates octahedra perperdicular to a-axis, within the easy bc magnetic plane, hence spins ''freewheel'' around a, with a negligible magnetic response to the displacement. Differently, Fe-chains form on the external layer, robustly AF coupled (Ja~ 70 meV), with a easy-axis, and a weak spin canting along c axis (0.11 $B/Fe pair). Interestingly, the c component of the magnetization is here sensitive to the AFD modes, such as the Pc polarization component, hence showing magnetoelectricity mediated by O-octahedra rotations. [1] M. Scarrozza et al, PRL 109, 217201 (2012); EPJB, to appear (2013); to be published. [2] J.Iniguez, PRL 101, 117201 (2008).

PS2.12 Magnetic chains embedded in a layered ferroelectric titanate

M. Scarrozza*, A. Filippetti, and V. Fiorentini CNR-IOM e Dipartimento di Fisica, Università di Cagliari, Italy (*now at CNR SPIN, UOS L'Aquila, Italy) !

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Corresponding author: Marco Scarrozza, CNR SPIN, UOS L'Aquila, via Vetoio 10, 67100 Coppito, L’Aquila, Italy - email: [email protected]

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PS2.13 Domain, strain and interface engineering in BiFeO3 compound Vilas Shelke*, Shreeja Pillai, Rama Yadav, Deepika Bhuwal Novel Materials Research Laboratory, Department of Physics, Barkatullah University, Bhopal 462 026 India

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Corresponding author: [email protected]; Ph. No. 0091-9407547879

Recently, ‘multiferroics’ emerged as a new class of materials, which incipiently exhibits ferroelectric and magnetic ordering. Bismuth ferrite is a classic multiferroic material with additional functionalities like photovoltaic, photostriction, modulation of conduction current, metal-insulator transition, terahertz radiation, etc. A lead free composition with room temperature functioning make it an ideal choice for variety of ferroelectric, spintronics and energy applications. We have studied several aspects of multiferrocity in BiFeO3 compound. Significantly high ferroelectric polarization values are routinely obtained in BFO thin films. We deposited several thin film samples using pulsed laser deposition technique. We exploited domain engineering approach using vicinal substrates to achieve the lowest ever reported coercive field. This finding is very important to design effectively switchable low voltage devices. We used strain engineering to stabilize a nearly-tetragonal phase and also demonstrated reversible ferroelastic switching between rhombohedral and nearly-tetragonal phases of BFO thorough a tip-induced bias field. We investigated composites of two antiferromagnetic perovskite oxides BiFeO3 and LaMnO3 to study change in bulk magnetic behavior. Composites with nominal compositions (1-x)BiFeO3-xLaMnO3 (x = 0, 5, 10, 20 wt. percent) were synthesized by solid state reaction route. The structural analysis performed using X-Ray diffraction and Raman spectroscopy indicated presence of phase separated compounds. Significant enhancement in magnetic moment is observed in composite samples, which was attributed to the uncompensated spins at the interface of two distinct antiferromagnetic phases.

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PS2.14 Investigation of ZnO as a Tunnel Barrier using First Principle Calculation Gokaran Shukla, Stefano Sanvito and Thomas Archer.

School of Physics, Trinity College Dublin, Ireland.

One of the key stumbling points of spintronics is the ability to inject a spin from one material to another. With the recent research in ZnO and as a possible DMS, the quality of ZnO films have improved significantly, allowing the growth of insulating layers of ZnO capable of coherent tunneling. In this work we investigate ZnO as possible martial for spin-injection, using density functional theory and the non-equilibrium Green’s function transport formalism with the LDA+ASIC(atomic self interaction corrected) exchange and correlation functional. We show that the transport is governed by the 2-point and that symmetry filtering removes all incident states bar states with 31 symmetry. Making ZnO ideal for spin injection in the next generation of spintronics devices.

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Flexoelectricity is the linear polarization response of an insulator to a strain gradient. Unlike piezoelectricity, flexoelectricity is symmetry-allowed in any insulator. Although hardly detectable in macroscopic samples, this effect has attracted considerable interest from the experimental community in recent years, as nanoscale geometries (where huge strain gradients can occur) hold concrete promise for practical device applications. From the theoretical point of view, however, a comprehensive theory is still lacking, despite some notable recent achievements. In this presentation I will show how to overcome some of the remaining formal difficulties, e.g. concerning the relationship between dynamical and static coefficients, and the electronic versus lattice-mediated effects. The proposed formalism is based on a rigorous long-wavelength expansion of well-defined response functions, which can be readily calculated from first principles in the framework of density-functional perturbation theory. Applicability to both bulk materials and low-dimensional nanostructures (e.g. surfaces) will be discussed.

PS2.15 Linear-response theory of flexoelectricity

Massimiliano Stengel

ICREA - Instituci´o Catalana de Recerca i Estudis Avan¸cats, 08010 Barcelona, Spain Institut de Ci`encia de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain

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PS2.16 Hybrid improper ferroelectricity in a Multiferroic and Magnetoelectric Metal-Organic Framework A. Stroppa1, P. Barone1, P.Jain2 J.M. Perez-Mato3, S. Picozzi1 1CNR-SPIN. L'Aquila, Italy 2 Los Alamos National Lab, 30 Bikini Atoll Rd Los Alamos, NM 87545-0001 (505) 664-5265 3Departamento de Fisica de la Materia CondensadaFacultad de Ciencia y Tecnologia, UPV/EHU, Bilbao (Spain)

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Corresponding author: Alessandro Stroppa, CNR-SPIN, L'AQUILA, [email protected]

Metal-organic frameworks (MOFs) show increasing promise as materials for applications in catalysis, gas storage and molecular recognition, and are also interesting from fundamental condensed-matter physics point of view. Of particular interest are MOFs with the perovskite architecture, which show hydrogen bonding-related multiferroic phenomena, exhibiting both magnetic and ferroelectric ordering. By using state-of-the-art- ab-initio calculations, we show that in [C(NH 2)3]Cr(HCOO)3 MOF, interaction between the cooperative antiferro-distortive Jahn-Teller distortions and the C(NH 2)3 cations breaks the inversion symmetry through hydrogen-bonding and induces a ferroelectric polarization. Interestingly, the polar behavior arises due to a trilinear coupling between two unstable modes, namely a Jahn-Teller and a rotational mode, and one stable polar mode. Therefore, this compound represents the first example of hybrid improper ferroelectric in the family of metal-organic compounds. Since rotational modes in perovskite-inorganic compounds usually freeze-in at elevated temperatures (Tc >300°K), the trilinear coupling in MOF compounds may provide an interesting route towards room temperature multiferroic. Last but not least, we show that switching of polarization direction implies the reversal of the weak ferromagnetic component, which is predicted to be as large as 1 µB/formula unit. Based on a perturbative approach of the spin-orbit coupling, an explanation of weak-ferromagnetism and its coupling to ferroelectricity is proposed, underlying the central role of Jahn-Teller distortions. These results therefore support the idea that MOFs show promising new routes for achieving multiferroic properties and other unprecedented possibilities otherwise not achieved in pure inorganic materials. References

A. Stroppa et al, Adv. Mat. (2013), in press. !

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PS2.17 Role of the dielectric / electrode interface on the electrical properties of (Ba,Sr)(Ti,Zr)O3 based thin film capacitor J. Wolfman, A. Ruyter, B. Negulescu, J. Sakai, C. Autret, F. Gervais

Laboratoire GREMA N, UMR CNRS 7347, Université François Rabelais, Parc de Grandmont, 37200 Tours, France

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(BaxSr1-x)TiO3 (BST) based materials are potential candidates for microwave applications relying on their voltage tunable high dielectric permittivity. However high frequency applications require BST thin films to be sandwiched between highly conductive metal electrodes and integrated onto silicon. This leads to a polycrystalline structure of the dielectric films correlated to lessen performances compared to their bulk counterparts. Understanding the structure-properties relationship and its modulation by chemical doping and / or ordering is a possible route to optimized thin films dielectric performances. Another important factor impacting capacitances performances in term of leakage current and tunability symmetry versus polarity is the matching between dielectric and electrodes materials. In this work we grew Ba1-xSrxTi1-yZryO3 (BSTZ) composition spread thin films and multi-layers libraries by combinatorial pulsed laser deposition. Ruthenate, nickelate and manganites electrodes promoting BSTZ epitaxial growth with various strain state on SrTiO3 were investigated. Combinatorial variation of the electrode composition close to the electrode / dielectric interface was used to modulate the electrode work function and the leakage current. A comparison with polycrystalline BSTZ films grown on silicon with Pt or IrO2 electrodes is made. Thin films structures and compositions were characterized by X-rays diffraction, TEM, EDX, and near field microscopy, while electrical properties of a few thousands parallel plates capacitors were statistically studied. X-rays diffraction and EDX showed an effective control of the phase and stoichiometry across the libraries. Continuous variations of dielectric permittivity (!), tunability (T), losses (tan ") and dc leakage current were mapped. Strong influences of the dielectric microstructure and / or of the nature of the electrode on electrical properties were observed. Epitaxial BSTZ films demonstrated non-monotonic variations of electrical properties with composition. On the contrary polycristalline samples showed more monotonic behaviours, mimicking bulk material properties’ dependence on composition. A 6 orders of magnitude variation in leakage current was evidenced in epitaxial Ba1-xSrxTiO3 going from La0.7Sr0.3Mn03 to LaSrNiO4 electrodes. We also showed that symmetrical tunability versus voltage polarity could be obtained adjusting Zr content in Ba0.6Sr0.4Ti1-yZryO3, i.e. that C(V) curves offset does not solely depends on electrodes work function difference.

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PS2.18 Electromagnons and phonons in multiferroic CaMn7O12 !

S. Kamba, V. Goian, F. Kadlec, and P. Van!k Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic!

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Corresponding author: Stanislav Kamba, Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic, [email protected]!

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CaMn7O12 is an unusual material which exhibits a rich sequence of structural, magnetic and ferroelectric phase transitions. At high temperatures, (CaMn3)Mn4O12 crystallizes in the cubic perovskite structure (space group Im3) and upon cooling below Tc1 ~ 450 K it undergoes a phase transition to a trigonal structure with the space group R3 (Z=3). Below Tc2 =250 K, orbital ordering occurs and the trigonal1structure becomes incommensurately modulated with a propagation vector qc=(0,0,2.077). Upon further cooling, below TN1=90 K, an incommensurately modulated magnetic structure emerges with a magnetic modulation vector qm = 1⁄2qc.1 Very recently, it was reported2 that the magnetic structure is also ferroelectric with a spontaneous polarization attaining up to Ps=2870 !Cm-2 oriented parallel to the c axis. This is the highest spin-induced polarization among all multiferroics. At TN2=50 K, another magnetic transition occurs, giving rise to a second propagation vector along the c axis. Both propagation vectors qm1 and qm2 change with temperature, but their average value corresponds exactly to 1⁄2qc. We have investigated far-IR reflectivity and time-domain THz transmission spectra of CaMn7O12 between 10 and 550 K and found significant changes in the spectra at structural and magnetic phase transitions. The IR reflectivity changes abruptly at Tc1 = 450 K, giving evidence of the first-order character of the phase transition. Moreover, the conductivity seen in the high-temperature IR spectra disappears below Tc1, proving the metal-insulator character of this phase transition due to charge ordering. New polar phonons appear in the spectra in the incommensurate phase below 250 K due to a change of IR selection rules. Close to TN1, a new mode develops near 50 cm-1; another one near 30 cm-1 appears below TN2. Since the modes appear near the magnetic phase transitions, they can be spin waves (magnons). Nevertheless, IR reflectivity spectra reveal a transfer of the oscillator strengths from the phonons to the excitations seen below 50 cm-1. Thus, the modes at 30 and 50 cm-1

contribute to the dielectric permittivity and not to the magnetic susceptibility. This is a typical signature of electromagnons, i.e. electric active magnons, which are activated in THz dielectric spectra due to dynamical magnetoelectric coupling. The origin of magnetoelectric coupling in CaMn7O12 will be analyzed and discussed. 1. N.J. Perks, R.D. Johnson, C. Martin, L.C. Chapon, and P.G. Radaeli, Nature Commun.

3, 1277 (2012). 2. R.D. Johnson, L.C. Chapon, D.D. Khalyavin, P. Manuel, P.G. Radaeli, and C. Martin,

Phys. Rev. Lett. 108, 067201 (2012).

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PS3.1 Stepwise behaviour of magnetization temperature dependence in iron nanoparticle assembled films V Iannotti ,5, S Amoruso1, G Ausanio1, R Bruzzese1, L Lanotte1, A C Barone2, G Margaris3, K N Trohidou3 and D Fiorani4 1 CNR-SPIN and Department of Physical Sciences, University of Naples “Federico II”, Piazzale V. Tecchio 80, I-80125 Napoli, Italy 2 CNR-ISTEC, Via Granarolo 64, I-48018 Faenza (RA), Italy 3 Institute of Materials Science, NCSR "Demokritos", Aghia Paraskevi, 15310 Athens, Greece 4 ISM - CNR, Area della Ricerca1, Via Salaria km 29.300, C.P. 10, 00016 Monterotondo Scalo (RM), Italy

E-mail: [email protected]

An unusual stepwise behavior is reported in the temperature dependence of the Zero Field Cooled magnetization in iron nanoparticle dense films produced by ultra-short pulsed laser deposition assisted by irradiation of nanoparticles with a nanosecond UV laser pulse, appropriately delayed, during their flight from the target to the substrate. This behaviour, induced by the particle system’s morphology, characterized by clusters of tightly coupled nanoparticles as well as by some voids between them, is ascribed to the competition between Zeeman energy density, intracluster anisotropy energy density and intercluster exchange energy density. A phenomenological model and Monte Carlo simulations are reported which support the proposed interpretation.

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PS3.2 Magnetic and structural properties of Sr4Ru3O10 Veronica Granata1,2, Lucia Capogna3,4, Manfred Reehuis5, Rosalba Fittipaldi1,2, Bachir Ouladdiaf4, Sandro Pace1,2, Mario Cuoco1,2and Antonio Vecchione1,2 1CNR-SPIN, I-84084 Fisciano, Italy 2Dipartimento di Fisica ‘E R Caianiello’, Università degli Studi di Salerno, Via Ponte don Melillo I-84084 Fisciano, Italy 3CNR-IOM OGG, 6 rue J Horowitz, F-38042 Grenoble, France 4Institut Laue-Langevin, 6 rue J Horowitz, F-38042 Grenoble, France 5Helmholtz-Zentrum Berlin f¨ur Materialen und Energie, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany

The magnetic properties of the triple-layered Sr4Ru3O10 have been investigated by means of neutron scattering diffraction. At zero field we find that the magnetic moments are ferromagnetically coupled and oriented along the c-axis with no signatures of either long-range antiferromagnetic order or ferromagnetic components in the ab-plane. The field dependence of the reflection intensity points to a metamagnetic response involving only the planar magnetic moments. The structural refinement indicates a distinct rearrangement of the unit cell as a function of both temperature and in-plane applied field. We show that at the temperature of about 50 K, below which the metamagnetic behavior is observed, the c-axis lattice parameter exhibits a rapid increase while the in-plane amplitude saturates. A similar upturn of the in-plane lattice parameter after the quench of the c-axis amplitude occurs above a critical magnetic field. !

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PS3.3 Charge and magnetic excitations in hole- and electron-doped infinite layer cuprate superconductors G. Dellea1, M. Minola1, C. Mazzoli1, L. Braicovich1, G. Ghiringhelli1, A. Galdi2, P. Orgiani3, L. Maritato2, D. Di Castro4, A. Tebano4, G. Balestrino4, C. Aruta5, M. Moretti Sala6, N. Brookes6, C. Jia7, T. Devereaux7, D.G. Schlom8 1CNR-SPIN, CNISM and Dipartimento di Fisica, Politecnico di Milano, Italy 2CNR-SPIN and DIEII-Dipartimento di Ingegneria Elettronica ed Ingegneria Informatica Università degli Studi di Salerno, Italy 3CNR-SPIN and Università degli Studi di Salerno, Italy 4CNR-SPIN and Università di Roma Tor Vergata, Italy 5CNR-SPIN and Università di Napoli “Federico II”, Italy 6European Synchrotron Radiation Facility, Grenoble, France 7SLAC National Accelerator Laboratory and Stanford University, USA 8Material Science and Engineering Department, Cornell University, USA

Corresponding author: Greta Dellea, Ph.D. student Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy Phone (Milano): (+39) 02 2399 6189, e-mail: [email protected]

Infinite layers (IL) present the simplest crystallographic structure among layered cuprates (RCO2, where R is an alkaline-earth-metal cation), containing the CuO2 planes considered fundamental for achieving high-Tc superconductivity (HTS). The doping charge needed for the superconducting transition, can be provided, in bulk, by a change of stoichiometry1 or, for films, by growing superlattices (SLs), where interfaces play the role of charge reservoir2-4. By these methods it is possible to achieve superconductivity (both n- and p-type) in some IL compounds. High resolution resonant inelastic x-ray scattering (RIXS) at Cu-L3 edge has recently been proved, both experimentally5 and theoretically6-10, to be a powerful tool for measuring ligand field (dd) and magnetic excitation in cuprates. In particular, magnon dispersions have been measured in undoped insulating cuprates6-8, while superconducting samples present damped spin excitations (paramagnons)9-10. These paramagnons have dispersions and spectral weights comparable to those of insulating compounds and represent a strong candidate as glue for Cooper’s pairs in HTS. Here we present Cu-L3 RIXS measurements on insulating and superconducting IL systems. In particular, we compare spectra measured for the two possible doping mechanisms, n- and p-type. (CaCuO2)m/(SrTiO3)n superlattices are characterized by hole doping, while Sr1-xLaxCuO2 grown in O3 atmosphere and on a GdScO3 substrate presents electron doping. Beside several analogies, the two systems show noticeable differences in the evolution, with the doping level, of the charge excitation continuum and of the magnetic peak intensity and damping. These trends can be found also in the corresponding theoretical calculations, in which the RIXS cross-section is determined using a single band Hubbard model plus 2p core level with spin-orbit coupling. 1. S. Karimoto, K. Ueda, M. Naito, T. Imai, Physica C 378–381, 127–130 (2002) 2. A. Gozar, G. Logvenov, L. Fitting Kourkoutis, A. T. Bollinger, L. A. Giannuzzi, D. A. Muller and I. Bozovic, Nature 455, 782 (2008) 3. G. Balestrino, S. Martellucci, P. G. Medaglia, A. Paoletti, G. Petrocelli, and A. A. Varlamov, Phys. Rev. B 58, R8925 (1998) 4. D. Di Castro, M. Salvato, A. Tebano, D. Innocenti, P. G. Medaglia, M. Cirillo and G. Balestrino, arXiv:1107.2239v2 (2012) 5. L. J. P. Ament, G. Ghiringhelli, M. Moretti Sala, L. Braicovich, and J. van den Brink, Phys. Rev. Lett., 103, 117003 (2009) 6. L. Braicovich et al., Phys. Rev. Lett. 102, 167401 (2009) 7. Guarise et al., Phys.Rev. Lett. 105, 157006 (2010) 8. L. Braicovich et al., Phys. Rev. B 81, 174533 (2010) 9. L. Braicovich et al., Phys. Rev. Lett., 104, 077002 (2010) 10. M. Le Tacon et al., Nature Physics 7, 725 (2011)

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PS3.4 Mechanical detection of phase transition in VO2 thin films N. Manca1,2, L. Pellegrino2, T. Kanki3, S. Yamasaki3, H. Tanaka3, R. Buzio2, A. S. Siri1,2, D. Marre1,2 1 Dipartimento di Fisica, Università di Genova, Genova (Italy) 2 CNR-SPIN, Genova (Italy ) 3ISIR, Osaka University, Osaka (Japan)


Correlated electron systems like Transition Metal Oxides (TMO) exhibit a wide range of physical phenomena thanks to the interplay between spin, charge and lattice degrees of freedom. TMO have broad lattice compatibility and can be grown in complex heterostructures having high structural quality. TMO heterostructures show also enhanced or even new properties that arise from the mechanical interactions between the single layers or by electrical or magnetic couplings. Phase transitions in TMO often determine multiple changes of their physical properties, including mechanical ones. Within correlated electron materials, VO2 attracted lot of attentions due to its still debated Metal-Insulator transition above room temperature. It shows more than three-orders-of-magnitude hysteretic change of electrical resistance nearby 68 °C associated with a change in its crystal structure from monoclinic to rutile-type. In this work, we present a study on the detection of mechanical resonances of microcantilevers based on crystalline VO2/TiO2 heterostructures grown on MgO(001) substrates. Device fabrication combines Pulsed Laser Deposition (PLD) and micrometric optical lithography. The mechanical resonance frequency of these microcantilevers changes during phase transitions due to the different Young’s modulus of the metallic and insulating phases of VO2. We will report the fabrication process of VO2/TiO2 microcantilevers and their electrical properties [1]. Details of the experimental setup for mechanical detection, based on optical lever method will be given. Our system is able to measure shifts of the resonance frequency of about 1 Hz in different atmospheres (vacuum, gas) and from -10°C to 150°C. This frequency shift method is based on the same principles of Frequency Modulation Atomic Force Microscopy and allows very precise detection of external forces and bulk properties. We will discuss measurements on the detection of mechanical resonance frequencies in mixed metallic/insulating states of VO2. Interplay between built-in strain in free-standing structures, metallic domain distribution and the local temperature on the cantilever will be discussed in the framework of VO2 material properties and finite element analysis. [1] L. Pellegrino, N. Manca, T. Kanki, H. Tanaka, M. Biasotti, E. Bellingeri, A. S. Siri, D. Marré, Adv. Mater. 24, 2929–2934, 2012.

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PS3. 5 Influence of size and shape of antiferromagnetic Cr2O3 nanoparticles in the magnetic behavior of Fe3O4-Cr2O3 nanocomposites Ruth Otero-Lorenzo, Verónica Salgueiriño Dpto. Física Aplicada, Universidade de Vigo, 36310 Vigo, Spain

Corresponding author: Verónica Salgueiriño Departamento de Física Aplicada, Universidade de Vigo, 36310 Vigo (Spain). E-mail: [email protected]

Nowadays the combination of materials with different physical properties at the nanoscale is being hardly investigated, especially to study surface effects and to combine the mentioned properties to create multifunctional hybrids. Accordingly, we propose to combine antiferromagnetic Cr2O3 and ferrimagnetic Fe3O4 nanoparticles, in order to induce interface phenomena such as the widely known exchange bias effect, on which there are still main open questions. With the aim of studying this effect, Fe3O4-Cr2O3 nanocomposites were synthesized via different methods looking for several combinations of the two materials, changing the interface in order to prove any influence in the final magnetic behavior. These nanocomposites were synthesized by a well-known solvothermal method using an autoclave system on which we have observed subtle loop shifts and increase in coercivity. Additionally and consequently, new routes of synthesis to obtain narrower size distributions and new shapes were proposed, permitting the production of diverse Cr2O3

nanoparticles to induce a different interface.

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PS3.6 Electrical spin injection in all-oxides crystalline heterostructures. F.Telesioa,b, E. Espositoc, I. Pallecchib, L. Pellegrinob, D. Marréa,b a Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy b CNR-SPIN, Corso Perrone 24, 16152, Genova, Italy c Istituto di Cibernetica E. Caianiello del CNR, Via Campi Flegrei 34, I-80078 Pozzuoli (NA), Italy

The search for a suitable semiconducting non magnetic material for spin injection devices is so far still unaddressed. The crucial role of interfaces in spin depolarization has been partially understood only recently, in the case of electrical spin injection from ferromagnetic electrodes. Indeed, such issues as conductivity mismatch, electron wave function symmetry, band alignment, presence of disorder and traps may severely suppress the spin polarization of injected charge carriers. In this context, the realization of fully crystalline devices with epitaxial ferromagnetic electrodes and active channels may yield high quality interfaces and thus enhanced spintronic performances. In this work we present pioneering experiments of spin injection in fully crystalline oxide heterostrutures, either in vertical or planar spin valve geometries. The ferromagnetic electrodes are made of perovskite manganites, while for non-equilibrium spin transport we test semiconducting Cu2O and SrTiO3. These prototype devices have the two-fold aim of measuring the spin diffusion length in Cu2O and SrTiO3, as well as of testing the actual spintronic device feasibility.

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PS4.1 Resistance fluctuation processes in LaAlO3/SrTiO3 interfaces C. Barone, F. Romeo, S. Pagano, E. Di Gennaro*, F. Miletto Granozio*, U. Scotti di Uccio* Dipartimento di Fisica "E.R. Caianiello" and CNR-SPIN Salerno, Università di Salerno, I-84084 Fisciano (SA), Italy *!CNR-SPIN Napoli and Dipartimento di Scienze Fisiche, Complesso Universitario di Monte Sant’Angelo, I-80125 Napoli, Italy

Corresponding author: Carlo Barone

Dipartimento di Fisica “E. R. Caianiello” – Università degli Studi di Salerno Via Ponte don Melillo – 84084 Fisciano (SA) ITALY Tel: +39089968212 - Fax: +39089969658 - e-mail: [email protected]

Complex oxides show a broad spectrum of intrinsic functionalities, which can be used and combined in electronic devices that are based on epitaxially grown heterostructures. One example is formed at the interface between the two insulating, dielectric perovskites, LaAlO3 and SrTiO3. The high electron mobility, the complex ionic structure, and particular interactions found at such an interface are expected to promote novel electronic phases that are not always stable as bulk phases. The discovery of this electron gas at the interface between two insulators has generated an impressive amount of experimental and theoretical work. However, very few electric noise investigations are found for these type of heterostructures, as well as for the twin interfaces based on III-V semiconductors. In this respect, we report on new voltage-spectral density measurements performed on these compounds. In particular, the spectral analysis at low frequencies reveals the presence of a clear 1/f noise contribution, whose quadratic bias current dependence indicates the activation of standard resistance noise fluctuations. The existence of a temperature crossover in the resistivity behavior at about 100 K, probably due to specific properties of the crystal structure of the involved wide-bandgap insulators, is confirmed by sensitive voltage-noise investigations. The meaning of these experimental findings has to be fully understood and clarified. Therefore, an explanation in terms of existing theoretical models concerning the transport mechanisms in quasi-two-dimensional electron gases is tentatively proposed and discussed.

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PS4.2a Resistive switching and nanoscale electronic transport of Au/Nb-doped SrTiO3 Schottky junctions R. Buzio1, A. Gerbi1, E. Bellingeri1, A.S. Siri1,2, D. Marré1,2 1CNR-SPIN Institute for Superconductivity, Innovative Materials and Devices, C.so Perrone 24, 16152 Genova, Italy 2Physics Department, University of Genova, Via Dodecaneso 33, 16146 Genova, Italy

Corresponding author: Renato Buzio, CNR-SPIN Institute for Superconductivity, Innovative Materials and Devices, C.so Perrone 24, 16152 Genova Italy, Tel: +39 010 6598 731, E-mail: [email protected]

Whenever a Schottky barrier is formed at the junction between large work function metals and electron-doped SrTiO3 (STO) samples, the macroscopic rectifying transport is accompanied by a resistance switching (RS) behavior. This is likely due to local, field-induced accumulation or depletion of oxygen vacancies at the vicinity of the metal/STO interface, that in turn might lead to redox processes and trapping effects responsible for the appearance of resistance switching [1]. Metal/Nb-doped STO junctions represent a model system for the elucidation of the physical mechanisms driving RS. Several investigations recently addressed the RS dependence on few intrinsic and extrinsic physical parameters. There is no doubt that a deeper insight was gained when the macroscale studies were complemented by the use of local probes, addressing junctions response at the nanoscale [2]. Here we describe the fabrication and electrical characterization of Au/Nb:STO single-crystal junctions with nanometer thick metal electrodes. We observe an unexpected phenomenon, namely the coexistence within the same device of highly rectifying properties - under laboratory air - and bipolar RS - under reducing vacuum conditions. We explore this phenomenology by systematically studying the junction response under different oxygen and inert gas atmospheres [3]. Since nanometer-scale alterations of the Schottky barrier represent one of the microscopic mechanisms proposed to explain RS, we report on novel Scanning Tunnelling Microscopy - Ballistic Electron Emission Microscopy (STM-BEEM) experiments aimed to directly visualize and quantify the local inhomogeneities of the effective Schottky barrier height. This approach represents an original strategy to attempt a correlation with the macroscopic response of the studied system [4]. [1] R. Waser et al. Adv. Mater. 21, 2632 (2009). [2] K. Szot et al. Nat. Materials 5, 312 (2006). [3] R. Buzio, A. Gerbi, A. Gadaleta et al., Appl. Phys. Lett. 101, 243505 (2012). [4] A. Gerbi, R. Buzio, A. Gadaleta et al. “Hot electron transport in Au/Nb:SrTiO3 structure studied by ballistic electron emission spectroscopy” in preparation

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PS4.2b Transport properties of organic-on-inorganic Schottky diodes by current-voltage characteristics and ballistic-electron-emission-microscopy R. Buzio1, A. Gerbi1, D. Marré1,2, F.V. Di Girolamo3, A. Cassinese3 1CNR-SPIN Institute for Superconductivity, Innovative Materials and Devices, C.so Perrone 24, 16152 Genova, Italy 2Physics Department, University of Genova, Via Dodecaneso 33, 16146 Genova, Italy 3CNR-SPIN and Physics Department, University of Naples Federico II, Piazzale Tecchio 80125 Naples, Italy

Corresponding author: Renato Buzio, CNR-SPIN Institute for Superconductivity, Innovative Materials and Devices, C.so Perrone 24, 16152 Genova Italy, Tel: +39 010 6598 731, E-mail: [email protected]

Metal-semiconductor contacts are still actively investigated because of their present and potential applications in technology. The electrical properties of such junctions depend on the interface states between the metal and the semiconductor and also on the material parameters. The control of these properties can be obtained in different ways, including the use of a suitable organic interlayer [1,2]. The performance and reliability of organic-based Schottky barrier diodes especially depends on the formation of the organic interfacial layer at the metal-semiconductor interface, interface states density at organic layer/Si interface, series resistance and Schottky barrier spatial inhomogeneity. Experiments complementing macroscopic methods with local probes, addressing junctions response at the nanoscale, represent an invaluable strategy to address the above issues. Perylene and its derivatives are undoubtedly one of the most interesting classes of n-type semiconductors, since they provide high carrier mobility and have been used in many applications such as photovoltaic solar cells, laser dyes, organic light emitting diodes. We have fabricated Au/perylene diimide PDI-8CN2/n-Si organic Schottky diodes by thermal evaporation [3] of a few nanometers thick PDI layer on n-Si semiconductor wafers. The current-voltage characteristics of such devices have been investigated in a wide temperature range between 80K and 300K, showing good rectifying behavior at all temperatures. The electronic parameters, e.g. ideality factor and barrier height, have been determined and their temperature dependence discussed in detail. Furthermore, we have performed Ballistic-Electron-Emission-Microscopy/Spectroscopy (BEEM/BEES) studies at 300K and 80K by STM. The presence of the organic semiconductor substantially decreases the ballistic transmittance compared to reference Au/Si diodes. We analyze experimental data according to the standard hot-electron transport models as well as accounting for interference of photocurrents, that relate to photons creation at the STM tip-metal interface followed by photons absorption across the Si and PDI band gaps [4]. [1] O.F. Yuksel, M. Kus et al., J. Appl. Phys. 110, 024507 (2011). [2] F. Yakuphanoglu, Synth. Met. 160, 1551 (2010). [3] M. Barra, F.V. Di Girolamo, F. Chiarella et al. J. Phys. Chem. C 114, 20387 (2010). [4] E. R. Heller, J. P. Pelz, Appl. Phys. Lett. 82, 3919 (2003).

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PS4.3 Strain effect on magnetic properties of La0.7Ca0.3MnO3/SrRuO3

Superlattices Sujit Das, Andreas Herklotz, Kathrin Doerr IFW Dresden, Postfach 270116, 01171 Dresden, Germany Institute for Physics, MLU Halle-Wittenberg, 06099 Halle, Germany

Corresponding author: Sujit Das, Electronic mail: [email protected]

Coherent interfaces between magnetic oxides such as La0.7Sr0.3MnO3 and SrRuO3 may induce an intense magnetic coupling [1]. Recent work indicated an impact of elastic strain on the strength and even the sign of the coupling [2]. Superlattices (SL) of La0.7Ca0.3MnO3/SrRuO3

with layer thicknesses below 10 unit cells were grown by pulsed laser deposition simultaneously on SrTiO3 (001) (STO), LaAlO3 (001) (LAO) and piezoelectric 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 (001) (PMN-PT) substrates and structurally characterized by X-ray diffraction (XRD). On LAO, the SL assumes a compressive strain state, i. e. the lattice parameter is larger out-of-plane than in-plane, whereas on PMN-PT it shows a tensile strain state and on STO an intermediate strain value. Magnetization measurements demonstrate an antiferromagnetic (AFM) coupling in all SLs below Curie temperature of the SrRuO3 layers which is attributed to the superexchange interaction between Ru4+ (4d4) and Mn4+ (3d3) ions via oxygen ions. The AFM coupling seems to be strong in case of SL grown on LAO substrate and it decreases under tensile strain. The coupling is weaker on PMN-PT, probably because of a somewhat lower crystalline quality of the interfaces. In order to probe the effect of elastic strain directly, magnetization loops in reversibly controlled strain states have been recorded for SLs on PMN-PT and reveal the increasing strength of the antiferromagnetic superexchange induced by the in-plane compression. [1] M. Ziese et al., PRL 104, 167203 (2010), [2] J. W. Seo et al., PRL 105, 167206 (2010) !

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PS4.4 Surface doping in T6/ PDI-8CN2 Heterostructures investigated by transport and photoemission measurements F. V. Di Girolamo1, L. Aversa2, R.Verucchi2, R. Tatti2, M. Barra1, F.Ciccullo1, A. Cassinese1 and S.Iannotta2,3

1 CNR-SPIN and Department of Physics Science, University of Naples “Federico II”,P.le Tecchio 80, 80125 Naples, Italy 2 Istituto dei Materiali per l’Elettronica ed il Magnetismo, IMEM-CNR, sezione FBK di Trento, Via alla Cascata 56/C - Povo, 38123 Trento, Italy 3Istituto dei Materiali per l’Elettronica ed il Magnetismo, IMEM-CNR, Parco Area delle Scienze 37/A - 43124 Parma, Italy

Corresponding author: [email protected], CNR-SPIN and Department of Physics Science, University of Naples “Federico II”,P.le Tecchio 80, 80125 Naples, Italy

The creation and control of electronic properties of organic/organic interfaces is receiving great attention for several applications in electronics and optoelectronics. The electronic properties of these systems are ruled by the alignment of the energy levels of the organic species, showing narrower and spatially more localized bands with respect to inorganics. In particular, the Fermi energy difference between the dopant and the semiconductor can drive a charge transfer process which leads to the enrichment of the majority charge carriers in the semiconductor. This occurrence enhances the conductivity of the overall system and consequently represents an efficient way to dope organic semiconductors, improving their electrical properties [1, 2]. The presence of charge transfer and accumulation layers is currently under investigation for several pairs of organic conjugated materials. To this aim, useful hints can be given by electrical measurements; indeed, in a previous report, it was shown that the formation of an accumulation junction is able to explain the physical and electrical properties exhibited by T6/PDI-8CN2 heterostructures [2]. However, the most viable approach to verify and quantify their occurrence is the Ultraviolet Photoemission Spectroscopy (UPS) [1]. Here we have analyzed the surface doping effect produced by the evaporation of few nanometers of PDI-8CN2 on the top of 15 nm thick sexithiophene (T6) films, acting as active channels of field-effect transistors. Both electrical transport and photoemission measurements were performed. The electrical characterization evidenced the large enhancement of the channel conductance and the shift in of threshold voltages as extracted from the transfer-curves in the PDI-8CN2 doped T6 transistors. These findings suggest the occurrence of band-bending at the interface between T6 and PDI-8CN2, as confirmed by the Ultraviolet Photoelectron Spectroscopy (UPS) investigation, that also put in evidence the presence of interface dipoles and the upward shift of the Highest Occupied Molecular (HOMO) Level in T6 and the downward shift of the calculated Lowest Unoccupied Molecular Orbital (LUMO) in PDI-8CN2 [3]. The results have been confirmed also by UPS measurements in situ on sputtered and heated gold and on heated silicon. [1] Shenghao Wang, Takeaki Sakurai, Ryusuke Kuroda, and Katsuhiro Akimoto Appl. Phys. Lett. 100, 243301 (2012). [2] F.V. Di Girolamo, M. Barra, F. Chiarella, S. Lettieri, M. Salluzzo, A. Cassinese, Phys. Rev. B 85, 125310 (2012). [3] L. Aversa, R.Verucchi, R. Tatti, F. V. Di Girolamo, M. Barra, F.Ciccullo, A. Cassinese and S. Iannotta Appl. Phys Lett., 101, 233504 (2012).

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PS4.5 Inkjet printed Perylene Diimide based transistors: electrical response and bias stress effect study A.I. Grimaldi1, M.Barra2, A. Carella3, F.V. Di Girolamo2, F. Loffredo1, C. Minarini1, F. Villani1, A. Cassinese2 1Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Portici Research Center, Piazzale Enrico Fermi 1, 80055 Portici (Naples), Italy. 2 CNR-SPIN and Department of Physics Science, University of Naples Federico II, Piazzale Tecchio 80125, Naples, Italy. 3 Department of Chemistry, University of Naples ”Federico II”, Via Cinthia, I-80126 Naples,

Corresponding author: Immacolata Angelica Grimaldi Tel.: +39 081 7723386; fax: +39 081 7723344. E-mail address: [email protected]

N-type organic thin film transistor have received much attention in recent years because the development of organic semiconductors with high charge mobility, stable under air condition and soluble in common organic solvent. In particular, the easy solution processability of the organic semiconductors allows for selective deposition of precise quantities of materials via inkjet printing technique. The most promising n-channel candidates for transistor applications are the Cyanated Perylene Carboxylic Diimide derivatives which are soluble in many organic solvents (chlorobenzene, dichlorobenzene, chloroform, etc) and allows the fabrication of devices showing mobility higher than 1 cm2/Vs in air. In the present work, we report the printing of the n-type Perylene Diimide semiconductor (PDI-8CN2, Polyera ActivInk N1200) onto Si (gate)/SiO2 (dielectric)/Au (contacts) substrates for the fabrication of hybrid organic/inorganic bottom gate/bottom contacts transistors. The semiconductor was dissolved in mixtures of dichlorobenzene (DCB) and chlorobenzene (CB) at different mixing volume ratios, in order to investigate the effect of solvents and their mixtures on the electrical performances (including the bias stress phenomena) of the manufactured transistors. The results showed that the solvent mixture approach, with the optimized mixing ratio, is able to better ensure the film uniformity and to limit the degradation of the device performances when larger printed channel areas are considered.

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PS4.6 Structural characterization and phase-dependent photoluminescence in TiO2 nanoparticles produced by ultrafast laser ablation E. Orabona1,2, S. Amoruso1,2, D. Pallotti1,2, S. Tuzi2, C. Aruta1, R. Bruzzese1,2, F. Chiarella3, R. Fittipaldi3, A. Sambri2, A. Vecchione3, P. Maddalena1,2, S. Lettieri1 1 CNR-SPIN, UOS Napoli, Via Cintia I-80126 Napoli (Italy) 2Dipartimento di Fisica, Università degli Studi di Napoli “Federico II”, Napoli 3 CNR-SPIN UOS Salerno, via Ponte don Melillo, I -84084 Fisciano (SA), Italy

Corresponding author: Emanuele Orabona - Dipartimento Scienze Fisiche - University "Federico II", Complesso Universitario di Monte S. Angelo -Via Cintia, I-80126 Napoli (ITALY); e-mail: [email protected]

Structural and luminescence properties of titanium dioxide (TiO2) nanoparticles (NPs) prepared by femtosecond pulsed laser deposition (fs-PLD) in oxygen ambient gas are investigated, evidencing multiple photoluminescence bands whose relative intensities are correlated with the TiO2 crystalline phase. By varying the background pressure, various regimes of TiO2 plumes propagation are identified, leading to different sample morphologies and crystal phases. These outcomes are particularly relevant to tune oxide NPs films characteristics for applications where the nanometer-scale porosity and large surface-to-volume ratio can be beneficial to facilitate reactions with the interacting medium. The influence of TiO2 crystal phase on optical transitions is investigated by photoluminescence analysis. Various electron-hole processes are found to contribute to the overall PL emission and up to four PL emission bands are observed, spanning from near-infrared (NIR), red, green and blue range of electromagnetic spectrum. By crossing the PL results with the structural evidences given by X-ray diffractometry, we evidenced that the specific emission bands are correlated with both surface morphology and crystal phase of TiO2 NPs.

An overall interpretation is proposed with regard to the nature of different radiative recombination pathways occurring in rutile, anatase and amorphous TiO2. In particular, we discuss the involvement of: amorphous TiO2 surface and band-tail states to near-band edge radiative recombination (blue/violet emission, about 3.0 eV), surface states of each crystal phases to green emission (about 2.3 eV), self-trapped electrons and holes at anatase and rutile surface sites to red (1.9 eV) and near-infrared (1.5 eV) emission bands, respectively.

Finally, some issues and preliminary results regarding the possibility to use TiO2 NPs as bi-parametric (optical plus electrical) chemical nanosensors will be addressed.

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PS4.7 From Magnetic Nanoparticles to Nanopillars Grown in Mesoporous Si Templates A.L. Dolgiy, S.V. Redko, V.P. Bondarenko, V.G. Baev*, K.I. Yanushkevich#, S.L. Prischepa Belarusian State University of Informatics and RadioElectronics, Minsk, Belarus *National Scientific and Educational Centre of Particle and High Energy Physics, Belarusian State University, Minsk, Belarus #Scientific Practical Materials Research Center, National Academy of Sciences, Minsk, Belarus

Corresponding author: S.L. Prischepa. Belarusian State University of Informatics and RadioElectronics, P.Browka str. 6, Minsk 220013, Belarus. Tel. +375-17-2932317. Email: [email protected].

Both Ni nanoparticles and nanopillars with different density of distribution were electrochemically deposited into pores of mesoporous silicon templates under the stationary galvanostatic regime. The morphology and structural properties of samples have been exhaustively studied by using scanning electron microscopy, X-ray .-2. diffraction, X-ray microanalysis and Raman spectroscopy. The magnetic properties have been investigated by studying the temperature dependence (77 K – 700 K) of the specific magnetization / and the magnetization hysteresis loops. The ferromagnetic resonance spectroscopy was also applied to evaluate the magnetic anisotropy of the samples. The crossover from magnetic nanoparticles to magnetic nanopillars occurs by changing the time of deposition tNi. At the initial stage (tNi < 15 min) Ni is deposited in the form of isolated spherical-like nanoparticles with the average diameter around 40 – 60 nm. While increasing the deposition time the conglomerates of Ni nanoparticles start to be growth and, finally, at large deposition time (tNi > 60 min) the dense metallic nanopillars are formed. The diameter and the length of the formed nanopillars are defined by the diameter and the length of pores in mesoporous silicon template and were equal to 100 nm and 10 µm, respectively. This gives the high value of the aspect ratio, a.r. = 100. By means of Raman spectroscopy it was established that, the presence of amorphous Si (a-Si) on the pore walls facilitates the formation of non-magnetic metallic Ni2Si silicide at ambient temperature. This is caused by much less values of the activation energy for the growth of the Ni2Si phase at the Ni/a-Si interface (4.48+10-20

J) with respect to the Ni/c-Si one (2.4+10-19

J). The nickel silicide formation reduces both the specific magnetization and the Curie temperature. The effect is less pronounced for nanopillars and can be governed by temperature. Nevertheless, the specific magnetization measurements reveal surprisingly high values, close to that of bulk Ni, / # 58.9 A+m2+kg-1, even for nanoparticles. The Curie temperature is less than for bulk material and depends on the deposition time, approaching the value for bulk Ni at tNi = 60 min. The magnetization versus magnetic field measurements in the temperature range 6 K – 300 K confirm the ferromagnetic nature of all the samples and reveal the perpendicular magnetic anisotropy for Ni nanopillars. The ferromagnetic resonance measurements at different angles of the magnetic field with respect to the substrate plane indicate the perpendicular magnetic anisotropy and allow evaluating the anisotropy field and g-factor of Ni nanopillars. Moreover, by increasing the packing density of nanopillars the crossover from perpendicular easy axis to parallel easy plane magnetization was experimentally observed.

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PS4.8 Compositional and structural study of homoepitaxial-STO based oxides heterostructures M. L. Reinle-Schmitt, C. Cancellieri, A. Cavallaro*, S. J. Leake, C. W. Schneider, J. A. Kilner*, P. R. Willmott Paul Scherrer Institut, 5232 Villigen, Switzerland *Department of Materials, South Kensington Campus, Imperial College London, London SW7 2AZ, United Kingdom

Corresponding author: M. L. Reinle-Schmitt, [email protected], Paul Scherrer Institut, WBBA/011, 5232 Villigen, Switzerland

Complex oxide heterostructures, and in particular the interface between LaAlO3 (LAO) and SrTiO3 (STO), have been the subject of intensive research in the last years. The unexpected interfacial conductivity between these band insulators may be particularly interesting for future integrated oxide-based electronic devices. Attempts to grow derivatives of this system, such as LAO on homoepitaxial layers of STO, or multilayers have been made. However as far as we know the new devices do not generally exhibit similar interfacial properties as standard LAO grown on STO. In particular no evidence of more than one conducting interface was found in multilayers structures. In this work we try to explain why LAO grown on top of homoepitaxial STO behaves differently than when LAO is grown on top of bulk STO. !

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PS4.9 Preparation of two-dimensional electron gases at the surfaces of transition metal oxides by chemical and thermal treatment T.C. Rödel, C. Bareille, F. Fortuna, F. Bertran*, M.Gabay**, M. Rozenberg**, T. Maroutain***, P. Lecoeur***, A.F. Santander Syro CSNSM, Université Paris-Sud and CNRS/IN2P3, Bâtiments 104 et 108, 91405 Orsay cedex, France *Synchrotron SOLEIL, L’Orme des Merisiers,Saint-Aubin-BP48, 91192 Gif-sur-Yvette, France **Laboratoire de Physique des Solides, Université Paris-Sud, Bâtiment 510, 91405 Orsay, France ***Institut d’Electronique Fondamentale, Université Paris-Sud, Bâtiment 220, 91405 Orsay, France !

Corresponding author: T.C. Rödel; CSNSM, Université Paris-Sud and CNRS/IN2P3, Bâtiments 104 et 108, 91405 Orsay cedex, France; email: [email protected]!

The two-dimensional electron gases (2DEGs) in transition-metal oxide (TMO) interfaces are a field of current intense research. They can present remarkable properties, such as superconductivity or magnetoresistance. However, the microscopic origin of such interfacial metallic states is still controversial. The recent discovery, via angle-resolved photoemission spectroscopy (ARPES), of a 2DEG that is formed at the (100) surface of vacuum-fractured single crystals of insulating SrTiO3 opened a new avenue for the understanding and fabrication of 2DEGs in TMOs. To explore 2DEGs at surfaces of TMOs other than SrTiO3, fracturing the sample is not always a feasible technique. As many TMOs have no natural cleaving plane, fracturing leads in many cases to disordered surfaces, thus preventing the formation of a 2DEG. To circumvent this, we prepared ex-situ different oxide surfaces by chemical and thermal treatments. A subsequent in-situ vacuum annealing cleans the surface and creates the oxygen vacancies whose electrons potentially form the 2DEG. For SrTiO3, we compared the electronic structure of the 2DEG obtained in vacuum-fractured samples with the ones obtained for different thermally-prepared surface reconstructions. !

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PS4.10 Competing misfit relaxation mechanisms in epitaxial correlated oxides F. Sandiumenge1, J. Santiso2, Ll. Balcells1, Z. Konstantinovic1, J. Roqueta2, A. Pomar1, J.P. Espinós3 and B. Martínez1 1Instituto de Ciencia de Materiales de Barcelona, CSIC. Campus UAB. Bellaterra 08193. SPAIN 2Centre for Nanoscience and Nanotechnology, CIN2 (CSIC-ICN). Campus UAB Bellaterra 08193. SPAIN 3 Instituto de Ciencia de Materiales de Sevilla (CSIC-University of Seville). c/Américo Vespucio 49, E-41092 Seville. SPAIN

Corresponding author: Felip Sandiumenge. ICMAB-CSIC, Campus UAB. Bellaterra 08193-Spain. E-mail: [email protected]

Strain engineering of functional properties in epitaxial thin films of strongly correlated oxides exhibiting octahedral-framework structures is hindered by the lack of adequate misfit relaxation models. Here we present unreported experimental evidences of a three-stage hierarchical development of octahedral-framework perturbations resulting from a progressive imbalance between electronic, elastic and octahedral tilting energies in La0.7Sr0.3MnO3 epitaxial thin films grown on SrTiO3 substrates. Electronic softening of the Mn - O bonds near the substrate leads to the formation of an interfacial layer clamped to the substrate with strongly degraded magnetotransport properties, i.e. the so-called dead layer, while rigid octahedral tilts become relevant at advanced growth stages without significant effects on charge transport and magnetic ordering.

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PS4.11a Thermomagnetic instabilities inside the superconducting regime controlled by strain in superconductor-piezoelectric-superconductor hybrids D Stamopoulos*,1, M. Zeibekis1, H. Luo2, E. Aristomenopoulou1 1Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems, NCSR ”Demokritos”, 153-10, Aghia Paraskevi, Athens, Greece 2Key Laboratory of Inorganic Functional Material and Device, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China

Corresponding author: Tel. +302106503330, e-mail: [email protected]!!

In recent years much effort has been focused on the modulation of the properties of a superconductor (SC) by magnetic means provided by an adjacent ferromagnet (FM) [1-3]. In another class of hybrids, recently it has been shown that the properties of a FM can be controlled by means of strain provided by an adjacent piezoelectric (PE) substrate [4-5]. The possibility to control the properties of a SC by means of strain has not been explored so far. Here we present introductory results on a SC-PE-SC hybrid that constitutes of a 0.7Pb(Mn1/3Nb2/3)O3–0.3PbTiO3 single crystal (5x5x0.7 mm3) and two Nb thin films (20 nm).

Regarding the PE crystal it has a rhombohedral to tetragonal phase transition at TRT=80oC and a ferroelectric to paraelectric phase transition at TC=140oC. The PE crystal is cut along [001] because the maximum d coefficient is shown along this direction, d33～2000 pC/N and was left unpoled. The Nb outer layers are high-quality SC exhibiting a relatively high critical temperature Tc

SC=6.5 K. To optimize the properties of the SC layers attention was paid on the elimination of any residual oxygen in the sputtering chamber; the Nb layers were deposited at 3x10-3 Torr of Ar (99.999%) only when a base pressure in the range of 10-8 Torr was established. Except for being high-quality SC, the Nb outer layers act as voltage electrodes for the application of an electric field to the PE crystal, so that the motivated strain will be eventually experienced by the Nb thin films.

Detailed magnetization measurements were performed by means of a SQUID magnetometer (Quantum Design) below Tc

SC=6.5 K. Isothermal magnetization loop m(H) measurements provided evidence that under controlled variation of the externally applied voltage (that is electric field) we can systematically induce thermomagnetic instabilities (TMI), that is vortex avalanches, in the SC thin films. Future goals and possible practical applications of the observed TMI are discussed. Acknowledgements: Two of the authors, M. Zeibekis and E. Aristomenopoulou, would like to acknowledge the ‘A.G. Leventis Foundation’ for support through a scholarship.

[1] D. Stamopoulos et al., J Supercond Nov Magn Doi 10.1007/s10948-012-1864-y (2013) [2] A. Angrisani Armenio et al., Physical Review B 76, 024515 (2007) [3] D. Stamopoulos et al., Physical Review B 75, 184504 (2007) [4] S. Polisetty et al. Physical Review B 82, 134419 (2010) [5] S. Brivio et al. Applied Physics Letters 98, 092505 (2011)

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PS4.11b Minimization of the magnetoresistance effect in Co-Nb-Co trilayers: weakening the magnetostatic coupling of the outer Co layers by exceeding the finite range of stray fields

E Aristomenopoulou, D Stamopoulos Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems, NCSR ”Demokritos”, 153-10, Aghia Paraskevi, Athens, Greece !

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Corresponding author: Tel. +302106503330, e-mail: [email protected]!

Superconductivity (SC) and Ferromagnetism (FM) are generally considered as antagonistic phenomena. In recent years the FM-SC-FM trilayers (TLs) have attracted much interest due to their exotic transport properties [1-5]. In brief, the transport properties of the SC interlayer can be controlled by the magnetic configuration of the outer FM layers, whether we refer to in-plane (the so-called superconducting spin valve effect [SSVE]) or out-of-plane (superconducting magnetoresistance effect [SMRE]), in respect to the TL’s surface. Former experiments of ours conducted on NiFe-Nb-NiFe TLs [5] have shown that the SMRE peaks correlate nicely with the peaks observed in the out-of-plane magnetization components of the outer FM layers. Those experiments illustrated that the transverse magnetostatic coupling of the outer FM layers through stray fields that pierce the SC interlayer is the most relevant underlying mechanism motivating the SMRE. Moreover, our recent experiments on Co-Nb-Co TLs [1] not only confirm the stray-fields scenario but also reveal the great impact of Co’s shape anisotropy on SMRE.

Here we study the dependence of the SMRE on the thickness of the SC interlayer in Co-Nb-Co TLs. We focus on the SMRE appeared on Co(30nm)-Nb(dsc,1)-Co(30nm) and Co(10nm)-Nb(dsc,2)-Co(10nm) TLs by investigating a wide range of Nb thickness, dsc,1=10-200 nm and dsc,2=17-100 nm, respectively. Based on the stray-fields scenario we expect that, as the thickness of the SC interlayer increases, the range of the stray fields is exceeded and thus the magnetostatic coupling of the outer FM layers should become weaker leading to lower SMRE values. The experimental data indicate that as the dsc increases a region free of stray fields is introduced at the center of the SC interlayer and the SMRE decreases to a baseline value that equals the one observed in the relevant bilayers (BLs). In both cases, Co(30nm)-Nb(dsc,1)-Co(30nm) and Co(10nm)-Nb(dsc,2)-Co(10nm), we have found the critical SC thickness, dsc,crit, above which the TLs behave as two independent and magnetostatically uncoupled BLs.

Acknowledgements: One of the authors, E. Aristomenopoulou, would like to acknowledge the A.G. Leventis Foundation for support through a scholarship.!

[1] D. Stamopoulos et al., J. Supercond. Nov. Magn. DOI 10.1007/s10948-012-1864-y (2013) [2] V. N. Kushnir et al., Phys. Rev. B 84, 214512 (2011) [3] C. Cirillo et al., Phys. Rev. B 84, 054536 (2011) [4] A. Angrisani Armenio et al., Phys. Rev. B 76, 024515 (2007) [5] D. Stamopoulos et al., Phys. Rev. B 75, 184504 (2007)

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PS4.12 Depth-Dependent Distribution of Magnetic Moments in [La2/3Sr1/3]n±1MnnO3n±1/SrTiO3 A. Steffen1, S.Pütter1, S. Mattauch1, W. Zander 3, J. Schubert3, and Th. Brückel2

1 Jülich Centre for Neutron Science JCNS, Forschungszentrum Jülich GmbH, Outstation at FRM II, Lichtenbergstr. 1, 85747 Garching 2 Jülich Centre for Neutron Science JCNS and Peter Grünberg Institut PGI, JCNS-2, PGI-4: Scattering Methods, Forschungszentrum Jülich GmbH, 52425 Jülich 3 Peter Grünberg Institut PGI, PGI-9: Semiconductor Nanoelectronics, Forschungszentrum Jülich GmbH, 52425 Jülich

Corresponding author: Alexandra Steffen Jülich Centre for Neutron Science JCNS , Forschungszentrum Jülich GmbH Außenstelle am FRM II Lichtenbergstraße 1 85747 Garching Germany [email protected]

Transition metal oxide thin films give rise to a huge variety of fascinating phenomena like ferromagnetism at interfaces of non-magnetic materials as found in SrTiO3/KTaO3[1]. To analyze these effects quantitatively, a well defined layer structure is needed. By molecular beam epitaxy high quality epitaxial films can be achieved with high purity, low intrinsic defect concentrations and atomic-layer control. The stoichiometry of the films may be varied easily.

La1-xSrxMnO3 provides a huge variety of fascinating effects due to its spin, charge, orbital, and lattice degrees of freedom which lead to a highly interesting magnetic phase diagram[2]. Due to its high Curie temperature of 350K and its nearly perfect spin polarisation La2/3Sr1/3MnO3 is under intense investigation. Thin film on top of SrTiO3 show an inhomogeneous magnetization[3]. On the contrary to the intensively investigated different Sr doping levels in La1-xSrxMnO3 we focused on different [La/Sr] to Mn ratios to investigate the magnetic characteristics.

We determined the magnetic depth profile of [La2/3Sr1/3]2Mn3O8/ SrTiO3 and [La2/3Sr1/3]3Mn2O7/SrTiO3 and compared these results with [La2/3Sr1/3]1Mn1O3/SrTiO3 at the polarized neutron reflectometer TREFF@FRM II. The samples for this study were grown utilizing the oxide molecular beam epitaxy system which is operated in the JCNS thin film laboratory at the FRM II. The stoichiometry was adjusted in-situ via quarz-crystal balance monitoring and cross-checked with Rutherford Backscattering Spectrometry while the growth mode was observed via Reflection High Energy Electron Diffraction. By Low Energy Electron Diffraction, X-Ray reflectivity and X-Ray diffraction, the crystalline sample quality was studied. The magnetic properties were determined via SQUID magnetometry.

[1] R. Oja et al., Phys. Rev. Lett. 109, 127207 (2012)

[2] J. Hemberger et al., Phys. Rev. B 66, 094410 (2002)

[3] F. Ott et al., J. Magn. Magn. Mater. 211, 200-205 (2000)

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PS4.13 When the top layer become amorphous – a look beyond the all-crystalline LaAlO3/SrTiO3 F. Trier, D.V. Christensen, Y.Z. Chen, and N. Pryds Department of Energy Conversion and Storage, Risø Campus, Technical University of Denmark, DK-4000 Roskil

During the last decade, the electron gas formed at the interface between crystalline LaAlO3 and SrTiO3 has attracted a huge interest. Unexpectedly, recent discovery has revealed that a similar electron gas is formed when depositing amorphous rather than crystalline LaAlO3 leading to several advantages. Through oxygen vacancy defect control, we here present different strategies to fashion the interface state between amorphous LaAlO3 and crystalline SrTiO3 (a-LaAlO3/SrTiO3) from metallic to insulating. This control is achieved by means of carefully modifying: i) the impinging plasma plume on the substrate during deposition either by the background gas pressure or electric fields. ii) the a-LaAlO3/SrTiO3 sample storage environment. iii) the interface conductivity using electric fields through a resistive switching transition.

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PS4.14 Electrical transport in nano-patterned quasi-2D electron gas at the LaAlO3/SrTiO3 interface

Pier Paolo Aurino,1 Alexey Kalabukhov,1 Nikolina Tuzla,2 Eva Olsson,2 Tord Claeson,1 and Dag Winkler1 1Department of Microtechnology and Nanoscience – MC2, Chalmers University of Technology, SE412 96 Gothenburg, Sweden 2Department of Applied Physics, Chalmers University of Technology, SE412 96 Gothenburg, Sweden

We present an investigation of electrical transport properties of different structures patterned in the quasi-two-dimensional electron gas at the interface between LaAlO3 (LAO) films and SrTiO3 (STO) substrates.[1] The electron gas was formed by epitaxial growth of 10 unit cells of thick LAO films using pulsed laser deposition on the STO substrates. The interface conductivity was patterned using low-energy Ar+ ion beam irradiation.[2] We will present measurements of the Hall bars structures with dimensions from 30 $m wide down to 100 nm. The temperature dependence of the resistance was sensitive to the angle between the electrical current direction and crystallographic axes of the substrate. Moreover, the Hall mobility measured in these structures was higher than the one measured in van der Pauw configuration in non-patterned samples. Nanowires with widths down to 50 nm were fabricated and all showed metallic properties. It was also found that the sheet resistance of those structures significantly increased when the width decreased below 1 $m. Our findings indicate that the conductivity of the interface has possibly a non-homogeneous in-plane distribution. [1] A.Ohtomo and H.Y. Hwang; Nature (London) 427, 423 (2004). [2] P. P. Aurino,!A. Kalabukhov, N. Tuzla, E. Olsson, D. Winkler and T. Claeson; (submitted).

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PS4.15 Observation of strontium segregation in LaAlO3-SrTiO3 and NdGaO3-SrTiO3 oxide heterostructures by X-ray photoemission spectroscopy

U. Treske, N. Heming, M. Knupfer, B. Büchner, A. Koitzsch Institute for Solid State Research, IFW-Dresden, P.O. Box 270116, DE-01171 Dresden, Germany E. Di Gennaro*, U. Scotti di Uccio*, F. Miletto Granozio* *CNR-SPIN, Dipartimento di Scienze Fisiche, Universitá di Napoli “Federico II”, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy

Uwe Treske, IFW-Dresden, P.O. Box 270116, DE-01171 Dresden, Germany, [email protected]

Pulse laser deposition grown LaAlO3 and NdGaO3 on Ti-terminated SrTiO3 with different film thicknesses were investigated by soft X-ray photoemission spectroscopy. By varying the photon energy h' and emission angle . the surface sensitivity of the measurements has been tuned. In contrast to the core levels of the other elements the Sr 3d line shows an unexpected splitting for higher surface sensitivity. This can be assigned to an additional, chemically distinct from SrTiO3 Sr component. It seems that a small fraction of Sr diffuses out of the substrate, possibly forming strontium oxide and segregates at the surface during the growth process.

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