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Laboratory for

Quantum Magnetism

TP lab presentation 2009

Henrik M. Ronnow (EPFL since Jan. 2007)

How many body physics?

One of the simplest problems:

H = J Si Sj

1 spin: trivial

2 spins: singlet state |↑↓> - |↓↑>

4 spins: back-of-the-envelope calc.

16 spins: 10 seconds on computer

40 spins: World record ! (A. Läuchli, EPFL)

1023 spins: Antiferromagnet (Louis Neel 1932)

Fluctuating singlets (PW Anderson 1973,1987)

1023 – some electrons: High-Tc superconductivity

– THE enigma of modern solid state physics

CuO S= 1/22

A small contribution

There are fluctuating singlets in the

ground state !

Quantum Magnetism

Theoretical models Novel materials

theNeutron Bulk methods scattering big magnets,

low temperature,

high pressure

of physics

Physics of Interacting Systems

• A challenge on all length scales Classical n-body problem (from 3 to

galaxies)

Neural networks

Spin-models

= QUANTUM EFFECT ?

Maybe the “Big Bang” was powered by

“Vacuum Quantum Fluctuations” ?

(Hawkins et al.)

Novel electronic materials

• Strongly correlated electrons

• Often magnetism plays a (leading?) role - e.g.:High-Tc superconductors Colossal magnetoresistance

La2-xBaxCuO4 La2-2xSr1+2xMn2O7

Doped spin ½ antiferromagnets Intrinsic spin valves

Building models

• SpinsLength: |S|=1/2

Quantum / classical

Dimension: Ising, XY, Heisenberg

• Architecture

Dimension

Connectivity

• InteractionsCu2+ O 2px Cu 3dx2-y2

H = J Si Sj

Anti-/Ferromagnetic

• Extentions

Randomness

Charge, orbit, lattice...

HsatCuGeO3

(Hpip)2CuBr4

(d6-5CAP)2CuCl42DHAF

CuGeO3

Magnetic measurementsM

ag

ne

tization

S

usceptibili

ty

NM

R, μ

SR

etc

. S

pecific

heat

Neutron Scattering

intensity cross-section correlation function wave-function overlap

2

f

2

0)(),(f

fE

dEd qSfSdEd

dI Q

Experiment Theory

fi kkQ

m

k

m

k

22

2

f

22

i

2

(Crystal) momentum transfer

Energy transfer

Mais les Neutrons, ils sont où ?

All ways lead to Rome…

Reactor or spallation sources:

6-10 in Europe

~2008 next-generationin US & Japan

European Spallation

Source (ESS) ?

Last decade:x10 in fluxx10 in detection

Can study samples and phenomena not previously possible

ILL, Grenoble

EPFL

SINQ, PSI

Bern

Start: Villigen

Via: Lausanne

Ziel: Grenoble

400.2 km 3:04 h

Quantum Magnetism - Quo vadis ?

• Entanglement & quantum information theory

– New notation or new resource ?

• Quantum phase transitions:

– Different quantum phases, universal behaviour etc.

• Controlled quantum magnets:

– “Pump” dynamically to obtain and control “new semiconductor”

• Bulk Restricted geometries

– Finite size quantization devices ?

Driver of new theories and pictorial explanations

Correlated Electron Technologies ?

The laboratory

• Activities:

~ 40% neutron scattering (at international facilities)

~ 60% in-house activities

– Sample synthesis and study of new materials

– Sub-kelvin measurements (susceptibility etc.)

– High-pressure cells (quantum phase transitions)

– Theory and simulations

some stars of LQM:

Neutrons

Low-T & high-P

Susceptibility and Specific heat

Metal-organic and hydrothemal synth

Real-time Laue,

0.3K SQUID end mar

„Horizontal team strategy‟ – competence based

The laboratories:

• Halle Bernard Vittoz

– 9 tesla cryomagnet

– Dilution fridge

– Dip-stick, 3He

Susceptometry,

specific heat,

high-pressure

Future:

– New magnetometer

– 18 tesla system

– 400μW fridge

The laboratories:

• The abyss (hosting the SQUID magnetometer)

– SQUID magnetometer

– Synthesis lab.

Copper

acetate

Cu(C5D5NO)6(11BF4)2

Synthesis

Crystal growth

SamplesMeasurements

TP-projects

• Past projects and present suggestions:

1. Synthesis of spin-dimer systems (Farley)

2. Adiabatic cooling for magnetometer (dalla Piazza)

3. Low-T susceptometer (Piatek)

4. High-pressure susceptometry of SrCu2(BO3)2

5. Simulation of novel neutron spectrometer

6. New iron-pnictide and dichalcogenide superconductors

7. Theoretical modeling of spin systems

8. Magnetometer design for Swiss company (non-disclosure restriction)

9. Nano-devices of correlated electron materials (collaboration with STI)

10. New materials synthesis and quantitative crystal growth

Visit lqm.epfl.ch/publications for examples of past reports

Example: High-pressure susceptometry

• Recent TPIV

project:

Laurent Cevey

Quantum phase transition

at 20-25kbar !

• magnetometer based on

commercial hall probes

• Measure SrCu2(BO3)2

• Compare to neutron and ESR

New cell:

30kbar

New materials synthesis

Simulation of Novel Neutron Spectrometer

Continuous Angle Multiple Energy Analysis (CAMEA)

Hybrid for mapping excitation spectectra:

• 60º continuous angle coverage x15(over conventional TAS)

• 5 successive analysers x 4.5

• Better resolution x 3

• Estimated improvement x 200 !

• Prove improvement

• Develop actual design

Sample analysers

detectors

TP-projects

• General philosophy:

– Foreseeable outcome in one semester

– Related to real research (linked to ongoing projects)

– Can be extended to Dimploma/Master‟s project

– Defined together with student

• Any questions?

• If interested,

schedule a discussion

– henrik.ronnow@epfl.ch