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Condensed MatterJournal of Physics:

TOP PAPERS 2006 SHOWCASE

Research journals are only worthreading if the quality of thescience in them is good enough.Top Papers 2006 is our annualattempt to give an impression ofthe diversity, range and scientificquality of papers in Journal ofPhysics: Condensed Matter(JPCM). Choosing these examplesis one of the hardest but mostpleasurable tasks for the Editors,Board Members and Publishingstaff.

The choice is difficult becauseJPCM has succeeded in attracting manyexcellent and innovative authors. Our veryfair but rigorous referees have also ensured aconsistently high standard of papers in thejournal. Many more papers were singled outwith the support of referees (identifying workof the very highest importance), of readers(through high numbers of full-textdownloads) and of our Board members(through recommending articles they foundespecially valuable) than could be featuredhere. The final choice by us as Editors in Chiefcan only give an indication of papers thatseemed to us to be exciting and of especialinterest.

These papers give a flavour of the excitingwork published in JPCM, showing just howvigorous and lively the condensed matter fieldis at present. They also reflect the highlyinternational nature of our authorship.Perhaps your paper will be one chosen for TopPapers 2007.

Marshall Stoneham and David Ferry

From molecular elevator…

A two-component molecular device, 4H39+, that behaves like a

nanometre-scale elevator. It consists of a tripod componentcontaining two different notches at different levels in each leg. Thelatter are interlocked by the tritopic host, which plays the role of aplatform that can be made to stop at the two different levels.

See Artificial nanomachines page 17.

…to space elevator

Will it break?

See Space elevator: not all it’s cracked up to be? page 11.

In this issue

Full contents overleaf

Professor David FerryEditor-in-Chieffrom 2007

Professor MarshallStonehamEditor-in-Chief2002- 2006

Condensed Matter: Top Papers 2006 Showcase

2 C o n d e n s e d M a t t e r : Top Papers 2006 Showcase

P A P E R S

Molecular motors for lithography 4A molecular motor consisting of a self-assembled monolayer of ‘chemical fuel’ may offer a route to molecular lithography

New route to spintronic materials 4Magnetic semiconductor materials can be produced in a controllable way by inducingoxygen vacancies in TiO2 films

Coherence of spin qubits in silicon 5P donors in isotopically pure 28Si have sufficiently long spin relaxation times to be candidates for qubits in a quantum computer

Complementary media of electrons 5The concept of complementary media, analogous to negative refractive index materials, for electron waves is feasible

Negative refraction in multilayers 6Metal–insulator multilayer systems can show negative refractive index behaviour

How life (un)folds? 6This paper explores how networks of molecular interactions can have given rise to life

Nucleation of nano-domains 7A new defect-free model of nucleation in ferroelectrics and ferromagnets has significant implications for the ultimate switching speed in thin-film memory devices.

Wetting problem solved? 7A new coarse-grained description of wetting in the 3D Ising model accounts for missing physics in the original interfacial description

In situ measurement of ZnO nanowires 8A TEM method enables in situ measurement of mechanical properties of ZnO nanowires

Testing DFT for surfaces 8Monte Carlo simulations test different approximation schemes in DFT for metal and oxide surfaces and for desorption

Coherent phonons in semimetals 9At a high level of excitation, phonons in semimetals show interesting quantum interference effects

A new relaxor ferroelectric 9Barium substitution enhances the properties of a relaxor ferroelectric for applications

Theory of glasses in transition? 10Experiments at extremely high pressure are proposed to resolve the controversy over the driving force behind the glass transition

Fighting cancer with nanoparticles 10Magnetic nanoparticles can be used in cancer therapy either for drug delivery or for destroying the tumour by heating

Space elevator: not all it’s cracked up to be? 11It is argued that a proposed space elevator cable made of carbon nanotubes would contain enough defects to cause it to break

Quantitative description of exchange bias 11Taking into account spin disorder at the interface accounts for experimental results forexchange bias

Stretching proteins in a force-clamp 12Stretching proteins in a force-clamp yields information on the statistics of unfolding events


Condensed Matter: Top Papers 2006 Showcase 3

R E V I E W A R T I C L E S

Quantum IT clouded by uncertainty 15Issues affecting how a quantum information technology industry may develop in the future are discussed

Ultrafast magneto-optics 16The use of magneto-optical spectroscopy in investigating spin-related phenomena in (III,Mn)V ferromagnetic semiconductors is reviewed

Single-molecule experiments in biological physics 16Experimental techniques for manipulating and measuring single molecules and their applications are reviewed

Nanoemulsions: clearly superior 17Nanoemulsions have quite different physical properties from microscale emulsions, which gives them great potential in a wide range of industries

Artificial nanomachines 17Recent progress in constructing simple prototypes of artificial molecular machines is reviewed

Si-Ge nanostructures 18Epitaxial growth and characterization of Si1-2xGex islands and Ge dots on (001) Si are reviewed

Simulating smart biomaterials 18New multiscale techniques for simulating smart biomaterials are reviewed

THz biosensing devices 19THz devices offer marker-free biomolecule detection on functionalized surfaces in dry and fluid environments

Organic superconductors 19Organic charge transfer salts are reviewed, especially their superconducting properties

Amorphous ices 20Recent experiments and computer simulations of the different phases of water are reviewed

Light damage in a-Si:H 12Structural changes from exposure of a-Si:H to intense light have been successfully simulated

Landau theory corrected 13The phase behaviour of certain polymers depends crucially on certain non-local terms in the Landau free energy expansion that have been neglected before.

Separating particles by their surface properties 13Particles, proteins and DNA may be separated by their surface properties using a liquid crystal as solvent

Liquid silicon under pressure 14An ab initio study accounts for experimentally found pressure-induced structural and dynamical changes in liquid silicon

Decomposing food 14Spinodal decomposition in a food colloid–biopolymer mixture shows evidence for a linear regime

Condensed Matter: Papers

Molecular motors for lithography

A molecular motor produces a controlleddisplacement of matter at the molecularscale, under the influence of a suitableenergy-supplying process (e.g. a chemicalreaction).

Vincent Huc of Université de Paris-Sud hassuggested that some transition-metal-basedcatalysts can be converted to molecularmotors. The molecules to be used by thecatalyst as ‘chemical fuel’ can be grafted ontoa surface as a self-assembled monolayer(SAM), resulting in a periodic 2D moleculararray. The catalytic motor may move alongthis array by jumping from one molecule tothe next.

One possibility is to introduce an electriccharge to the catalytic motor and to controlthe motion by using an electric field tochange the direction of the catalyst’sdisplacement.

Another possibility is to place a liquidcrystalline phase above the SAM of ‘fuel’molecules (see figure).

Another approach is to induce a change inthe favoured direction of motion byreversibly modifying the organization of the‘fuel’ molecules on the surface, for exampleby irradiating the surface with a laser.

This ‘molecular chessboard’ approach

may open the possibility to build complexmolecular objects on a surface in a veryprecise way (at the sub-nanometre scale)under the influence of a macroscopiccontrol, opening the way to a true ‘molecularlithography'.

Transition metal catalysts for molecularmotors: towards molecular lithographyVincent HucJ. Phys.: Condens. Matter 18 (2006)S1909–S1926

This article is part of a special section on molecularnanomachines. To find out more about molecularnanomachines, see Journal of Physics: CondensedMatter Vol 18, No 33.

Controlling the motion of the catalyst with a liquid crystalline phase; this has a highly anisotropic orientation of molecules which can be easilytuned using an electric field or polarized light, inducing a preferential direction for the motion of the catalyst.

4 Condensed Mat te r : Top Papers 2006 showcase

A molecular motor consisting of a self-assembled monolayer of ‘chemical fuel’ may offer a route to molecular lithography

New route to spintronic materials

Magnetically ordered semiconductormaterials are sought because of theirpotential new applications in spintronicsdevices (e.g. spin diodes and spin-FETs).

Vincent Harris of Northeastern Universityand co-workers have induced magnetism infilms of TiO2-d. The amount of inducedmoment is systematically controlled by theoxidation pressure employed during PLDgrowth and by defects generated at thefilm–substrate interface.

Stoichiometric TiO2 is a wide bandgapsemiconductor and contains only Ti4+ ionsso it is non-magnetic. Oxygen vacanciescreate a charge imbalance and unpaired 3delectrons in Ti3+ and Ti2+ ions can potentiallycause magnetism. The formation of Ti3+ andTi2+ requires the Ti ions to be stripped ofelectrons from the s band rather than the 3dband, so Ti3+ and Ti2+ ions have 3d1 and 3d2

electronic configurations, respectively. Harris et al demonstrate the ability to

control the concentration of oxygen defects

and to explore their relationship tomagnetism and transport in the oxidesemiconductor. Magnetism scales withconductivity, suggesting that a double

exchange interaction is active. Films havingthe highest moments have magnetictransition temperatures near 880 K.Furthermore, they demonstrate adependence of both conductivity andmagnetization with sample thickness wherethin films have the strongest magnetization,thus confirming the role of interfacial defectsin inducing magnetism in these films.

This represents a new approach in thedesign and refinement of magneticsemiconductor materials for spintronicsdevice applications above roomtemperature.

Oxygen-defect-induced magnetism to880 K in semiconducting anatase TiO2

filmsSoack Dae Yoon, Yajie Chen, Aria Yang,Trevor L Goodrich, Xu Zuo, Dario A Arena,Katherine Ziemer, Carmine Vittoria andVincent G HarrisJ. Phys.: Condens. Matter 18 (2006) L355–L361

Temperature dependence of the magnetization for the TiO2-8 filmdeposited at 0.3mTorr of oxygen pressure, The kink around 450Kshows a second transition at a lower temperature. The data was fittedto two functions with magnetic ordering temperature of 880K and asecond transition temperature of ~600K.

Magnetic semiconductor materials can be produced in a controllable way by inducing oxygen vacancies in TiO2 films

http://www.iop.org/EJ/abstract/0953-8984/18/33/S09

http://www.iop.org/EJ/abstract/0953-8984/18/27/L01


Condensed Mat te r : Top Papers 2006 showcase 5

Coherence of spin qubits in silicon

Semiconducting systems are obviouscandidates for quantum computers becauseof the huge base of knowledge andexperience built up through their use inclassical electronics. Many different statesinside a semiconductor have been proposedas qubits. One possible approach is to usespins as qubits: of nuclei, of electrons, orboth in combination.

A recent paper by S A Lyon (Princeton) andco-workers reviews progress inunderstanding spin relaxation for P donorsin natural and 28Si-purified silicon. In naturalSi the spin relaxation is strongly affected byspectral diffusion due to presence of 4.7%29Si magnetic nuclei which decohere theelectron spin in less than 0.5 ms. Very longrelaxation times (extrapolating to T2e = 60ms) have been found in isotopically pure 28Si.They have also demonstrated that the spinstates of both the electron and nucleus of a31P donor can be effectively controlled usingresonant microwave and RF pulses.

The bang-bang decoupling pulse protocolhas been successfully implemented through

the advantage of having coupled electronand nuclear spins in the donor. There are stillmany questions remaining to be answered.The implementation of two-qubit gates willrequire advanced processing, but the effectson spin coherence of this processing and oflocating the spins in device structures is notyet known, for example. However, work ondonor spins in silicon has established thatthis system can be considered a promisingcandidate for a future solid state quantuminformation processing technology.

Coherence of spin qubits in siliconA M Tyryshkin, J J L Morton,S C Benjamin, A Ardavan, G A D Briggs,J W Ager and S A Lyon.J. Phys.: Condens. Matter 18 (2006) S783–S794

This article is part of a special issue on quantuminformation processing. To find out more aboutquantum information processing, see Journal ofPhysics: Condensed Matter Vol 18, No 21.

(A) Free Rabi nutation of the 31P nuclear spin driven by a long RFpulse at �n1 can be stopped at will and locked into a particular spinstate (B) by applying a burst of the closely spaced 2� microwavepulses (�e1) to the electron spin. The position of the microwave pulsesis schematically shown with vertical arrows. The nuclear spinnutation can then be released for further evolution to be locked lateron in the opposite state (C) with a second burst of microwave pulses.

P donors in isotopically pure 28Si have sufficiently long spin relaxation times to be candidates for qubits in a quantum computer

Complementary media of electrons

Pendry and Ramakrishna introduced theconcept of a complementary medium - amaterial with permittivity �c =�� andpermeability µc = �µ, where � and µ are thepermittivity and permeability of the originalmedium - which reproduces the informationof amplitude and phase of electromagneticwaves in the original medium. This is theorigin of negative refraction and perfectlenses.

Katsuyoshi Kobayashi of OchanomizuUniversity, Japan, has extended theformulation of complementary media toelectron waves. He defines thecomplementary medium as having a transfermatrix which is the inverse of that of theoriginal medium.

He describes an application to subsurfaceimaging in scanning tunneling microscopy.He simulates the impurity images for asimple subsurface impurity and an impurityhindered from imaging by an obstacle andshows that the resolution can be improvedusing complementary media.

He extends the formulation ofcomplementary media to include interfacescattering. Because the electron wavelengthis of the order of atomic distances, the atomicstructure of the interface is important. The

extended formulation makes it possible toform effectively complementary systemsonly by tuning transmission properties atinterfaces.

Practical realization of complementarymedia for electrons is a difficult problem tobe solved in future, but Kobayashi has shownthat it is theoretically possible.

Complementary media of electronsKatsuyoshi KobayashiJ. Phys.: Condens. Matter 18 (2006) 3703–3720

The concept of complementary media, analogous to negative refractive index materials, for electron waves is feasible

Interface structures forming complementary systems of the graphitic lattice. They either interchange the A and B sublattices of the graphiticlattice while keeping the waves at points K and K0 in the Brillouin zone or vice versa. Closed circles show sites with only two �bonds.


http://www.iop.org/EJ/abstract/0953-8984/18/15/016



How life (un)folds?

A recent paper by Timothy R Lezon, JayanthR Banavar (Penn State) and Amos Maritan(Padova) ties together the biology of proteinswith a novel phase of matter that proteinstructures are housed in. The key idea is thatliving matter is governed by physical law andunderstanding the underlying physics is vitalfor progress.

All living organisms rely upon networks ofmolecular interactions to carry out their vitalprocesses. In order for a molecular system todisplay the properties of life, its constituentmolecules must themselves be endowedwith several features: stability, specificity,self-organization, functionality, sensitivity,robustness, diversity, and adaptability. Theauthors reflect upon how these molecularneeds are met by known organisms, and theydiscuss properties of conventional phases ofmatter that make them poorly suited for

supporting life. They postulate a novelmolecular phase of matter and demonstrate

that proteins, the functional molecules ofterrestrial life, are perfectly suited to thisphase. They explore, through anunderstanding of this phase of matter, thephysical principles that govern the operationof living matter. The new ideas presented inthis paper pertain to how advances innanoscience and supramolecular chemistrycan be harnessed for powerful applicationsand indeed for the ultimate development ofphysically-based artificial life itself.

This paper is a clear tutorial that is easilyaccessible to a broad audience and isspecially suited to condensed matterphysicists.

The origami of lifeTimothy R Lezon, Jayanth R Banavar andAmos MaritanJ. Phys.: Condens. Matter 18 (2006) 847–888

Ribbon views of haemoglobin, which contains four chains folded into� helical domains.

This paper explores how networks of molecular interactions can have given rise to life

Negative refraction in multilayers

There is currently much interest in left-handed and negative refractive indexmaterials. In left-handed materials thedirection of energy flow of anelectromagnetic wave is opposite to thewavevector. In negative refractive indexmaterial the component of the averagePoynting vector of an electromagnetic wavetangential to the interface changes sign afterrefraction.

S T Chui (University of Delaware) and co-workers have investigated multilayerstructures as negative refractive index andleft-handed materials. Multilayer structureshave two advantages: firstly, the propagationof electromagnetic waves can be calculatedexactly, allowing assessment of differentapproximations. Secondly, metal-insulatormultilayers are highly anisotropic, acting as ametal along the planes and an insulatorperpendicular to the planes, so providing awide range of incident angle within whichnegative refraction will occur.

A condition for negative refraction is thatthe interlayer spacing is much less than thewavelength, so the wave is not confined in asingle insulating region confined by the

metallic layers. This condition can be easilysatisfied at both microwave and infraredfrequencies.

Negative refraction comes about becausethe group velocity and the Poynting vectorhave a large component parallel to the layers,no matter what the angle of incidence of theincoming radiation is. This behaviour in turncomes from the large anisotropy of the phasevelocities. If one of the components is aferromagnetic metal, the system can be aleft-handed material above theferromagnetic resonance frequency.

Multilayer structures as negativerefractive and left-handed materialsS T Chui, C T Chan and Z F LinJ. Phys.: Condens. Matter 18 (2006) L89-L95

Another new class of negative refractive-index metamaterials made of metallicspheres arranged in a three-dimensionallattice of helicoidal symmetry is reported byVassilios Yannopapas (Patras).

The studied metamaterial possessesseveral frequency bands which give rise tonegative refraction. The proposed structuresconstitute a viable solution to realizingoptical metamaterials since they can exhibitnegative refraction in the frequency region ofthe surface-plasmon excitations of noblemetals.

Negative index of refraction in artificialchiral materialsVassilios YannopapasJ. Phys.: Condens. Matter 18 (2006) 6883–6890

The geometry of refraction from the multilayer.

Metal-insulator multilayer systems can show negative refractive index behaviour





Wetting problem solved?

The nature of the wetting transition of theIsing model is a long-standing unsolvedproblem. This phase transition occurs whenone of two coexisting phases (think liquidand vapour) is energetically preferred by theconfining walls of the system. The wettingtransition has deep implications since it isintrinsically linked to the vanishing of thecontact angle formed by the drop of one ofthe phases (liquid, say). The hard question is‘Does the wetting transition occurcontinuously or abruptly?’ or ‘What is theorder of the wetting transition for the threedimensional Ising model?’. This problem isparticularly appealing since the dimension d = 3 is the marginal case separating themean-field regime (d > 3) from thefluctuation dominated regime (d < 3), and theinfluence of fluctuations is not an easymatter to assess.

An exact solution of the 3D Ising model is

prohibitively difficult and theorists haveresorted to coarse-grained descriptionsbased on ‘interfacial models’ characterizedby a surface tension and an effectivepotential W. The predictions of this coarse-grained treatment are, however, completelyat odds with simulation studies of the Isingmodel.

A O Parry (Imperial College London) andco-workers in Madrid and Seville derive acoarse-grained description which appears toconsistently resolve this, and othercontroversies.

In this theory, the potential W has the aboveelegant diagrammatic representation whichaccounts for missing physics in the originalinterfacial description and allows one to ‘see’the shape of a free-energy.

The model also opens the door to more

systematic studies of wetting atmicropatterned and sculpted substrates,relevant to nanofluidics, which were notpossible in previous descriptions.

A Viewpoint article based on this articleappears in the same issue (J. Phys.: Condens.Matter 18 (2006) V11).

Parry et al expect to submit part II of thiswork to Journal of Physics: Condensed Matter in2007.

Derivation of a non-local interfacialHamiltonian for short-ranged wetting: I.Double-parabola approximationA O Parry, C Rascón, N R Bernardino and J MRomero-EnriqueJ. Phys.: Condens. Matter 18 (2006) 6433–6451

A new coarse-grained description of wetting in the 3D Ising model accounts for missing physics in the original interfacialdescription

Nucleation of nano-domains

A skyrmion is a quasiparticle correspondingto topological twists or kinks in a spin space.M Dawber (Geneva), A Gruverman (NorthCarolina State University) and J F Scott(Cambridge) have used skyrmions to modelhigh-field nucleation in ferroelectrics andferromagnets and test predictions regardingnucleation sites with new experimental data.

This nonlinear, defect-free model isqualitatively different from all knownmodels of ferroelectric nucleation andpropagation; nucleation in ferroelectrics hasbeen viewed as inhomogeneous, initiated atstatic impurity or defect sites that are fixed inspace; the present model is alsoinhomogeneous but involves nucleation atexisting domain walls, which are dynamicand not fixed in space. This defect-freeferroelectric nucleation model contrastswith the frequently invoked mechanism inthin-film switching of nucleation atelectrode–dielectric interfaces and thus hassignificant implications for the ultimateswitching speed in thin-film memorydevices.

Nucleation of ferroelectric domains inother systems has been shown to stronglyfavour nucleation sites on existing antiphaseboundaries rather than at impurity sites orelectrode interfaces. The rapid nucleation ofnano-domains in front of an advancingmacroscopic domain wall will produce asnow-plough effect that at high fields could

mimic supersonic domain wall velocities,whereas real phase velocities of domainwalls can remain subsonic.

Atomic force microscopy experimentshave supported the basic prediction of theskyrmion model, that nucleation, albeitinhomogeneous, does not occur atreproducible sites and hence is not defectinitiated.

Skyrmion model of nano-domainnucleation in ferroelectrics andferromagnetsM Dawber, A Gruverman and J F ScottJ. Phys.: Condens. Matter 18 (2006) L71–L79

J F Scott has also published a topical reviewof the physics of submicron ferroelectrics,describing the application considerations formemory devices (both as switching memoryelements for ferroelectric nonvolatilerandom access memories, FRAMs, and aspassive capacitors for volatile dynamicrandom access memories, DRAMs) as well asthe fundamental physics questionsregarding both the thickness and lateral sizeof present interest.

Nanoferroelectrics: statics and dynamicsJ F ScottJ. Phys.: Condens. Matter 18 (2006) R361–R386

Nano-domains ejected from the ferromagnetic domanin wall in iron garnet at different fields H and times t.

A new defect-free model of nucleation in ferroelectrics and ferromagnets has significant implications for the ultimate switchingspeed in thin-film memory devices

W = a1 + b1 +


http://www.iop.org/EJ/abstract/0953-8984/18/17/R02




In situ measurement of ZnO nanowires

Zinc oxide nanowires, being structurallyuniform and single crystalline, havepotential use as nanocantilevers andnanoresonators in nanoelectromechanicalsystems (NEMS), so it is important to studytheir mechanical resonance behaviour,especially their elastic bending modulus.

Measurement of the elastic bendingmodulus of a nanowire object with a lengthof a few micrometres is challenging becauseits small size prevents the use of establishedtechniques. A new experimental approach,based on electric-field-induced resonantexcitation, has recently been developed fordirect measurement of the mechanicalproperties of individual nano-objects suchas carbon nanotubes, silicon nanowires,silicon carbide–silica composite nanowires,and ZnO nanobelts by in situ transmissionelectron microscopy (TEM).

Yue Zhang of State Key Laboratory forSurface Physics in Beijing and co-workershave studied individual ZnO nanowires,grown by a solid–vapour phase thermalsublimation process. They studied themechanical resonances induced by analternating electric field, by an in situ TEM

method and characterized the mechanicalproperties, including elastic properties anddamping dissipation. They also built ananobalance and used it to measure the massof the nanoparticle attached at the tip of ananowire.

They concluded that the single crystalline,structurally controlled nanowires could beused as a new type of nanoresonator andnanocantilever, which is useful in NEMS andhighly functional nanodevices. The ZnOnanowires could also be used as functional

tips for scanning probe microscopy.A TEM method enables insitu

measurement of mechanical properties ofZnO nanowires.

In situ mechanical properties ofindividual ZnO nanowires and the massmeasurement of nanoparticlesYunhua Huang, Xuedong Bai and Yue ZhangJ. Phys.: Condens. Matter 18 (2006) L179–L184

Images showing mechanical resonances of nanowire of length 11.40 Tm, diameter 77 nm, fundamental frequency 269 kHz, bendingmodulus 59.7 GPa.

A TEM method enables in situ measurement of mechanical properties of ZnO nanowires

Testing DFT for surfaces

Density functional theory (DFT) is widelyused in surface science, but its predictionscan depend strongly on the approximationused for exchange-correlation energy. D Alféand M J Gillan (University College London)have tested recent suggestions that thegeneralized gradient approximation (GGA)is inferior to the local density approximation(LDA) for the surface formation energy � oftransition metals and oxides.

Their quantum Monte Carlo calculationsof � for the MgO(001) surface support theaccuracy of LDA, and indicate that GGA is~30% too low. This has importantimplications for nanoscience modelling.

In a further paper they present a techniquefor computing the absolute desorption rate �of adsorbate molecules from a surface forany coverage and temperature. Thetechnique is based on an exact expression for� in terms of the difference of non-configurational chemical potentials of gas-phase and adsorbed molecules. This

difference is expressed in terms of a potentialof mean force, which is computed byconstrained first-principles moleculardynamics.

They applied the technique to D2O on theMgO(001) surface at low coverage, using theGGA. Comparisons with experimental data

allow an assessment of the accuracy of theGGA.

This scheme can provide calculated valuesof frequency prefactors, which areimportant in interpreting experimentalresults, and provides a way of testing DFTexchange-correlation functionals againstexperiment. These methods also provide away of calculating adsorption isotherms byDFT simulation, which should provideanother way of probing the accuracy of DFT.

The energetics of oxide surfaces byquantum Monte Carlo. D Alfé and M J GillanJ. Phys.: Condens. Matter 18 (2006) L435–L440

Absolute rate of thermal desorptionfrom first-principles simulationD Alfé and M J GillanJ. Phys.: Condens. Matter 18 (2006) L451–L457

Mean force on water O atom against its height (relative to a fixedatom in the centre of the slab) above the surface at 100, 300, 600 and800 K (solid, dotted, chain and dashed curves, respectively). Barsshow statistical errors.

Monte Carlo simulations test different approximation schemes in DFT for metal and oxide surfaces and for desorption






Coherent phonons in semimetals

O V Misochko (Institute of Solid StatePhysics, Moscow), K Ishioka and M Kitajima(NIMS, Tsukuba) and Muneaki Hase(Tsukuba University) have performed acomprehensive investigation of the coherentphonons of different symmetry in thesemimetals bismuth and antimony undervarious temperature and excitationconditions.

Decreasing temperature can increase thenumber of phonon modes observed in time-domain since at low temperature and anyexcitation level, both A1g and Eg coherentphonons appear in time-resolvedreflectivity. The fact that Eg coherentphonons disappear at room temperature,whereas A1g phonons do not, can beexplained by a smaller thermal population ofthe latter mode.

The relative initial phase of the A1g and Eg

oscillations in bismuth testifies that the fullysymmetric mode is excited for the most partdisplacively, whereas the doubly degeneratemode is driven impulsively, suggesting thatthe A1g and Eg phonons respectively coupleto real and virtual charge-densityfluctuations.

At high excitation level, both modes arecoupled and behave in a similar way: aninitial fast decay of oscillations is followed byweak recurrence with much slower decay,but the characteristic times of the recurrenceare shorter for the doubly degeneratephonons.

Fully symmetric and doubly degeneratecoherent phonons in semimetals at lowtemperature and high excitation:Similarities and differencesO V Misochko, K Ishioka, Muneaki Hase andM KitajimaJ. Phys.: Condens. Matter 18 (2006)10571–10584

Coherent oscillations in bismuth (after the removal of non-oscillatingbackground) demonstrating collapse and revival in Bi at T = 7 K andF = 9.5 mJ/cm2.

Wavelet transform of the collapsing and reviving oscillations inbismuth.

At a high level of excitation, phonons in semimetals show interesting quantum interference effects

A new relaxor ferroelectric

Pb-based perovskites with the generalformula Pb(B’xB"1�x )O3 are typical relaxorferroelectrics and have importantapplications in optical modulators andinformation storage.

V Marinova (Hamburg) and co-workershave synthesized single crystals of a newrelaxor compound with chemical formulaPb0.78Ba0.22Sc0.5Ta0.5O3 (PBST). Its frequencydependence of the dielectric constant showsa strong dielectric dispersion over a widetemperature range and a dielectric-constantmaximum near 200 K at 10 kHz.

They probed the local atomicenvironment by Raman scattering andoptical absorption spectroscopy. The resultsshow that the incorporation of Ba deformsthe BO6 octahedra adjoining the BaO12

polyhedra along the <111> direction andshortens the Pb-O bond lengths next to theBaO12 polyhedra within the {111} planes.The random substitution of Ba for Pb leads toa wider distribution in the size and shape ofthe ferroic species in PBST compared withstoichiometric PbSc0.5Ta0.5O3. As a result, thenon-ergodic ‘true-relaxor’ state is enhancedon account of the proper long-range orderedferroelectric state, which considerably

broadens the dielectric-constant maximumand modifies the optical absorption edge.

The addition of Ba shifts the opticalabsorption edge to lower energies and givesrise to extra absorption peaks at 460 and 730nm. The latter peak is related to polar atomicrearrangements in the vicinity of the Ba ionsembedded into the PbSc0.5Ta0.5O3 matrix.

Structural, optical and dielectricproperties of relaxor-ferroelectricPb0.78Ba0.22Sc0.5Ta0.5O3

V Marinova, B Mihailova, T Malcherek,C Paulmann, K Lengyel, L Kovacs, M Veleva,M Gospodinov, B Güttler, R Stosch andU Bismayer J. Phys.: Condens. Matter 18 (2006) L385–L393

The atomic environment of an A-site cation and of a B-site cation neighbouring two different A-site cations. The swelling of the AO12

polyhedron when Ba substitutes for Pb should result in a shortening of the adjacent Pb–O bonds in the plane perpendicular to the <111>direction as well as to a deformation of the adjacent BO6 octahedra along the <111> direction.

Barium substitution enhances the properties of a relaxor ferroelectric for applications





Theory of glasses in transition?

The driving force behind the glass transitionis very controversial. It could be mainly afree-volume (or density, or pressure)phenomenon, since glass formers aretypically dense liquids where core-repulsioneffects are large, or it could be a dominantlyenergetic (temperature-driven)phenomenon, as it clearly is in bonding-driven network-forming glasses such assilica. This question is now being addressedin high-pressure experiments but the resultsare far from equivocal.

Th Voigtmann and Wilson Poon(University of Edinburgh) have suggestedthat conventional molecular glasses andcolloidal glasses have in common thephysical interpretation of the way their glasstransition changes with pressure andtemperature. This opens a connectionbetween the two kinds of systems that hasreceived little attention: it suggests that one

can use colloidal systems, taking advantageof the fine control they offer over theinteraction potential, to gain insight intosome open questions in the field ofmolecular glasses. As a first example, theyestimate the minimum pressure needed toreveal a properly density- or pressure-drivenglass transition in molecular glass formers.Their estimate is 10 GPa, higher than thepressures reached in previous dynamicexperiments at high viscosities, explainingwhy the controversy is still alive. Extremely-high-pressure experiments on molecularglass-forming liquids, accessing this regime,would be of great value.

Glasses under high pressure: a link tocolloidal science?Th Voigtmann and Wilson C K PoonJ. Phys.: Condens. Matter 18 (2006) L465–L469

The glass transition in the pressure–temperature plane for hardspheres with square-well attraction: * experimental data fromcolloid– polymer mixtures (the hard-sphere data point is indicated byan arrow to T �); other symbols are experimental data for small-molecular and polymeric glass formers. Lines are mode-couplingtheory predictions for the square-well system. The dashed lineindicates the soft-sphere asymptote, P � T 5/4. Inset: MCT lines in theusual density–attraction-strength representation. Points 1–3correspond in the two plots.

Experiments at extremely high pressure are proposed to resolve the controversy over the driving force behind the glass transition

Fighting cancer with nanoparticles

Two recent papers describe the use ofmagnetic nanoparticles in treating cancer,either as magnetic targetable carriers fordrug delivery systems or for localhyperthermia.

Biocompatible nanosized drug deliverysystems for specific targeting of therapeuticsare being developed, especially for thetreatment of cancer and diseases of thevascular system. In an experimental cancermodel, R Jurgons (Erlangen) and co-workersperformed targeted drug delivery and usedmagnetic iron oxide nanoparticles, bound toa chemotherapeutic agent, which wereattracted to an experimental tumour inrabbits by an external magnetic field(magnetic drug targeting). Complete tumourremission could be achieved. An importantadvantage of these carriers is the possibilityfor detecting these nanoparticles aftertreatment with common imagingtechniques (i.e., x-ray-tomography,magnetorelaxometry, magnetic resonanceimaging), which can be correlated tohistology.

Rudolf Hergt and co-workers in Jena havestudied loss processes in several types ofmagnetic iron oxide nanoparticles tooptimize the specific loss power (SLP) forapplication in tumour hyperthermia. Inmagnetically mediated hyperthermia,magnetic material is deposited in the tumourand heated by an external alternatingmagnetic field.

They studied the dependence of the SLP onthe mean particle size over a broad range

from superparamagnetic up to multidomainparticles. Particles with mean size 18 nm anda narrow size distribution proved mostuseful. In particular, very high heating powermay be delivered by bacterialmagnetosomes. Even higher power may beachievable by metallic magnetic particles.

These papers are part of a special issue onferrofluids. For more information on othermedical applications and all other aspects offerrofluids, see Journal of Physics: CondensedMatter Vol 18, No 38.

Drug loaded magnetic nanoparticles forcancer therapyR Jurgons, C Seliger, A Hilpert, L Trahms, SOdenbach and C AlexiouJ. Phys.: Condens. Matter 18 (2006)S2893–S2902

Magnetic particle hyperthermia:nanoparticle magnetism and materialsdevelopment for cancer therapyRudolf Hergt, Silvio Dutz, Robert Müller andMatthias ZeisbergerJ. Phys.: Condens. Matter 18 (2006)S2919–S2934

Radioactive 59Fe-distribution 60 min after intra-arterial 59Fe-ferrofluid application; with (dark) and without (light) external magnetic field.114 times more activity could be detected in the tumour region after magnetic drug targeting than without a magnetic field.

Magnetic nanoparticles can be used in cancer therapy either for drug delivery or for destroying the tumour by heating






Space elevator: not all it’s cracked up to be?

A space elevator has been proposedconsisting of a cable about 105 km long fixedgeosynchronously to the Earth’s surface as arevolutionary method of carrying payloadsinto space.

The cable requires a material with veryhigh strength and low density. Themaximum stress at the geosynchronousorbit for a material with the density ofcarbon is 63 GPa. Only recently, since thediscovery of carbon nanotubes, has such alarge strength been experimentallyobserved: ropes composed of single- ormultiwalled carbon nanotubes are expectedto have an ideal strength of about 100 GPa.

Nicola M Pugno of Turin Polytechnicargues that the presence of even a fewvacancies in a single nanotube play adramatic role. In such a huge cable pre-existing defects are expected both forstatistical reasons and as a consequence ofdamage nucleation, e.g. due tomicrometeorite or low Earth orbit object

impacts and atomic oxygen erosion. Hereviews the mechanics of the cable, andconsiders the effect on the strength of thetypes of damage mentioned. He predicts that

the strength of a megacable is reduced by afactor of at least 70% compared with thetheoretical nanotube strength, sufficient toplace in doubt the feasibility of the spaceelevator which he claims would break. This isconfirmed by experiments and atomisticsimulations based on molecular or quantummechanics.

On the strength of the carbon nanotube-based space elevator cable: fromnanomechanics to megamechanicsNicola M Pugno J. Phys.: Condens. Matter 18 (2006)S1971–S1990

This article is part of a special section onnanoscience and nanotechnology; see Journal ofPhysics: Condensed Matter Vol 18, No 33 formore papers in this area.

(a) Strength of defective SWCNT versus hole size (QFM, dashed line;rhombi, atomistic simulations), or versus crack length (QFM,continuous line; points, atomistic simulations). (b) Defect andfracture quantum sizes: n = 2 denotes a nanocrack formed onremoving two adjacent chemical bonds, m = 1 denotes a nanoholeformed on removing the first carbon atom perimeter (six atoms); theapplied tensile load is vertical.

It is argued that a proposed space elevator cable made of carbon nanotubes would contain enough defects to cause it to break

Quantitative description of exchange bias

The exchange bias system refers to the shiftof the ferromagnetic (F) hysteresis loop topositive or negative values when the F systemis in contact with an antiferromagnetic (AF)system and cooled in an applied magneticfield through the Néel temperature of the AFsystem. Exchange bias is associated with theinterfacial exchange coupling between F andAF spin structures, resulting in aunidirectional magnetic anisotropy, but theorigin of the enhanced coercive field is notyet well understood. The details of theexchange bias effect depend crucially on theAF/F combination chosen and on thestructure and thickness of the films.

Florin Radu and co-workers at RuhrUniversity, Bochum show that interface spindisorder is the main reason for thediscrepancy between model calculations andexperimental results. Taking into accountspin disorder at the interface between the AFand F layers, they achieved a compellingagreement between the azimuthaldependence of the coercivity and exchangebias field in the IrMn/FeCo systems.

The new key physical concept is a realisticstate of the interface characterized by areduced AF anisotropy. This disordergoverns the enhanced coercivity in theferromagnetic layer and reduces theexchange bias field to realistic values. They

believe that this is a general feature ofexchange bias systems. By controlling thedegree of spin disorder and the thickness ofthe interfacial layer, better control over theexchange bias of magnetic heterostructurescould be achieved.

Quantitative description of theazimuthal dependence of the exchangebias effectFlorin Radu, Andreas Westphalen, KatharinaTheis-Bröhl and Hartmut ZabelJ. Phys.: Condens. Matter 18 (2006) L29–L36

Azimuthal dependence of the coercive fields Hc1 and Hc2 (a) from the experimental hysteresis loops and (b) calculated. The coercive field andexchange bias field as a function of the azimuthal angle (c) experimental and (d) simulated.

Taking into account spin disorder at the interface accounts for experimental results for exchange bias





Stretching proteins in a force-clamp

Atomic force microscopy (AFM) is used toprobe biomolecules by stretching at aconstant speed. The force of resistance tostretching yields information about theirelastic structure. A new variant of AFM, aforce-clamp, allows one to maintain aconstant pulling force on the protein whilemonitoring the end-to-end distance as afunction of time. For polyubiquitin, thelength of the protein increased in a step-wisemanner with each step corresponding tounwinding of a single ubiquitin chain inagreement with a simple, i.e., two-state all-or-none, model of unfolding. The ensemble-averaged length of a protein is well describedby a single exponential with a characteristictime whose logarithm depends linearly onthe force.

P Szymczak and Marek Cieplak (Warsaw)have investigated the generality of theseresults. They show that the averageunfolding time needs not be exponential(especially at large forces) and they providean example of a protein with more than twosteps of unfolding at low force. A coarse-grained model of ubiquitin has log-normalstatistics above a threshold force andexponential statistics below it.

Correspondingly, the mean unfolding time isslowly varying and decreases exponentiallywith the force. The time dependences of theend-to-end distances are also distinct. Thetime sequence of unfolding events dependsweakly on force and much of it resemblesthat for stretching at constant speed. A morecomplicated time dependence is found forintegrin.

Stretching proteins in a force-clampyields information on the statistics ofunfolding eventsP Szymczak and Marek CieplakJ. Phys.: Condens. Matter 18 (2006) L21–L28

Left: three trajectories for integrin at various forces. The lower two trajectories lead to full unfolding at much longer time than the scale in thefigure. Right: force versus displacement traces for stretching at constant speed for ubiquitin (shifted upward by 1.7 to avoid overlap) andintegrin. For this trace, Fmax is near 2.45.

Stretching proteins in a force-clamp yields information on the statistics of unfolding events

Light damage in a-Si:H

Hydrogenated amorphous silicon (a-Si:H) isan important material for solar photovoltaic(PV) devices. It is inexpensive and serves animportant niche in energy markets. Animpediment to its use is the so-calledStaebler–Wronski effect (SWE) (the light-induced creation of carrier traps causingreduced energy conversion efficiency).Exposure of a-Si:H to intense light leads tostructural changes, with a serious impact onthe performance of a-Si:H cells.

T A Abtew and D A Drabold (OhioUniversity) have used ab initio moleculardynamics to simulate the SWE. They obtainimproved microscopic understanding ofphotovoltaic operation, compute the motionof H atoms, and modes of light-induceddegradation of photovoltaics.

Highly accurate simulations in a light-induced state lead to the formation of a classof new configurations, consistent with (a)recent NMR experiments and the authors’previous studies, and (b) the hydrogencollision model of Branz and other paired-Hmodels. In contrast, simulations in theelectronic ground state do not exhibit thetendency to paired-H final states. For the first

time, they show the detailed dynamicpathways that arise from light-inducedoccupation changes, and provide oneexplicit example (see figure) of defectcreation and paired-H formation.

This work should be extended in a numberof ways, including: exploration of longertime scales, large models, and a thoroughstudy of possible photo-excitations and thestatistics of associated events.

Atomistic simulation of light-inducedchanges in hydrogenated amorphoussiliconT A Abtew and D A DraboldJ. Phys.: Condens. Matter 18 (2006) L1-L6

Motion of key atoms from photo-excited ab initio MD simulation ofa-Si:H. Small atoms depict H and large atoms represent Si. (lightblue: ‘typical’ Si atoms; white: ‘typical’ H atoms; red: H atoms thateventually pair; green: initial Si DB sites; dark blue: Si atoms thatend as DB.) (a) Initial conformation, (b) one H dissociates and formsa DB, (c) mobile H attaches to a Si DB, (d) the other (red) H becomesmobile, (e) atomic rearrangements near defect sites, (f) SiH2 isformed.

Structural changes from exposure of a-Si:H to intense light have been successfully simulated





Landau theory corrected

Landau theory is commonly used to describethe thermodynamic behaviour ofpolydisperse heteropolymer systems. Allcalculations up to now have retained onlythe fourth-order non-local term in the freeenergy expansion. S I Kuchanov and S VPanyukov (Moscow) have shown that theneglect of some non-local terms of sixthorder is responsible for the erroneous resultsreported.

In particular they have shown that theaddition of two such terms, ignored in allpreceding publications, is indispensable.This assertion stems from the inspection ofthe phase diagram they calculated for themelt of binary Markovian multiblockcopolymers. It differs markedly from thephase diagram constructed earlierdisregarding these terms (see figure).

Key results they obtained for Markovianheteropolymers can be verified for otherpolydisperse multiblock copolymers, sincetheir formulae hold at arbitrary values of theparameters of the vertex functions inexpansion of the Landau free energy.

The central idea of Kuchanov andPanyukov far exceeds the limits of the

construction of the phase diagram ofheteropolymer liquids. This idea may wellfind application when considering thethermodynamic behaviour of varioussystems with ‘quenched’ structural disorderin terms of the Landau theory of phasetransitions.

A correct account of the non-local termsin the Landau theory of phase transitionsin polydisperse heteropolymersS I Kuchanov and S V PanyukovJ. Phys.: Condens. Matter 18 (2006) L43–L48

Phase diagram of the melt of binary Markovian copolymers: (a) calculated by Kuchanov and Panyukov and (b) reported earlier. Lines separatehomophase regions (coloured white) from two-phase ones (coloured grey).

The phase behaviour of certain polymers depends crucially on certain non-local terms in the Landau free energy expansion thathave been neglected before

Separating particles by their surface properties.

Electrophoresis and sedimentation arepowerful means for separating particles,proteins, and DNA, via differences in particlecharge, mass, and size. Surface properties ofcolloids and proteins are closely related totheir physical, chemical, and biologicalfunctions, so the selection of particles bytheir surface properties is highly desirable.

Takeaki Araki and Hajime Tanaka (Tokyo)report a new principle of surface-sensitiveparticle selection by using nematic liquidcrystal as a solvent. When colloidal particlesare immersed in a nematic liquid crystal, thedirector field of the nematic solvent aroundthem is distorted due to the anchoring on theparticle surface. The elastic couplingbetween particles and nematic liquid crystalleads to rich static and dynamic behaviourwhich may be used for surface-sensitive

particle separation. Using a special type oflyotropic liquid crystal that is water-basedbut non-surfactant and non-toxic, it may bepossible to disperse colloidal particles orbiological molecules in a liquid crystalhomogeneously.

Numerical simulations incorporatingboth elastic and nematohydrodynamiccouplings show that the surface anchoringproperties change both direction and speedof motion of a particle driven in an orientednematic liquid crystal. This principle may beused for separating particles in terms of theirsurface properties.

Surface-sensitive particle selection bydriving particles in a nematic solventTakeaki Araki and Hajime TanakaJ. Phys.: Condens. Matter 18 (2006) L193–L203

Surface-sensitive particle separation. Particles with differentanchoring strengths are dragged by an external force F = 6 pN (a)and F = 20 pN (b) in a nematic liquid crystal aligned such that E |_F.The anchoring strengths of the blue, green, and red particles (bottom to top) are W = 1.5, 2.0 and 2.5 x 10-4 N m -1,respectively.

Particles, proteins and DNA may be separated by their surface properties using a liquid crystal as solvent





Liquid silicon under pressure

Upon melting, silicon undergoes asemiconductor–metal transition, its localstructure changing from fourfold toapproximately sixfold coordination.

A Delisle and M J Stott (Queen’sUniversity, Canada) and D J González(University of Valladolid) have investigated l-Si using orbital-free ab initio moleculardynamics (OF-AIMD) combined with a first-principles local pseudopotential.

Their results for the static structurequalitatively follow the experimental trendswhen the pressure is increased from 4 to 23GPa, namely an increase of intensity andposition of the S(q)'s main peak, along with aprogressive vanishing of its shoulder. Thestatic structures of l-Si from the triple pointup to 8 GPa are very similar, but between 8and 14 GPa it transforms to a denserstructure typical of a liquid simple metal.

The structural changes are also reflected inthe dynamic structure factors and the

transverse current correlation. Thecalculated self-diffusion and shear viscosity

transport coefficients are also affected by thestructural changes occurring between 8 and14 GPa.

These results for the static and dynamicproperties of compressed l-Si underscore thecapability of the OF-AIMD method to tackleliquid systems encompassing a range ofbonding and structure which evolves frommild remnants of covalent bonding to ametallic one. Further improvements will befocused on developing more accurateelectron kinetic energy functionals and localionic pseudopotentials.

Pressure-induced structural anddynamical changes in liquid Si–an abinitio studyA Delisle, D JGonzález and M J StottJ. Phys.: Condens. Matter 18 (2006) 3591–3605

Static structure factor of l-Si at different high pressures: full circles:experimental x-ray diffraction data; continuous line: OF-AIMDsimulations.

An ab initio study accounts for experimentally found pressure-induced structural and dynamical changes in liquid silicon

Decomposing food

Colloidal suspensions are ideal modelsystems to study fundamental issues such asliquid ordering, crystallization and glassformation. Food is a classical example ofcomplex soft condensed matter.

Suresh Bhat and Peter Schurtenberger(University of Fribourg) and Remco Tuinier(Jülich) have investigated phase separationand structural evolution in a complex foodcolloid (casein micelles) and biopolymer(xanthan) mixture using small-angle lightscattering. They show that phase separationis induced by a depletion mechanism, andthat the resulting coexistence curve can bedescribed by osmotic equilibrium theory formixtures of colloids and polymer chains in abackground solvent, taking into accountinteractions between the polymer chains inthe excluded volume limit. The lightscattering pattern of an unstable mixtureexhibits the typical behaviour of spinodaldecomposition, confirming the validity ofdynamic similarity scaling.

They find three distinct regimes for the

decomposition kinetics that differ in the timedependence of the peak position of thestructure factor. In the initial linear regime,the peak position remains constant and theamplitude grows. The existence of spinodal-like decomposition and the validity ofuniversal scaling in the intermediate andtransition stages have been found inprevious studies, but to the authors’knowledge the initial linear regime has neverbeen observed in colloidal suspensions.They attribute this at least partly tohydrodynamic interactions which areefficiently screened in their system as themeasurements were performed at highpolymer concentrations, i.e. in the semi-dilute regime.

Spinodal decomposition in a foodcolloid–biopolymer mixture: evidencefor a linear regimeSuresh Bhat, Remco Tuinier and PeterSchurtenbergerJ. Phys.: Condens. Matter 18 (2006) L339–L346

Evolution of the characteristic domain size obtained from theposition qmax(t) of the peak of the scattered intensity as a function oftime. The inset shows the time evolution of the logarithm of thescattered intensity I (q, t) at qmax.

Spinodal decomposition in a food colloid–biopolymer mixture shows evidence for a linear regime



Condensed Matter: Review Articles


Quantum IT clouded by uncertainty

The research fields of quantum informationprocessing and communication are now wellestablished, although still growing anddeveloping. A Viewpoint article by T P Spillerand W J Munro of Hewlett-PackardLaboratories, Bristol discusses how aquantum information technology (QIT)industry may develop in the future.

As electronic devices become smaller, theywill suffer errors from quantum fluctuationsso, unless we can control these effects,nanoscale conventional IT devices will fail towork. As a result, IT companies areresearching quantum effects, and this couldhave a spin-off in a revolutionary new QIT,based on storing, processing andcommunicating information according tothe laws of quantum physics, utilizing effectssuch as superposition, entanglement andquantum measurement.

Spiller and Munro discuss the issues toconsider with regard to the start of a QITindustry, including the need for a market for

QIT. Building a factoring machine in orderto break ‘secure’ communications is a much-quoted example. Other possible areas arequantum communication (key exchange,cryptography, etc), quantum(-improved)sensing and detecting (i.e. quantummetrology), and quantum simulation.

They then consider some of the candidatetechnologies and associated problems such

as interconversion between travellingoptical qubits and static (presumably matter-based) qubits, and processing qubits. Theyconclude that there is currently noconsensus on the best route to scalablequantum computing.

There is now a growing expectation of realquantum information technology. Certainlythere are some promising avenues forseeding and growing a QIT industry.

Towards a quantum informationtechnology industryT P Spiller and W J MunroJ. Phys.: Condens. Matter 18 (2006) V1–V10

To find out more about quantum informationprocessing see special issue on quantuminformation processing, Journal of Physics:Condensed Matter Vol 18, No 21.

Moore’s Law, illustrated through the number of electrons peroperating device. This is decreasing exponentially as devices shrink insize. Quantum effects become increasingly important as thequantum regime is approached.

Issues affecting how a quantum information technology industry may develop in the future are discussed

1. High ImpactJPCM’s Impact Factor is on anupward trend. It is currently 2.154and as the journal continues toattract the very best submissionsis expected to rise further.

2. Fair refereeingWith a 2006 rejection rate of 51.5%,JPCM’s fair but rigorous refereeingensures the journal publishesresearch of the highest quality.

3. Global visibilityJPCM is available in over 1800 institutions and is read byresearchers at the top researchorganisations across the world.

4. Rapid publication processIn 2006 the median number of days from receipt to first decision was only 41 days.

FOUR REASONSWHY TOPAUTHORS PUBLISH IN JPCM

Do you want to know more? For more information about submitting to the journal, including full author guidelines, visit www.iop.org/journals/authorsubs

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Ultrafast magneto-optics

Junichiro Kono (Rice University) andcolleagues from Virginia Tech, UCSD,Florida University and Tokyo Institute ofTechnology have reviewed various ultrafastoptical processes in ferromagnetic (III, Mn)Vsemiconductors induced by femtosecondlaser pulses. Magneto-optical spectroscopycombined with femtosecond laser pulsesprovides direct time-domain informationabout magnetic properties of excited stateswith high temporal resolution and finecontrol.

They describe newly developed two-colour methods of MOKE and MCDspectroscopy and present experimentalresults on charge dynamics, phonondynamics, and spin dynamics.

Time-resolved two-colour transientreflectivity measurements with a mid-infrared pump and a single-colour near-infrared probe yield very short carrierlifetimes (~2 ps) and multilevel charge decaydynamics. Carrier relaxation of (III, Mn)Vsemiconductors show some uniqueproperties such as large-density mid-

bandgap states and heavy Mn p-doping.The transient reflectivity technique also

provides direct information about phonondynamics. An oscillatory component in thetransient reflectivity, whose period,amplitude and damping depend strongly onthe probe laser energy, is associated withpropagating coherent acoustic phononwavepackets. The CAP in (III, Mn)Vs issimilar to that in GaN/InGaN epilayers, but

its generation mechanism is via adeformation potential coupling mechanism,instead of screening of piezoelectric fields.

Both fast (<1 ps) and slow (~100 ps)demagnetization processes are observed inferromagnetic InMnAs and GaMnAs. Thefast dynamics is attributed to spin heatingthrough sp–d exchange interaction betweenphoto-carriers and Mn ions while the ~100ps component is interpreted as spin–latticerelaxation.

They also give a qualitative picture ofultrafast photoinduced softening inferromagnetic InMnAs.

Ultrafast magneto-optics inferromagnetic III–V semiconductorsJigang Wang, Chanjuan Sun, YusukeHashimoto, Junichiro Kono, Giti AKhodaparast, L ukasz Cywinski, L J Sham,Gary D Sanders, Christopher J Stanton andHiro MunekataJ. Phys.: Condens. Matter 18 (2006) R501–R530

Three reservoirs (carriers, spins, lattice) taking part in the light-induced demagnetization process. The descriptions in italics arespecific to the (III, Mn)V case. Spin–lattice and carrier–phonon arequite well understood and direct carrier–spin coupling is a subject ofcurrent research. In (III, Mn)V ferromagnetic semiconductors thecarrier–spin mechanism is the sp–d interaction between the spins ofdelocalized carriers and the localized Mn spins.

The use of magneto-optical spectroscopy in investigating spin-related phenomena in (III, Mn)V ferromagnetic semiconductors isreviewed

Single-molecule experiments in biological physics

Recent technological developments haveprovided the tools to design and buildscientific instruments of high enoughsensitivity and precision to manipulate andvisualize individual molecules and measuremicroscopic forces. Using single-moleculeexperiments (SMEs) it is possible tomanipulate molecules one at a time andmeasure distributions describing molecularproperties, characterize the kinetics ofbiomolecular reactions and detect molecularintermediates.

SMEs provide additional informationabout thermodynamics and kinetics ofbiomolecular processes that complementsinformation obtained in traditional bulkassays. In SMEs it is also possible to measuresmall energies and detect large Browniandeviations in biomolecular reactions,offering new methods and systems toscrutinize the basic foundations of statisticalmechanics.

F Ritort (Barcelona) has reviewed SMEs ata very introductory level. The reviewdiscusses SMEs from an experimentalperspective, first describing the mostcommon experimental methodologies andlater presenting various molecular systemswhere such techniques have been applied. Itbriefly discusses experimental techniquessuch as atomic-force microscopy, laser

optical tweezers, magnetic tweezers,biomembrane force probes and single-molecule fluorescence. It then presentsseveral applications of SME to the study ofnucleic acids (DNA, RNA and DNAcondensation) and proteins (protein–proteininteractions, protein folding and molecularmotors). Finally, it discusses applications ofSMEs to the study of the nonequilibriumthermodynamics of small systems and theexperimental verification of fluctuationtheorems and concludes with a discussion ofopen questions and future perspectives.

Single-molecule experiments inbiological physics: methods andapplicationsF RitortJ. Phys.: Condens. Matter 18 (2006) R531–R583

Experimental set-up for RNA pulling experiments using laseroptical tweezers.

Force-extension curve of RNA hairpin in a pulling cycle. During thestretching (red) the elastic response of the handles is followed by thesudden release of extension and a drop in the force corresponding tothe unfolding of the RNA molecule. During relaxation (blue) theRNA molecule folds back again.

Experimental techniques for manipulating and measuring single molecules and their applications are reviewed





Artificial nanomachines

Molecular machines are supramolecularstructures designed to perform a functionthrough the mechanical movements of itscomponents, which occur underappropriate external stimulation. Theyoperate via nuclear rearrangements andrequire an energy input. Through progressin chemistry, and better understanding ofbiological molecular machines, it hasbecome possible to design and constructsimple prototypes of artificial molecularmachines. Alberto Credi (Bologna) reviewsrecent progress and describes someexamples based on rotaxanes, catenanes,and related interlocked molecules.

The review covers: (i) the design andconstruction of more sophisticated artificialmolecular motors and machines; (ii) the useof such systems to do tasks such asmolecular-level transportation, catalysis,and mechanical gating of molecularchannels; and (iii) the possibility ofexploiting their logic behaviour forinformation processing at the molecularlevel and, in the long run, for theconstruction of chemical computers.

Apart from more or less futuristicapplications, the extension of the concept ofmotor and machine to the molecular level isof interest not only for the development ofnanotechnology, but also for the growth ofbasic research. Looking at molecular and

supramolecular species from the viewpointof functions with references to devices of themacroscopic world is indeed a veryinteresting exercise which introduces novelconcepts into chemistry as a scientificdiscipline.

Artificial nanomachines based oninterlocked moleculesAlberto Credi J. Phys.: Condens. Matter 18 (2006)S1779–S1795

This article is part of a special section on molecularnanomachines. To find similar related articles seeJournal of Physics: Condensed Matter Vol 18, No33.

Chemical formula (a) and cartoon representation (b) of the rotaxane 56+, showing its modular structure. Molecular models show that arotaxane molecule is approximately 5 nm long, and the diameter of the ring is about 1.3 nm.

Recent progress in constructing simple prototypes of artificial molecular machines is reviewed

Nanoemulsions: clearly superior

Extreme emulsification methods can be usedto produce nanoscale dispersions of dropletsof one liquid in another immiscible liquid.These microfluidic and ultrasonicapproaches of rupturing larger microscaledroplets into nanoscale droplets provideinteresting non-equilibrium systems ofstructured liquids. The physical properties ofnanoemulsions can be quite different fromthose of microscale emulsions.

Nanoemulsions hold great promise asuseful dispersions of deformable nanoscaledroplets that can have flow propertiesranging from liquid to highly solid andoptical properties ranging from opaque tonearly transparent. They will play anincreasingly important role commercially,since they can typically be formulated usingsignificantly less surfactant than is requiredfor nanostructured lyotropic microemulsionphases.

This review by T G Mason and co-workersat UCLA summarizes procedures forproducing ‘nanoemulsions’ comprised ofnanoscale droplets, methods for controllingthe droplet size distribution andcomposition, and interesting physicalproperties of nanoemulsions. In contrast tomore common microscale emulsions,nanoemulsions exhibit optical transparencyat high droplet volume fractions, �,surprisingly strong elasticity at low �, andenhanced diffusive transport and shelfstability. For these reasons, nanoemulsionshave great potential in a wide range ofindustries including pharmaceuticals, foods,and personal care products.

Nanoemulsions: formation, structure,and physical propertiesT G Mason, J N Wilking, K Meleson, C BChang and S M GravesJ. Phys.: Condens. Matter 18 (2006) R635–R666

Transmitted intensity, Itrans, of nanoemulsions having <a> = 40 nmin a 0.2 mm thick cell as a function of wavelength, �, in UV andvisible for a series of volume fractions. The high Itrans at visiblewavelengths indicates that the nanoemulsions are nearly transparent,yet they scatter significantly in the UV. Inset: 1 cm diameter glassvials contain a nanoemulsion that is nearly transparent in the visiblespectrum, and a microscale emulsion that appears white and opaquedue to multiple scattering of visible light.

Nanoemulsions have quite different physical properties from microscale emulsions, which gives them great potential in a widerange of industries





Simulating smart biomaterials

Biological membranes are ‘smart’ materialswhose properties emerge from thepropagation across many scales of themolecular characteristics of theirconstituents. Artificial smart materials, suchas drug delivery vehicles and biosensors,often rely on modifying naturally occurringsoft matter, such as polymers and lipidvesicles, so that they possess usefulbehaviour. However, the complexity ofnatural membranes, both in their staticproperties, exemplified in their phasebehaviour, and in their dynamic properties,as in the kinetics of their formation andinteractions, hinders their rationalmodification. Mesoscopic simulations, suchas dissipative particle dynamics, allow in silicoexperiments to be easily performed oncomplex, soft materials requiring as inputonly the molecular structure of theconstituents at a coarse-grained level. Theycan therefore act as a guide to experimentersprior to performing costly assays. Also,mesoscopic simulations provide the only

currently feasible window on the length- andtimescales relevant to important biophysicalprocesses such as vesicle fusion.

Julian Shillcock and Reinhard Lipowsky(Max Planck Institute of Colloids andInterfaces) review the development ofcomputational models of bilayermembranes, and in particular the use ofmesoscopic simulations to follow themolecular rearrangements that occur during

membrane fusion.It seems likely that in the future several

techniques will be combined into a ‘multi-scale self-parametrizing model’ in which keyparameters for the larger length- andtimescales are continuously determinedfrom data generated at smaller scales. Thiswill be of immense benefit in visualizing andrationally designing the smart biomaterialsof tomorrow.

The computational route from bilayermembranes to vesicle fusionJulian C Shillcock and Reinhard LipowskyJ. Phys.: Condens. Matter 18 (2006)S1191–S1219

This article is part of a special section onbiomembranes. To find more papers onbiomembranes see Journal of Physics: CondensedMatter Vol 18, No 28.

Sequence of snapshots of a 28 nm diameter vesicle fusing with a100x100 nm2 planar membrane patch.

New multiscale techniques for simulating smart biomaterials are reviewed

Si-Ge nanostructures

Semiconductor three-dimensional (3D)islands and quantum dots have potential innew electronic, photonic, or optoelectronicdevices. Accurate control of the size, shape,and position of the dots is crucial.

J-M Baribeau, X Wu, N L Rowell and D JLockwood (National Research CouncilCanada) review recent progress in thegrowth and characterization of Si1- Ge

islands and Ge dots on (001) Si. They discussthe evolution of the island morphology withSi1- Ge coverage, and the effect of growthparameters or post-growth annealing on theshape of islands and dots. They outline someof the structural, vibrational, and opticalproperties of Si1- Ge islands and reviewrecent advances in the determination of theircomposition and strain distribution.

They present an analytical electrontransmission microscopy study of the Gespatial distribution in Ge dots and Si/Si1- Ge

island superlattices grown by molecularbeam epitaxy and ultra-high vacuumchemical vapour deposition. They describethe use of undulated Si1- Ge islandsuperlattices for infrared detection attelecommunication wavelengths.

Finally, they discuss various approachescurrently being investigated to engineerSi1- Ge quantum dots and, in particular,

control their size, density, and spatialdistribution. As examples, they show howcarbon pre-deposition on Si(001) caninfluence nucleation and growth of Geislands and how low-temperature Si homo-epitaxy can lead to a particular surface cuspmorphology that may promote dotnucleation.

Despite all the challenges that remain inthe understanding and control ofsemiconductor nanostructures, these

artificial materials constitute a key enablingtechnology leading to the development ofquantum devices and eventually quantumcomputing.

Ge dots and nanostructures grownepitaxially on SiJ-M Baribeau, X Wu, N L Rowell and D J LockwoodJ. Phys.: Condens. Matter 18 (2006) R139–R174

AFM images showing the surface morphology of Si/Si1�xGex island superlattices prepared by (left) MBE and consisting of ten periods of 13nm Si/3.4 nm Si0.54Ge0.46 and (right) UHV-CVD and consisting of ten periods of 15 nm Si/3 nm Si0.50Ge0.50. The inset is a power spectraldensity map of the image.

Epitaxial growth and characterization of Si1- Ge islands and Ge dots on (001) Si are reviewed





Organic superconductors

Organic charge transfer salts are excellentmodel strongly correlated electron systems.Band structure suggests they should bemetals at all pressures and temperatures, butobserved phases include Mott insulators,Néel antiferromagnets, spin liquids,(unconventional) superconductors, Fermiliquids, a pseudogap and a ‘bad metal'.

B J Powell and Ross H McKenzie(University of Queensland) review the role ofstrong electronic correlations in quasi-two-dimensional organic charge transfer salts.They identify two classes: one is stronglydimerized and is described by a half-filledHubbard model; the second is not stronglydimerized and is described by a quarter-filledextended Hubbard model.

They explore the phase diagram of thehalf-filled quasi-two-dimensional organiccharge transfer salts, focusing on the metallicand superconducting phases. The metallicphase, which has both Fermi liquid and ‘badmetal’ regimes, is well described bydynamical mean field theory. They criticallyreview experimental studies of the

superconducting state, which is stillcontroversial, focusing on the key resultsthat may distinguish between rival theoriesof superconductivity, particularly probes of

the pairing symmetry and measurements ofthe superfluid stiffness. They then discusssome strongly correlated theories ofsuperconductivity, in particular theresonating valence bond theory.

Finally they discuss some majorchallenges, including parameterizingminimal models, the evidence for apseudogap from nuclear magneticresonance experiments, superconductorswith low critical temperatures and extremelysmall superfluid stiffnesses, the possiblespin-liquid states in �-(ET)2Cu2(CN)3 and �-[Pd(dmit)2]2Z, and the need for high-qualitylarge single crystals.

Strong electronic correlations insuperconducting organic charge transfersaltsB J Powell and Ross H McKenzieJ. Phys.: Condens. Matter 18 (2006) R827-R866

Schematic zero temperature phase diagram of the Hubbard model onthe anisotropic triangular lattice at half filling, which provides anexcellent qualitative description of the �, �, � and � phase organiccharge transfer salts.

Organic charge transfer salts are reviewed, especially their superconducting properties

THz biosensing devices

In the last few years THz technology hasbecome increasingly important forbiological applications. It is particularlyinteresting for biosensing applicationsbecause numerous characteristic vibrationalmodes of macro-molecules, like proteins orDNA, are located in the THz part of thespectrum. The energy levels at THzfrequencies are low (4 meV at 1 THz),enabling low-invasiveness probing ofbiological samples. THz probes haverecently been applied to identifyconformational states of proteins, monitorreceptor binding events and for DNAanalysis and imaging of cancerous tissue.

M Nagel, M Först and H Kurz of RWTHAachen have presented different concepts ofintegrated THz-sensor devices for marker-free biosensing applications. They showedthat functionalized resonators can detectspecific DNA sequences at probe moleculesimmobilized on the sensor surface via atime-resolved photoconductive samplingtechnique with femtomole sensitivity. Thisprinciple is suitable for the parallel executionof several hybridizing experiments on a chipwith an array arrangement of differentlyfunctionalized sensors.

They also introduced a compact THztransceiver device for fluidic analysis whichhas been applied for the characterization of

water–ethanol mixtures. Theimplementation of asynchronous opticalsampling into the biochip reader system is animportant development step that stronglyincreases the data throughput for this type ofsensing devices.

THz biosensing devices: fundamentalsand technologyM Nagel, M Först and H KurzJ. Phys.: Condens. Matter 18 (2006) S601–S618

This article is part of a special section onbiodevices; see Journal of Physics: CondensedMatter Vol 18, No 18 for more papers in this area.

Schematic drawing of two types of time-domain pump/probe measurement set-ups. A fs laser and a delay scanning stage are applied tofacilitate generation and time-resolved detection of transmitted THz signals. (a) Quasi-optical variation using propagation of free-space THzradiation generated and detected with photoconductive antennas (see inlay). (b) Equivalent circuit diagram representing a waveguide-basedscheme using integrated photoconductive switches.

THz devices offer marker-free biomolecule detection on functionalized surfaces in dry and fluid environments





Amorphous ices

Water has a very complex phase diagram; forexample, ice exists in at least 15 differentphases, including metastable phases such ascubic ice, ice IV and ice XII. In the(metastable) amorphous solid state (alsocalled glassy water), water showspolyamorphism, i.e., the presence of morethan one amorphous state. In addition toLDA (low-density amorphous ice), a secondamorphous state, HDA (high-densityamorphous ice), was discovered abouttwenty years ago. Since then,polyamorphism has been observed in manyother substances such as SiO2, GeO2, Si, andGe. Five years ago, experimental resultssuggesting the existence of a thirdamorphous state, VHDA (very high-densityamorphous ice), were reported, opening thepossibility that more than two amorphousstates could be observed in other substancesas well.

A consistent phase diagram of glassywater does not yet exist. Such a phasediagram is necessary if one also wants tounderstand the anomalous behaviour ofsupercooled liquid water. Since the discoveryof HDA, a large amount of work based on

experiments and computer simulations hasappeared. Thomas Loerting (Innsbruck) andNicolas Giovambattista (Princeton) review

such studies with special emphasis oncomparing the experimental and simulationresults. In particular, they review the recentstudies concerning VHDA, its nature, anddiscuss the main open questions relating tothe phase diagram of glassy water.

Answering these questions will require athorough understanding of transformationsbetween disordered states at lowtemperatures, which will probably require afew more decades of effort, e.g., to learnmore about the effects of cooling/heatingand compression/decompression rates inthe glass and liquid states.

Amorphous ices: experiments andnumerical simulationsThomas Loerting and NicolasGiovambattistaJ. Phys.: Condens. Matter 18 (2006) R919–R977

Effect of heating decompressed VHDA at constant pressure andconstant volume from molecular dynamics simulations using theSPC/E model. �, � :liquid isotherms; thick solid line: compressioncurve of LDA at T = 77 K; blue thick line: decompression curve ofVHDA at T = 77 K. Blue and red arrows (at P ~ 0 and ~ -0.3 GPa,respectively) show the density change upon annealing decompressedVHDA up to T ~ 170 K. Only at P ~ 0.3 GPa does VHDA seem totransform to LDA. Green arrow indicates the pressure change uponheating isochorically HDA at L ~ 1.6 g/cm3. At the present heatingrate, VHDA does not transform to HDA and the system gets trappedin the liquid isotherms before reaching the pressure corresponding toHDA.

Recent experiments and computer simulations of the different phases of water are reviewed

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