PUBLISHED BYIOP P UBLISHING Journalof Physicsej.iop.org/pdf/jpcm/2008_top_papers.pdfPUBLISHED BYIOP P UBLISHING TOP PAPERS 2008 SHOWCASE In the 20th Anniversary year of Journal of

P U B L I S H E D B Y IOP P U B L I S H I N G

TOP PAPERS 2008 SHOWCASEIn the 20th Anniversary year of Journal of Physics:Condensed Matter (JPCM) I am delighted to present aselection of top papers published in the journalthroughout 2008. We continue to receive anincreasing number of top quality submissionsspanning the field from our authors and once again,

this selection gives an impression of the diversity, range, andquality of papers in JPCM.

As usual, the task of preparing this collection was difficult, asour continuing effort to raise the acceptance standard for papersin JPCM has resulted in an enormous number of candidates, evenas our referees have made their standards more rigorous. Manymore papers were singled out by our referees (through theiridentification of work of the very highest importance), byreaders (through their downloads), and by our board members(through their recommendations) than could be featured here.The final choice was difficult, but I feel that all of these papershave been chosen for their excellent science, although we shouldnot forget that popular appeal also plays a role.

This year, the most cited paper was on transport through singlemolecules and self-assembled monolayers and one of the mostdownloaded papers reported on new diffraction experiments onZnO which revealed the pyroelectric behavior and the polaroctopole moment of the cation. Of course, these papers appearhere, and are just two of the papers which deserve the heightenedcoverage that this brochure provides.

Picking the above papers was the easy part. It would have beennice if the rest of the selection was as easy, but of course it wasn’t.I hope that this collection of highlights from 2008 are as excitingto you, the readers, as they were to me in picking them. Finally,we hope that they really do represent a flavour of the range ofcondensed matter physics that we publish in JPCM.

David Ferry

Editor-in-Chief

I N T H I S I S S U E

FULLCONTENTSOVERLEAF

Professor David FerryEditor-in-Chief

Curved Carbon Fragments

Journal of PhysicsCondensed Matter

Molecular electronics page 4

Polar multipoles in ZnO page 5

Theory meets industry page 8

Metamaterials in visible regime page 10

Atomic chains on surfacespage 11

Insights into protein foldingpage 12

...and much more

Condensed Matter: Top Papers 2008 Showcase 1

Condensed Matter: Top Papers 2008 Showcase

2 C o n d e n s e d M a t t e r : Top Papers 2008 Showcase

P A P E R S

Molecular electronics 4Different aspects of molecular electronics are reviewed

Ballistic magnetoresistance? 5Novel magnetoresistance effects are found in magnetic systems on the nanometer scale

Polar multipoles in wurtzite-like crystals 5Simulation of x-ray diffraction of ZnO and GaN shows polarization effects

New magnetic phase transitions 6Two unexpected magnetic phase transitions have been found in the multiferroic BiFeO3

Simulation of phase diagrams 6Computer simulation of fluid–solid and solid–solid equilibria are reviewed

Realistic modeling of correlated materials 7Realistic modeling is a powerful new tool for studying the properties of strongly correlated materials

Theory meets industry 8Modern materials science has a growing need to understand the phenomena determining the properties of materials on an atomistic level. Density-functional theory (DFT) represents a decisive step forwards in our efforts to develop tools for ab initio atomistic simulations of complex materials, preparing the way towards computational materials design

Vacancy formation in TiO2 9Simulation of vacancy formation may lead to effective defect engineering

Epitaxial multilayer graphene 10Unusual properties of epitaxial multilayer graphene make it a strong candidate for all-carbon electronics

Negative index materials 10Development of metamaterials into the visible region and future prospects are reviewed

Atomic chains on surfaces 11One-dimensional atomic chains will have great importance for future technological applications

Imaging activated carbon 12Imaging studies suggest that activated carbon has a fullerene-like structure

Understanding ensemble protein folding at atomic detail 12Simulation and experiments together provide an insight into protein folding on the atomic scale

Pressure-induced superconductivity in CaFe2As2 13Pressure may induce superconductivity in arsenides without the need of chemical disorder

Modular assembly on metal surfaces 13Directed assembly of supramolecular compounds and arrays on solid surfaces is reviewed

Understanding jamming 14The study of dynamical heterogeneities clarifies the slow dynamics of gels and glasses

Colloidal glasses 14Experiments help to clarify the properties of two types of colloidal glass

Atomic scale friction 15New experiments show that temperature plays a crucial role in atomic scale friction

Heating effects on nanostructures 15Theory of local heating effects in nanostructures gives good agreement with experiment

The anomalous Hall effect 16Semiclassical theories of the anomalous Hall effect are reviewed

Oxide surfaces 16A special issue in Journal of Physics: Condensed Matter (20, No 26) aimed to convey to the reader the most up-to-date understanding of the physics of the surfaces, interfaces, and thin films of complex metal oxides, in a clear and accessible manner. Here are summaries of two of the highlights.



Vortex ferroelectric domains 17Vortex domains are observed during switching of ferroelectric capacitors and modeled theoretically

Holey metallic films 17Unusual phenomena emerge when metallic films are perforated with periodic arrays of holes

The 0.7 anomaly 18An unusual feature in the conductance of 1D conductors is the subject of intense study

Quantum oscillations in SrFe2As2 18Study of quantum oscillations in their parent compound helps understanding of FeAs superconductors

Capillary adhesion 19Humidity has a strong effect on the adhesion between randomly rough surfaces

Optical properties of quantum dots 20The journal published a special section containing papers from the Proceedings of the 15th International Winterschool on New Developments in Solid State Physics (Mauterndorf (Bad Hofgastein), 18–22 February 2008) (J. Phys.: Condens. Matter 20No 45). It published papers on transport in semiconductor nanostructures, quantum dots—optics, growth andcharacterization of nanostructures, photonic crystals, magnetism in semiconductors, graphite and graphene and nanowires. Here are summaries of two of the papers on quantum dots

Condensed Matter: Papers

Molecular electronics

Ongoing miniaturization in themicroelectronics industry has led to intenseinterest in so-called molecular electronics,though it cannot yet compete with silicon onall the requirements of operation speed(especially), reliability, stability, powerconsumption and production costs.

The use of molecules has severaladvantages over top-down approaches:• The use of single molecules automatically

leads to structural dimensions at theatomic scale.

• Molecules self-organize and self-assemblein topologies that reflect the interactionsbetween them. By controlling thechemistry of the molecules, differentstructures can be produced.

• Chemical synthesis can be used to producemolecules of precisely defined properties.

• Molecules can have specific functions, forinstance switching between differentstates.The future of molecular electronics may

be in low-end applications where theoperation requirements are less critical butprocessing costs should be as low aspossible. Self-assembled monolayers (SAMs)might be used instead of single molecules.

Three topical reviews describe progress indifferent areas of molecular electronics.

Akkerman and de Boer (University ofGroningen, The Netherlands) subdivided ageneralized metal–molecule–metal junctioninto different components to determine theirinfluence on the values obtained for theresistance of alkane-based molecules. Thesemolecules make an ideal benchmark formolecular electronics, but show a largespread in conduction per molecule of up toeight orders of magnitude for differentmolecular junction geometries due to the

different nature of the contacts.What technique to use in molecular

electronics will thus depend on themotivation for the work. Fundamentalstudies might be best represented by single-

molecule techniques from the low-resistance group, whereas application-oriented research is more promising withreproducible techniques from the medium-resistance group. Macroscopic devices basedon a single molecular layer can be realized byusing one of the techniques from the high-resistance group and might lead tointeresting low-end applications. Alltechniques promise interesting new resultsin the field of molecular electronics.

Electrical conduction through singlemolecules and self-assembledmonolayersHylke B Akkerman and Bert de Boer2008 J. Phys.: Condens. Matter 20 013001(20pp)

Kröger, Néel and Limot (University of Kiel,Germany) review experiments using the tipof a scanning tunnelling microscope tocontact atoms and molecules adsorbed onsurfaces. They address conductancequantization upon forming or breaking acontact between the tip and surfaces as wellas between the tip and specifically chosenatoms and molecular orbitals. Imaging thecontact area prior to and after contactmeasurements allows one to monitor thestatus of the contacted object as well as thatof the contacting electrodes. Spectroscopywith the tip in contact with individual atomsor molecules reveals the reproducibility ofand control over such experiments.

They also study charge transfer andconcomitant energy dissipation in atomic-

sized contacts. These microscopic processesare considered to be at the origin of friction,adhesion, and wear at the macroscopic scale.

Contact to single atoms and moleculeswith the tip of a scanning tunnellingmicroscopeJ Kröger, N Néel and L Limot2008 J. Phys.: Condens. Matter 20 223001(16pp)

Grill (Free University of Berlin, Germany)reviews recent experiments on speciallydesigned molecules, acting as model systemsfor molecular nanotechnology. Scanningtunnelling microscopy (STM) enablesimaging of molecules with sub-molecularresolution and can also be used as a tool tomanipulate single molecules in a controlledway. The results focus on functionalizedmolecules, which represent model systemsfor several components in futureapplications of molecular nanotechnologyand molecular electronics, adsorbed onmetal surfaces. The substrate is acting on the

one hand simply as a supporting surface andon the other hand as an electrode (while theother electrode is given by the STM tip).He focuses on key functionalities: lateralrolling and hopping motion on a supportingsurface, the switching behaviour ofazobenzene derivatives by using the STM tipand the controlled reactivity of molecularside groups, which enable the formation ofcovalently bound molecularnanoarchitectures.

Functionalized molecules studied bySTM: motion, switching and reactivityLeonhard Grill2008 J. Phys.: Condens. Matter 20 053001(19pp)

Schematic depiction of a mechanical break junction set-up. A piezocontrolled pushing rod bends the substrate with µm control. Thelarge reduction factor between the Z-movement and the elongationin-plane allows for sub-nm control of the electrode distance.

Principle of the lateral manipulation of an adsorbate on a surface byusing the STM tip. Either repulsive or attractive forces are driving theprocess, leading to a ‘pushing’ (a) or ‘pulling’ mode (b), respectively.The tip is moving in the schematics (left) according to the arrow. Themanipulation signal (current curve at constant tip height), plotted tothe right, reveals characteristic shapes and the periodicity do of thesubstrate.

Conductance-displacement curve on clean Cu(111) surface at 8 K.Conductance is in units of the quantum of conductance. Threeregimes are discernible: tunnelling (I), transition (II), and contact(III). Contact formation between tip and flat surface is shown below.In the transition regime (II) adhesive forces between the tip andsurface lead to relaxations of the tip and surface crystal structure. Thecontact regime (III) reflects ballistic electron transport through asingle atom.


Different aspects of molecular electronics are reviewed



Ballistic magnetoresistance?

Using the spin of conduction electronsopens new possibilities for electronicdevices. It is possible to realize magneticheterogeneous structures, where themagnetization orientation can be controlleddown to the nanometre range. If amodification of the magnetizationconfiguration can be realized within adistance over which the conductionelectrons keep the memory of their spinorientation, a related change of resistance ofthe sample occurs. The related so-calledgiant magnetoresistance (GMR) propertiesare well understood. The phenomenologyand understanding of GMR drasticallychange for samples of dimensions smallerthan the electron mean free path. In this

ballistic regime of conduction, one expectsthe band structure of the material to governthe magnetoresistance (MR) properties.

Doudin ((IPCMS, Starsbourg, France) andViret (CEA Saclay, France) review work onelectric transport in metallic magneticnanocontacts of sizes reaching a single atom.Considering fabrication methods exemptfrom mechanical instabilities, they find twoexperimental consensuses inmagnetoresistance measurements. Firstly,magnetoresistance does not exceed a fewtens of per cent in atomic size constrictions.They attribute these modest values to thesignificant number of opened conductionchannels expected in contacts of 3D metals.Secondly, anisotropic magnetoresistance is

observed for all types of samples, withamplitudes at least one order of magnitudelarger than those found in bulk samples.Abrupt resistance changes with field angleconfirm the occurrence of discreteanisotropic magnetoresistance levels. Theeffect is attributed to enhanced spin–orbitcoupling at the atomic scale, resulting inpossible opening or closure of conductancechannels when varying the angle betweencurrent and applied magnetic field.

Ballistic magnetoresistance?B Doudin and M Viret2008 J. Phys.: Condens. Matter 20 083201(18pp)

Novel magnetoresistance effects are found in magnetic systems on the nanometer scale

Polar multipoles in wurtzite-like crystals

Wurtzite-like crystals are both pyroelectricand piezoelectric and possess a uniquehexad axis of symmetry which is necessarilythe direction of the pyroelectric moment.

Zinc oxide is a wide-band-gapsemiconductor of current interest for a rangeof applications including solar cells andtransparent transistors. Fractionalsubstitution of Zn by Li produces aferroelectric while substitution by Niproduces a magnetic modification. ZnOemits light in the blue-to-UV range withefficiency superior to that of gallium nitride(GaN). GaN is a key component ofoptoelectronic devices working in theblue–violet range, quantum-well structuresand nanowires.

Lovesey (Rutherford AppletonLaboratory, UK) and Balcar (ViennaUniversity of Technology, Austria) havepresented a theoretical analysis andsimulation of resonance-enhanced x-rayBragg diffraction by wurtzite-like crystalsfor parity-even and polar (parity-odd)atomic resonant processes.

They represented electronic properties ofa crystal as ground-state expectation valuesof atomic multipoles, and their analysisrespects all selection rules imposed ondiffraction by atomic and crystal symmetry.They focused on weak, space-groupforbidden reflections that are a result of

angular anisotropy in the cation electrondistribution from covalency (thebacktransfer of electrons between a cationand surrounding anions). They simulatedexpected variations of the diffractedintensity with orientation of the crystal dueto angular anisotropy of the electrondistribution. The crystal symmetry is suchthat the intensity couples to circularpolarization in the primary beam and,conversely, circular polarization is created indiffraction. They provide illustrativeexamples for both of these polarizationeffects, which are unique probes ofelectronic structure. Moreover, they provethat recent diffraction experiments on ZnOreveal the pyroelectric (dipole) and polaroctupole moment of the cation.

Polar multipoles in wurtzite-like crystals(ZnO, GaN)Stephen W Lovesey and Ewald Balcar2008 J. Phys.: Condens. Matter 20 122201 (4pp)

Simulation of x-ray diffraction of ZnO and GaN shows polarization effects

With pure � polarization in the primary x-rays (P3 = +1) the totaldiffracted intensity is the sum of intensities in channels withpolarization states �� and ��. Simulations of the total intensityfor the (1, 1, 3) reflection for resonant processes E1–E2, E2–E2 and amixture of these two processes.



Simulation of phase diagrams

The study of phase transitions has alwaysbeen a hot topic in computer simulation.Vega and co-workers (ComplutenseUniversity, Spain) review the computersimulation of phase diagrams, particularlyfluid–solid and solid–solid equilibria. Theydiscuss the methodology to compute the freeenergy of solid phases, particularly theEinstein crystal and Einstein moleculemethodologies, which both yield the samefree energies. The free energies of solidphases present noticeable finite size effectsfor which approximate corrections can bemade.

The computation of free energies of solidphases can be extended to molecular fluids.They describe in detail the procedure tocompute free energies of many differentsolid phases of water (ices). They provideinitial coexistence points leading to thedetermination of the phase diagram of water.They also discuss other methods to estimatethe melting point of a solid, such as the directfluid–solid coexistence or simulations of thefree surface of the solid. They show that themelting points of ice Ih obtained from free-energy calculations, direct coexistence

simulations and free surface simulationsagree within their statistical uncertainty.

Phase diagram calculations can help toimprove potential models of molecularfluids. They review some recent work on thephase diagram of the simplest ionic model,the restricted primitive model. Althoughoriginally devised to describe ionic liquids,the model is used to describe the behavior ofcharged colloids. They also discuss thepossibility of obtaining fluid–solid equilibriafor simple protein models. In these primitive

models, the protein is described by aspherical potential with certain anisotropicbonding sites (patchy sites).

Determination of phase diagrams viacomputer simulation: methodology andapplications to water, electrolytes andproteinsC Vega, E Sanz, J L F Abascal and E G Noya2008 J. Phys.: Condens. Matter 20 153101(38pp)

Phase diagram of water as obtained from experiment (center) and from computer simulations for two different models. The filled circles on the right panel indicate the stability limit of the solid phases in NpT simulations (without interfaces). Notice the shift of 100 MPa in the right panel.

Computer simulation of fluid–solid and solid–solid equilibria are reviewed

New magnetic phase transitions

There is currently wide interest inferroelectromagnetic materials, i.e.multiferroics, which exhibit ferroelectric (orantiferroelectric) properties in combinationwith ferromagnetic (or antiferromagnetic)properties. They also exhibit thephenomenon called magnetoelectriccoupling, i.e. magnetization induced by anelectric field and electric polarization by amagnetic field. These phenomena also giverise to unusual dynamical effects, which canbe observed in optical experiments.Magnetic excitations in multiferroic polarcrystals are generally not pure spin waves butcontain significant contributions to theirRaman scattering cross sections fromelectric dipole matrix elements. These so-called electromagnons exhibit differentdynamical characteristics from magnonscattering in centrosymmetric lattices.

Bismuth ferrite has become an extremelypopular material these days because of itsrare or even unique properties of havingroom-temperature ferroelectric andmagnetic order.

Singh, Katiyar (University of Puerto Rico,USA) and Scott (University of Cambridge,UK) have presented high-resolution Ramandata showing two one-magnon branches inbismuth ferrite. The lower branch near

18.4 cm-1 is ferromagnetic-like and varieswith temperature as an S = 5/2 Brillouinfunction up to 0.44 TN, above which itbecomes overdamped or instrumentallyunresolved from the elastically scatteredlaser light. The higher-frequency magnonnear 26.6 cm-1 is weaker in intensity,temperature-independent, and it resemblesthe antiferromagnetic mode in orthoferrites.They unexpectedly found anomalies inmagnon frequency and temperature at 200and 140 K, which may suggest a spin-reorientation transition similar to thoseobserved in most of the rare earthorthoferrites.

This is the first investigation of its kind onBiFeO3, and prior to this work the magneticstructure was considered to be unchangedfrom ambient temperatures to cryogenictemperatures. Thus, the existence of twonew magnetic phase transitions was notpredicted or anticipated.

New magnetic phase transitions inBiFeO3

Manoj K Singh, Ram S Katiyar and J F Scott2008 J. Phys.: Condens. Matter 20 252203 (4pp)

Experimental correlation between the electromagnon frequencysoftening of the one-magnon branch at 18.2 cm-1 and the S = 5/2Brillouin function. Note that the data points near 140 and 200 K donot fall on the curve.

Two unexpected magnetic phase transitions have been found in the multiferroic BiFeO3



Realistic modeling of correlated materials

Density-functional theory (DFT) issuccessfully used to explore the ground-stateproperties of many systems. It is based onminimizing the total energy functional withrespect to the electron density. As the formof the exchange–correlation potential isgenerally unknown, it is usually treated inthe local-density approximation (LDA). Inso-called strongly correlated materials, inwhich the state of each electron stronglydepends on the state of the other electrons,LDA often fails in describing both the excitedand ground-state properties.

Solovyev (National Institute for MaterialsScience, Japan) reviews the main ideas andscope of a newly developing direction forstrongly correlated systems called ‘realisticmodeling’. It makes a bridge between DFT-based methods of first-principles electronicstructure calculations and many-bodymodels, describing properties of stronglycorrelated systems in terms of a limitednumber of the most relevant modelparameters and including information aboutall remaining electronic structure implicitlythrough the renormalization of these modelparameters.

Realistic modeling combines the accuracyand predictive power of first-principles

electronic structure calculations with theflexibility and insights of the model analysis.The review illustrates this idea on a series ofexamples, starting from conventionalelectronic structure calculations in the LDA,followed by the construction of anappropriate low-energy model, motivated bythese calculations, and finally by the solutionof this model and by the analysis ofproperties of strongly correlated systems interms of these model categories and trends.

The first applications are very

encouraging and realistic modeling isexpected to become a powerful tool fortheoretical analysis, design, and control ofthe properties of strongly correlatedmaterials.

Combining DFT and many-bodymethods to understand correlatedmaterialsI V Solovyev2008 J. Phys.: Condens. Matter 20 293201(33pp)

Realistic modeling is a powerful new tool for studying the properties of strongly correlated materials

Crystal structure and LDA density of states of the bct phase of KO2. The shaded area shows the contributions of the oxygen 2p states. Othersymbols show the positions of the main bands. Fermi level is at zero energy.

CELEBRATING 20 YEARS OF PUBLISHING EXCELLENCE

Journal of PhysicsCondensed Matter



Theory meets industry

A meeting in Vienna entitled ‘Theory meetsindustry’ brought together researchers fromacademia and industry with an interest incomputational materials design. Papersfrom this meeting are published in a specialissue of Journal of Physics: Condensed Matter 20No 6. Some of the highlights are featuredhere.

The SIESTA methodOver last two decades, first-principlessimulation of condensed matter systems hasexpanded spectacularly, based on both thesteady growth of computing power and thedevelopment of methods based on density-functional theory (DFT). However, standardmethods demand computer resourcesscaling as the number of atoms cubed. Therehas been much work on the problem of howto perform full DFT calculations with alinear-scaling effort. The SIESTA methodstarted in 1995 by merging Sankey’s finite-support atomic orbitals, with a 3D real-spacegrid representation of the density and thebasis functions.

The SIESTA method has been evolvingsince its inception, and has been applied byan ever expanding user community to a largevariety of problems in many different fields.Here the SIESTA team review recentdevelopments in and around the SIESTAmethod, with emphasis on (i) theapplicability of the method for large andvaried systems, (ii) efficient basis sets for thestandards of accuracy of density-functionalmethods, (iii) new implementations, and (iv)extensions beyond ground-statecalculations.

The SIESTA method; developments andapplicabilityEmilio Artacho et al2008 J. Phys.: Condens. Matter 20 064208 (6pp)

Modeling electron transportThe transition from the microscale to thenanoscale is challenging thesemiconductor device modeling tools asquantum effects and the atomic-scaledetail of the device must be taken intoaccount.

Stokbro (University of Copenhagen,Denmark) reviews new tools for atomic-scale modeling of the electrical properties ofemerging electronic devices. Such devicesinclude new materials like molecules, carbonnanotubes, nanowires or use new quantitieslike the electron spin to process information.These materials have complex electronicproperties that depend on the detailed devicegeometry so an accurate quantum chemicalmodel of the atomic-scale geometry isneeded.

Non-equilibrium Green’s functions(NEGF) are combined with electronicstructure methods to model the electricalproperties of nanoscale devices. Theunderlying formalism has O(N) scaling and itis a promising approach for studyingcomplete devices with many thousandatoms.He presents applications of the method tomodel the electrical properties of molecularelectronics devices and spin-dependentelectron transport in magneto-tunneljunctions.

To extend the methodology to fullsemiconductor device models there is a needfor new more efficient algorithms.Simulation of the electrical properties ofsystems of thousands of atoms seemsfeasible in the near future. Furthermore,including interaction between the electronsand phonons as well as photons showspromise for the NEGF-DFT approach to bethe basis of a new generation ofsemiconductor device modeling tools.

First-principles modeling of electrontransportK Stokbro2008 J. Phys.: Condens. Matter 20 064216 (7pp)

Computing novel hydrogen storagematerialsPractical hydrogen storage for mobileapplications requires materials thatexhibit high hydrogen densities, lowdecomposition temperatures, and fastkinetics for absorption and desorption.Unfortunately, no reversible materials arecurrently known that possess all of theseattributes.

Wolverton (Northwestern University,USA) and co-workers present an overview oftheir recent efforts aimed at developing afirst-principles computational approach tothe discovery of novel hydrogen storagematerials. Such an approach requires severalkey capabilities to be effective: (i) accurateprediction of decompositionthermodynamics, (ii) prediction of crystalstructures for unknown hydrides, and (iii)prediction of preferred decompositionpathways. They present examples thatillustrate each of these three capabilities: (i)prediction of hydriding enthalpies and freeenergies across a wide range of hydridematerials, (ii) prediction of low energycrystal structures for complex hydrides(such as Ca(AlH4)2 CaAlH5, and Li2NH), and(iii) predicted decomposition pathways forLi4BN3H10 and destabilized systems based oncombinations of LiBH4, Ca(BH4)2 and metalhydrides. For the destabilized systems, theypropose a set of thermodynamic guidelinesto help identify thermodynamically viablereactions. These capabilities have led to theprediction of several novel high-densityhydrogen storage materials and reactions.

Discovery of novel hydrogen storagematerials: an atomic scale computationalapproachC Wolverton, Donald J Siegel, A R Akbarzadeh and V Ozolins̆2008 J. Phys.: Condens. Matter 20 064228(14pp)

Modern materials science has a growing need to understand the phenomena determining the properties of materials on anatomistic level. Density-functional theory (DFT) represents a decisive step forwards in our efforts to develop tools for ab initioatomistic simulations of complex materials, preparing the way towards computational materials design

Matching radii for 3s (filled symbols) and 3p (open symbols) orbitalsacross period III of the periodic table. Circles correspond to basesobtained variationally, squares to standard procedures. The valuestowards the right are reasonably reproduced by the standardprocedures, if slightly lower, but deviate strongly towards the left.

Geometry of a two-probe system consisting of a central region incontact with two electrodes. The electrodes form a semi-infiniteperiodic lattice and it is assumed that the central region is sufficientlylarge that the electron density in the electrode region has retained itsbulk value. The properties of the electrode region can then be obtainedfrom a calculation of an electrode cell with periodic boundaryconditions.



Vacancy formation in TiO2

The process of defect formation is importantbecause defects play a significant role incrystalline transport, affect mechanicalproperties through interactions withdislocations and stacking faults, modifysurface chemistry, and alter electrical/opticalproperties. In transition metal oxides, whichplay an important role in heterogeneouscatalysis, photoelectrolysis andbiocompatibility, defects such as bulk andsurface oxygen vacancies often dominate theelectronic and chemical surface properties.

Brandon Keith (West Virginia University,USA) and colleagues present an efficientmethod for predicting vacancy formationfree energies in TiO2 from first principles,which can contribute to various applicationsin fundamental TiO2 defect science andapplied surface chemistry.

Bulk defects such as oxygen vacancies playa key role in visible-light photoactive TiO2,so greater visible light absorption rates maybe possible if effective defect engineering canbe achieved. To further this they have

developed ab initio methods to simulatevacancy formation in bulk TiO2. Initialresults show an entropic reduction in the freeenergy of vacancy formation of 2.3 eV over arange of 266 K. They illustrate use of thisresult by a ‘toy’ mass-action kinetics modelwhich offers insight into vacancyconcentration, rate constants, and enthalpyof reaction.

This work could be useful in studyingresistivity, conductivity, thermoelectricpower, diffusion and redox measurements,

and further enhance understanding ofvisible TiO2 absorption whether thoughultra high vacuum annealing or nitrogen,fluorine, or cobalt doping. These techniquesmay also be an effective way to verify that theassociated vacancies rather than the dopantsthemselves are what improvephotodegradation.

Ab initio free energy of vacancyformation and mass-action kinetics invis-active TiO2

J Brandon Keith, Hao Wang, Brent Fultz andJames P Lewis2008 J. Phys.: Condens. Matter 20 022202 (5pp)

Ab initio calculation of the oxygen vacancy formation free energyFO

V(T) in rutile TiO2 as a function of temperature.

Simulation of vacancy formation may lead to effective defect engineering

Density functional theoryDensity functional theory (DFT) has becomea valuable tool for studying solid statesystems for hydrogen storage. It has greatlyexpanded our understanding of theproperties of known hydrides, includingelectronic structure, hydrogen bondingcharacter, enthalpy of formation, elasticbehavior, and vibrational energetics.Moreover, DFT holds substantial promise forguiding the discovery of new materials.Hector and Herbst (General Motors R&DCenter, USA) discuss, within the context ofresults from their own work, some successesand a few shortcomings of state-of-the-artDFT as applied to hydrogen storagematerials.

Density functional theory for hydrogenstorage materials: successes andopportunitiesL G Hector Jr and J F Herbst 2008 J. Phys.: Condens. Matter 20 064229(11pp)

Chemistry versus geometry inmolecular conductionMany electrical properties of single-molecule devices are exponentially sensitiveto changes in the environment and to detailsof the contact to electrodes. Systematicexperimental studies can yield clear trendsassociated with generic molecular features.These studies reveal that geometry plays animportant role in controlling electrontransport through single molecules. Recent

experiments by Venkataraman et al on aseries of molecular wires revealed a lineardependence of the conductance on cos2�,where � is the angle of twist betweenneighbouring aromatic rings, independentof chemical composition.

A group (Lancaster University, UK) soughtto understand whether the dominance ofgeometry over chemistry can be widelyexpected or if this is a peculiar property ofthe set of molecules measured. They presenta first-principles theoretical study of thetransport properties of this family ofmolecules as a function of the chemicalcomposition, conformation and the contactatom and geometry.

They find that geometry is the dominantfactor when the Fermi level sits inside theHOMO–LUMO (highest occupied molecularorbital–lowest unoccupied molecularorbital) gap but when the Fermi levelapproaches either the HOMO or LUMOresonances chemistry comes into play,because the chemical composition of thepristine molecules causes the positions ofthe HOMO and LUMO resonances to differfrom molecule to molecule, destroying thelinear dependence of the conductance oncos2� and giving rise to non-monotonicbehaviour associated with the level structureof the different molecules. Their resultssuggest that the above experiments provide anovel method for extracting spectroscopicinformation about molecules contacted toelectrodes.

Conformation dependence of molecularconductance: chemistry versusgeometryChristopher M Finch, SkonSirichantaropass, Steven W Bailey, Iain M Grace, Víctor M García-Suárez and Colin J Lambert 2008 J. Phys.: Condens. Matter 20 022203 (5pp)

GGA-relaxed configurations of all molecules studied in this workcapped with NH2. The dark vertex in the backbone of molecule 3corresponds to N and the side groups of molecules 6 and 7 (otherthan H) correspond to F and Cl atoms, respectively.



Negative index materials

Metamaterials are designed to havestructures that make available properties notfound in Nature. Their unique properties(such as negative index of refraction) can beextended from GHz all the way to opticalfrequencies. They have negative electricalpermittivity (�), negative magneticpermeability (µ), and negative index ofrefraction (n) at a common frequency band.

Soukoulis and colleagues (AmesLaboratory, USA and FORTH, Greece)present results for the magnetic response ofmetamaterials for different frequencyregions, as well as results on structures thatwere fabricated and show a negative index ofrefraction. Most of the left-handedstructures at the GHz frequency region arebased on the split-ring resonators (SRRs) andcontinuous-wires design that was firstproposed by Pendry et al. There is a need foralternative, improved and simplified designsthat can be easily fabricated andexperimentally characterized. This wasrecently achieved in the GHz, THz visibleregions by the use of pairs of finite lengthwires (cut wires) and the fishnet topology.

Soukoulis et al have reviewed the status ofthe design, fabrication and characterizationof metamaterials that give a negative index ofrefraction. In addition they have presentedresults for the ratio of the wavelength overthe size of the unit cell size versus frequency

for different designs (SRRS, cut wires, cutwires and continuous wires and fishnet) thatgive a negative µ. Only seven years after thefirst demonstration, negative indexmetamaterials have been brought frommicrowaves to the visible regime. Theirdetailed studies have shown that one of themain difficulties in the further developmentof metamaterials is the existence of stronglosses, especially in the infrared and visibleregime. So there is a need for finding newdesigns or ways to reduce the losses. We are

now in position to move forward and furtherdevelop the materials and methods that willmake these novel materials useful forapplication.

The science of negative index materialsCostas M Soukoulis, Jiangfeng Zhou,Thomas Koschny, Maria Kafesaki andEleftherios N Economou2008 J. Phys.: Condens. Matter 20 304217 (7pp)

Geometries for cut-wire pair arrays (left panel) and the fishnet structure (right panel). Both consist of a patterned metallic double-layer (yellowouter layer, usually Au) separated by a thin dielectric (blue inner layer).

Development of metamaterials into the visible region and future prospects are reviewed

Epitaxial multilayer graphene

Epitaxial graphene has been studiedintensively as a candidate for all-carbonpost-CMOS electronics. The advantage ofgraphene over carbon nanotubes resides inits planar 2D structure that enables circuitdesign with standard lithographytechniques, enabling graphene to be cut withdifferent shapes and selected edge direction.The bandgap of semiconductor grapheneribbons can be tuned by tailoring the ribbonwidth.

Epitaxial graphene is directly grown onlarge area insulating or semiconductingsubstrates, the films are lithographicallypatterned and metal contacts applied tomake electronic devices. Multilayergraphene films grown on SiC showelectronic properties similar to an isolatedgraphene sheet, including a Berry’s phase of� weak anti-localization, and a square rootdependence of the Landau level energieswith applied magnetic field.

Hass, de Heer and Conrad (GeorgiaInstitute of Technology, USA) have reviewed

the current state of research on epitaxialgraphene grown on SiC. The structure andgrowth of epitaxial graphene are verydifferent on the two polar faces of hexagonalSiC: (0001) and (000-1), and these differencesare only now coming to light. Significantimprovements in the growth of epitaxialgraphene on hexagonal SiC have beendemonstrated, particularly for graphenefilms grown on SiC(000-1). These films are

fast approaching wafer-scale dimensionsmeaning that they are now beingcharacterized by the yield of operationalswitching devices per mm2 instead ofdomain sizes in units of µm2.

Epitaxial graphene can have electronicproperties like an isolated graphene sheet.The Schottky barrier between the metallicgraphene layer and the semiconductinginterface influences doping of the graphene.On the SiC (000-1) surface the unusualrotational stacking in multilayer grapheneexplains why these films behave like isolatedgraphene sheets even when they are stackedup to 60 layers. These properties areexpected to lead to an explosion of researchthat will exploit for application in new andunusual epitaxial graphene devices.

The growth and morphology of epitaxialmultilayer grapheneJ Hass, W A de Heer and E H Conrad2008 J. Phys.: Condens. Matter 20 323202(27pp)

(a) Graphene hexagonal structure of identical carbon atoms. Theunit cell (shaded) containing two carbon atoms is shown along withstandard unit cell vectors aG and bG. The ‘armchair’ edge and the ‘zig-zag’ directions are shown ([21] and [10] respectively). (b) Schematic

of the in-plane � bonds and the � orbitals perpendicular to the planeof the sheets.

Unusual properties of epitaxial multilayer graphene make it a strong candidate for all-carbon electronics



Atomic chains on surfaces

The physical and electronic properties ofatomic chains formed on various surfacesare the subject of intense study. Owing to theinherent one-dimensional nature of thesechains, various exotic physical phenomenaspecific to one-dimensional systems havebeen observed including Peierls instabilityand Luttinger liquid behavior. Oncel(Princeton University, USA) reviews thephysics of these one-dimensional atomicchains and gives some examples from therecently published literature.

In the near future the typical size of theindividual components of integrated circuitswill be scaled down to only a fewnanometers. The next step will be theconnection of these pieces in order toincorporate them into the circuit, ideallywith wires only one atom in diameter.Therefore, the fabrication of theseconnections and their physical propertiesare as important as the individualcomponents.

Studying the physics of these one-dimensional atomic chains will have greatimportance for future technologicalapplications; the functions of these atomicchains will not be limited to interconnectorsof various components of an integrated

circuit. One major opportunity, overlookedup to now, is to employ atomic chains astemplates or bases for manufacturinghierarchically higher order structures. In thisway, the size and the sharpness of the edgesof the individual features can approach theangstrom level, well below the limits of thetop-down approach.

Another important opportunity cancome from chemical properties of theseatomic chains. The catalytic properties ofmaterials depends strongly on their size anddimensionality, therefore highly organizedone-dimensional atomic chains can beemployed as catalysts of various importantreactions.

Atomic chains on surfacesNuri Oncel2008 J. Phys.: Condens. Matter 20 393001(26pp)

(a) A 6.75 nm x 6.75 nm STM image of a patch of five nanowires.The width of the troughs between the nanowires is 1.6 nm. (b) A 7.9nm x 7.9 nm STM image of a patch of Pt nanowires. In this STMimage, both 1.6 and 2.4 nm wide troughs are visible. The rectanglesare drawn to guide the eye and indicate the phase relations betweenneighboring nanowires.

One-dimensional atomic chains will have great importance for future technological applications

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Imaging activated carbon

Activated carbon is used in manyapplications such as gas and waterpurification, metal extraction and medicine.It is prepared from a variety of carbonaceousprecursors, including coal, peat andnutshells which are carbonized and then‘activated’, either by oxidization with CO2 orsteam, or by treatment with acids, bases orother chemicals. The resulting carbon canhave a surface area of 1500 m2 g-1 or more,explaining its huge adsorptive capacity. Itsprecise atomic structure, however, isunknown. Diffraction methods have beenextensively applied to the study ofmicroporous carbons, but cannot provide anunequivocal identification of their structure.

Harris (University of Reading, UK) and co-workers (AIST, Japan) show that thestructure of a commercial activated carboncan be imaged directly using aberration-corrected transmission electron microscopy.They present images both of the as-producedcarbon and of the carbon following heattreatment at 2000˚C. In the 2000˚C carbonthey found extensive hexagonal networksand also clear evidence for the presence of

pentagonal rings, suggesting that thecarbons have a fullerene-related structure.Such a structure would help to explain theproperties of activated carbon, and wouldalso have important implications for themodelling of adsorption on microporouscarbons.

The idea that microporous carbons have afullerene-like structure has importantimplications for the modelling of adsorptionon such carbons. Indeed, theoretical studies

have already been carried out which showthat a model structure containing fullerene-related elements provides a better basis forunderstanding adsorption on activatedcarbon than the traditional models.

Imaging the atomic structure ofactivated carbonPeter J F Harris, Zheng Liu and Kazu Suenaga2008 J. Phys.: Condens. Matter 20 362201 (5pp)

Illustration of curved carbon fragments, containing pentagonal and heptagonal rings as well as hexagons.

Imaging studies suggest that activated carbon has a fullerene-like structure

Understanding ensemble protein folding atatomic detailProtein folding is of great biological andmedical importance and a tremendousamount of effort has been put intounderstanding it. Protein folding transformsa polypeptide chain from an unfolded, high-entropy state into its unique, nativestructure, which in most cases is onlymarginally stable.

Wallin and Shakhnovich (University ofHarvard, USA) review recent advances inatomic-level protein folding simulations andthe new insight provided by them into theprotein folding process. Methods foranalyzing many separate folding trajectoriesare important for understanding ensemblefolding kinetics, and the authors discusstechniques developed to condense the largeamount of information contained in anensemble of trajectories into a manageablepicture of the folding process.

Many of these advances have beenachieved with relatively simple modelswhich combine relatively simple statisticalpotentials and all-atom representation of theprotein chain. This emphasizes the role of afew key physical aspects of the forces thatdrive folding, such as hydrogen bonding and

the hydrophobic effect. Furtherdevelopment of the existing models isneeded, as well as comparative studies ofdifferent modeling approaches.

It is important to combine experimentaland theoretical approaches. Experimentaldata play a key role in verifying proteinmodels, but true synergy between theoryand experiments is also being obtained. Forexample, protein engineering experimentsare combined with explicit-chainsimulations thus achieving atomisticallydetailed constructions of transition-stateensembles for folding.

Computer simulation of folding, with itsability to provide microscopic insights, can

reconcile kinetic experimental data onfolding which may, at first, appearcontradictory. Given the rapid developmentin both simulation and experimental areas,fundamental additional progress in ourunderstanding of the kinetics andthermodynamics of protein folding is boundto follow in the near future.

Understanding ensemble protein foldingat atomic detailStefanWallin and Eugene I Shakhnovich2008 J. Phys.: Condens. Matter 20 283101(11pp)

Folding from a denatured (D) state, which rapidly undergoes nonspecific collapse (C). There are several C states, characterized by increasingcompaction and helical content. After the protein becomes sufficiently helical, the chain extends through fluctuations to an expandedintermediate (I) state, which allows rearrangement of the helices, and is followed by the transition state (TS). A final collapse to a near native(NN) state ensues, which proceeds through specific side chain packing and energetic relaxation to the native (N) state. C1, C2, C3, and I, mayundergo rapid conversion.

Simulation and experiments together provide an insight into protein folding on the atomic scale



Pressure-induced superconductivity in CaFe2As2

The tetragonal ThCr2Si2 crystal structuresupports a wide range of elementalcombinations. Recently, the AFe2As2 (A =Ca, Sr and Ba) materials that form in thisstructure have received renewed attentiondue to their similarity to another family ofcompounds ROFeAs (where R is a rareearth). Both systems contain structurallayers of FeAs, undergo a tetragonal toorthorhombic transition below roomtemperature and supportantiferromagnetism in the orthorhombicstructure. Non-isoelectronic chemicalsubstitution in the out-of-plane layer, forexample, substituting K for Ba in BaFe2As2 orF for O in LaOFeA, introduces additionalcharge carriers, suppresses the structuraland magnetic transitions, and inducessuperconductivity at temperatures in thetens of kelvins. Of these various materials,none with FeAs layers is superconductingwithout electronic doping, though thenominally isoelectronic nickel phosphides,BaNi2P2 and LaONiP, with NiP layers dosuperconduct at relatively low (≤5 K)temperatures.

Thompson and colleagues (Los Alamos,USA and Sungkyunkwan University, Korea)report pressure-induced superconductivityin a single crystal of CaFe2As2. Atatmospheric pressure, this material isantiferromagnetic below 170 K but under anapplied pressure of 0.69 GPa it becomes

superconducting, with a relatively hightransition temperature Tc exceeding 10 K,approaching 25–50% of the highest Tcsreported for hole-doped AFe2As2 materials.The rate of Tc suppression with appliedmagnetic field is – 0.7 K T-1, giving anextrapolated zero-temperature uppercritical field of 10 – 14 T.

These observations show that pressureoffers a new route to superconductivity inthese and possibly the related ZrCuSiAsmaterials without the need to introduceextrinsic chemical disorder.

Pressure-induced superconductivity inCaFe2As2

Tuson Park, Eunsung Park, Hanoh Lee, T Klimczuk, E D Bauer, F Ronning and J D Thompson2008 J. Phys.: Condens. Matter 20 322204 (3pp)

Temperature dependence of the normalized resistance of CaFe2As2.Resistance divided by its room-temperature value is plotted againsttemperature for 1 bar (squares) and 0.69 GPa (circles). Arrowsindicate magnetic and superconducting transitions for 1 bar and0.69 GPa, respectively.

Pressure may induce superconductivity in arsenides without the need of chemical disorder

Modular assembly on metal surfacesThe directed assembly of supramolecularcompounds and arrays using discretemolecular building blocks is a topic ofintense research with tremendous potentialfor novel materials and functionalnanoscopic devices. An important factor inthe development, integration, andexploitation of such systems is the capabilityto prepare them on surfaces or innanostructured environments. Recentadvances in supramolecular design on metalsubstrates provide atomistic insight into theunderlying self-assembly processes, mainlyby scanning tunneling microscopyobservations.

Stepanow (Barcelona, Spain), Lin (MPIStuttgart, Germany) and Barth (Munich,Germany) review progress in non-covalentsynthesis strategies under ultra-highvacuum conditions employing metal ions ascoordination centers directing the molecularorganization.

They show that methodologies employingmetal-directed assembly protocols onsurfaces are promising for achieving unique

low-dimensional coordination systems.They are conceivable for a great variety ofsystems and can be applied to substrateswith different symmetries, as well as physicaland chemical properties. Because of theirhigh thermal stability the realized clusters,polymers, and networks constitute apromising route towards low-dimensional

magnetism in a broad temperature range.Also the redox properties of thecoordination centers present an appealingtopic which needs to be further explored.

Nanoporous metal–organic coordinationnetworks can be used to arrange guestspecies in well-defined nanoscaleenvironments, either for patterningpurposes or for investigations of surfacechemical reactions in controlledsurroundings. Furthermore, they bearpotential to control large biomolecules andtheir molecular motions in tunable spaces.Finally, they may serve as templates for theorganization of separated, regularlydistributed magnetic nanoclusters.

Modular assembly of low-dimensionalcoordination architectures on metalsurfacesSebastian Stepanow, Nian Lin and Johannes V Barth2008 J. Phys.: Condens. Matter 20 184002(15pp)

Illustration of a typical experimental setup for the synthesis andsubsequent STM analysis of metallosupramolecular structures atsurfaces.

Directed assembly of supramolecular compounds and arrays on solid surfaces is reviewed



Understanding jamming

Glasses, colloidal and chemical gels, granularmaterials and foams are examples of systemswhich, by changing the control parameters,exhibit slow dynamics followed by astructural arrest called jamming.Understanding this jamming transition is amajor problem in soft-matter physics. Arecent advance is the idea that the dynamicalheterogeneities (DH) play the same role ascritical fluctuations in ordinary criticalphenomena because the decay of densityfluctuations in glasses and jammed systemstakes place via the dynamically correlatedmotions of groups of particles. Coniglio(Napoli, Italy) et al give a brief review of theproperties of the dynamical heterogeneitiesin glasses then analyze the cases of chemicaland colloidal gels, which are still intenselydebated.

The study of DH allows us to clarify thenature of slow dynamics and structuralarrest observed in gels and glasses. Thebehavior of the dynamical susceptibility,which describes the DH, in chemical gelation

is quite different from that in hard-sphereglasses. It grows steadily and reaches aplateau whose value coincides with the meancluster size in the low-wavevector limit. Inparticular, the low-wavevector signal can

detect the critical behavior of the system atthe gelation transition. In colloidal gelationat low T , DH are associated with clustersmade of long-living bonds and thedynamical susceptibility reaches a plateau asin chemical gels, except that, at long time, itdecays to zero due to the finite lifetime of theclusters. At higher volume fraction the DHcross over to a different behavior wherecrowding effects start to dominate. Thisdifference gives rise to the differentdynamical behaviors found in gels andglasses. Whether and how it is possible toalso geometrically characterize the type ofDH typical of glasses is still an openquestion.

Dynamical heterogeneities: from glassesto gelsA Coniglio, T Abete, A de Candia, E Del Gadoand A Fierro2008 J. Phys.: Condens. Matter 20 494239 (7pp)

The study of dynamical heterogeneities clarifies the slow dynamics of gels and glasses

The relaxation time, calculated from the self-intermediate scatteringfunction at small wavevector, together with the lifetime of the bonds�b. At low volume fraction the lifetime of the bonds is much longerthan the relaxation time.

Colloidal glasses

Colloidal glasses are concentratedsuspensions of microscopic particles in aliquid in which the particles’ motions areconstrained; they retain some freedom forlocal Brownian motions but are unable todiffuse over large distances. At rest they aresoft solids, deforming elastically under smallapplied stresses, but yielding and flowingwhen stressed more strongly (think oftoothpaste).

In a prize-winning paper from the EPSLiquid Matter Conference, Pusey (EdinburghUniversity, UK) compares and contrasts thebehaviour of two types of colloidal glass:repulsive colloidal glasses, in which particlesare caged by their neighbours, and attractiveglasses, where interparticle bonding is thedominant influence.

Dynamic light scattering and rheologyexperiments measure average properties ofthe sample, but glasses are heterogeneous,exhibiting localized regions of dynamicactivity: ‘dynamic heterogeneities’ inquiescent glasses and the possibly related‘shear transformation zones’ in stressedglasses. It still remains to obtain a full pictureof glass behaviour that reconciles themacroscopic average properties with themicroscopic heterogeneity. More study ofthe microscopic processes in colloidalglasses under stress, by confocal microscopyfor example is needed.

In common with most colloidal systems,the particles here have a distribution of sizes(polydispersity). Even a small spread inparticle size can strongly influence both

crystallization and glass formation incolloidal systems. In fact, computersimulation suggests that an assembly ofequal-sized hard spheres does not even showa glass transition. Recent calculation of theequilibrium phase diagram of polydispersehard spheres shows a complicated structurefor polydispersities greater than ~0.05 whichincludes multiple crystal phases withdifferent lattice parameters. Further work onthe influence of polydispersity is necessaryto obtain the full picture.

Colloidal glassesP N Pusey2008 J. Phys.: Condens. Matter 20 494202 (6pp)

Experiments help to clarify the properties of two types of colloidal glass



Atomic scale friction

Whether friction depends on temperature iscontroversial but it is generally agreed thatatomic-scale friction is temperature-dependent. Krylov (Russian Academy ofSciences, Moscow) and Frenken (LeidenUniversity, The Netherlands) show that,under many typical conditions, thermaleffects play a crucial role.

The dependence of atomic scale frictionon temperature is physically rich. For certainvalues of the system parameters, a modestchange of temperature can lead to an order-of-magnitude change in friction. At differenttemperatures one observes physicallydifferent regimes of sliding, correspondingto essentially different scenarios of energydissipation.

Krylov and Frenken find an ultra-lowvalue of the effective mass of the nanoscalecontact (m ~ 10-20 kg), ten or more orders ofmagnitude smaller than the mass, M, of themacroscopic friction sensor, leading to avery rapid activated motion of m and thecorresponding, partial or complete,delocalization of the contact, which areresponsible for the puzzling behavior offriction observed.

Their results should stimulatetemperature-variable friction forcemicroscopy (FFM) experimentation. Thereare a rich variety of physically differentfriction regimes related to different types ofcontact delocalization, representingdifferent scenarios of energy dissipation. Astrong variation of friction with temperaturehas been observed in a very recent FFMexperiment. More detailed, quantitativemeasurements are necessary for comparisonbetween the theory and experiment.

An FFM tip is believed to model a singlenanoasperity, one of those that establish thecontact between macroscopic sliding bodies.If so, although direct experimentalverification of most of the authors’observations is still lacking, their resultssuggest that there is a much morepronounced role of thermally drivendynamics in macroscopic sliding than hasever been imagined.

The crucial role of temperature in atomicscale frictionSergey Yu Krylov and Joost W M Frenken2008 J. Phys.: Condens. Matter 20 354003 (7pp)

FFM measuring system (schematic) and the corresponding two-mass–two-spring model. K is the spring coefficient of the cantileverand k is the spring coefficient associated with the tip apex. M is themass of the cantilever + tip system and m is an effective mass,representing the tip apex. The tip–surface interaction is modeled by asinusoidal potential with a corrugation U0 and with the period a ofthe substrate lattice. The system is driven with a constant velocity V.

New experiments show that temperature plays a crucial role in atomic scale friction

Heating effects on nanostructures

Much progress has been made inexperimental techniques and theoreticalmodeling of building electronic devices fromnanostructures, which is very promising forsize reduction and power dissipation, butlocal heat production and dissipation inthese systems has attracted much lessattention. These effects have contributionsfrom both electron–phonon andelectron–electron interactions.

D’Agosta and Di Ventra (UC San Diego,USA) consider the effect of the local electronand ionic heating on the conductance ofnanoscale systems. They show that the non-linear dependence of the conductance on theexternal bias may be used to inferinformation about the local heating of bothelectrons and ions.

They discuss a novel hydrodynamicapproach to transport that allows thedescription of charge and heat flow in termsof the single-particle density and velocityfield of the electron liquid. The theory allowsthem to make predictions about the electron

flow past a nanostructure and itsdependence on the external bias (or thecurrent). One such prediction is the heatingof electrons locally at the nanojunction.They have considered the measurableconsequences of this effect on the inelasticconductance which shows a broadening atthe inelastic step larger than the oneexpected from the background nominal

temperature. They have compared theirtheory with experimental results and founda reasonable quantitative agreement for thecase of a D2 molecule between two Pt leads.For the case of a H2 molecule between thesame leads their theory is only in qualitativeagreement with the experimental findings.They also predict that the width of theinelastic conductance step should increaselinearly with bias, a fact that can be testedexperimentally.

Local electron and ionic heating effectson the conductance of nanostructuresRoberto D’Agosta and Massimiliano Di Ventra2008 J. Phys.: Condens. Matter 20 374102(10pp)

Comparison of (a) experimental results of the non-linear DCconductance of a H2 molecule between two electrodes and (b) plot ofcalculated GH2 as a function of the external bias.

Theory of local heating effects in nanostructures gives good agreement with experiment



The anomalous Hall effect

The anomalous Hall effect (AHE) is one ofthe most famous transport phenomena inmagnetic materials. The Hall resistance of amagnetic film has the usual contributionproportional to the applied magnetic fieldand another, anomalous, contributionobservable only in the ferromagnetic state,which is often proportional to themagnetization, M. Recent theoreticalresearch showed that the linear dependenceon M is not universal and the Hall resistivitycan show resonance features as a function ofvarious parameters.

Recent advances in spintronics revivedinterest in the AHE as a useful tool to controlspin-polarized currents and to characterizethe magnetization. There is renewedtheoretical interest through the newinterpretation of the anomalous Hallconductivity in terms of Berry phases andtopological defects in the crystal bandstructure.

The modern semiclassical theoryrigorously takes into account all importantcontributions in the model of electrons inBloch bands interacting with staticimpurities, and its predictions were verifiedwith rigorous quantum mechanicaltechniques. However, this semiclassicaltheory deals only with electrons that do notinteract with each other.

The important problem now is the effectof the nonzero Berry curvature on many-body interactions, which is reviewed bySinitsyn (Los Alamos, USA). The theory

operates only with gauge-invariant conceptsthat have a simple semiclassicalinterpretation and provides a cleardistinction between the variouscontributions to the Hall current. While theconstruction of such an approach to theanomalous Hall effect problem has longbeen sought, only the new semiclassicaltheory showed agreement with quantitativeresults of rigorous approaches based onGreen function techniques.

Semiclassical theories of the anomalousHall effectN A Sinitsyn 2008 J. Phys.: Condens. Matter 20 023201(17pp)

The 2D electron system considered.

Semiclassical theories of the anomalous Hall effect are reviewed

Oxide surfaces

Interfaces between insulatorsConducting interfaces between polar andnon-polar insulating perovskites showmany important effects.Many transition metal oxides are stronglycorrelated electron systems (SCESs), which,although chemically similar, range fromband insulator, through Mott insulator,semiconductor, metal, to superconductor,and also many unusual magnetic propertiessuch as colossal magnetoresistance. This isdue to the profound effect subtle structuralchanges have on the interplay between thevalence electrons.

A fundamental understanding ofcorrelated electron effects in surfaces andinterfaces is essential in the drive to fabricatefuture devices exploiting these effects. Two-dimensional electron gases insemiconductors are used in applicationssuch as optoelectronics, high-power radio-frequency and magnetoelectronic devices.The ability to grow heterostructures ofoxides exhibiting similar effects is asignificant step towards the fabrication ofall-oxide devices.

Pauli and Willmott (Paul Scherrer Institut,Switzerland) give an overview of recentstudies of two-dimensional electron gasesformed at the interface between polar andnon-polar perovskites. They discuss the

proposed explanations of the origin of theconductivity and properties of the groundstate.

The meteoric progress in the fundamentalunderstanding of thin film growth andatomic engineering of polar interfaces inoxide heterostructures over the last few yearshas laid the bedrock for the futurefabrication of integrated electronic devicesusing these exceptionally adaptablematerials. It is expected that the effect ofmetallicity at the interface betweeninsulators, a wonderfully illustrativeexample of how subtle changes in thestructure of these systems can lead tofundamental changes in physical properties,will play an important role in the futuresuccess of this technology.

Conducting interfaces between polarand non-polar insulating perovskitesS A Pauli and P R Willmott2008 J. Phys.: Condens. Matter 20 264012 (9pp)

Dawber (Stony Brook University, USA) andco-workers looked at what are the mostimportant factors that impact on thebehaviour of very thin ferroelectrics, and inparticular the role that one or manyinterfaces can play. They have especially triedto point to some areas where things are notfully understood and where some moreattention might be required, from the role ofstrain, to the finer details of electrostaticscreening. Finally and, perhaps mostimportantly, they have tried to stress some ofthe possibilities that exist in ferroelectricoxide materials, in particular in superlatticesystems where the interfaces can become themajority of the material and drive new,unexpected and fascinating behaviour.

New phenomena at the interfaces of verythin ferroelectric oxidesM Dawber, N Stucki, C Lichtensteiger, S Gariglio and J-M Triscone2008 J. Phys.: Condens. Matter 20 264015 (6pp)

(a) The polar catastrophe: the electric potential induced by thealternating charged layers increases with each layer of LaAlO3. In(b), divergence is avoided by transfer of half an electron per unit cell inthe top titanium layer.

A special issue in Journal of Physics: Condensed Matter (20, No 26) aimed to convey to the reader the most up-to-date understanding ofthe physics of the surfaces, interfaces, and thin films of complex metal oxides, in a clear and accessible manner. Here are summariesof two of the highlights



Holey metallic films

When metallic films are perforated withperiodic arrays of holes, three differentphenomena arise: (i) extraordinary opticaltransmission in single metallic layers, (ii) theappearance of surface electromagneticmodes (the so-called spoof surfaceplasmons) when an array of holes is drilledon the surface of a perfect electricalconductor and (iii) the negative refractiveindex behavior observed in double-fishnet(DF) structures in which a periodic holearray is perforated on ametal–dielectric–metal stack.

García-Vidal (Madrid, Spain) and co-workers have analyzed in detail the physicalmechanisms leading to the negativerefractive index response observed in DFstructures made of perfect electricalconductors. They have found that theelectric response is mainly controlled by thecut-off frequency of the hole waveguide,which marks the separation betweenpositive and negative values for the effectiveelectric permittivity. On the other hand, theresonant magnetic response that yields

negative values for the magneticpermeability is due to the excitation of gap-surface-plasmon-polariton-like modespropagating along the dielectric gap. Theyhave related these two properties to thephenomena of extraordinary opticaltransmission and spoof surface plasmonmodes in perforated perfect conductors.

They have also analyzed how the negativerefractive index response evolves whenmany DF units are stacked together. Multiplemagnetic resonances emerge in thesestructures originating from theelectromagnetic coupling between thedifferent gap surface modes of the dielectricgaps. Finally, they have shown that in a truly3D DF-based metamaterial the negativerefractive index behavior is maintained.

Plasmonic metamaterials based on holeymetallic filmsA Mary, Sergio G Rodrigo, L Martín-Morenoand F J García-Vidal2008 J. Phys.: Condens. Matter 20 304215 (8pp)

(a) Schematic picture of a DF structure: a square array of rectangularholes of side ax and ay perforated on two metallic films of thickness hm.The perforated films are separated by a dielectric medium of thicknesshd, characterized by a dielectric constant �d. Parameter d defines theperiod of the array. The structure with SIBC is illuminated by a p-polarized plane wave.

Unusual phenomena emerge when metallic films are perforated with periodic arrays of holes

Vortex ferroelectric domains

Over the past 60 years several physicists haveconsidered the possibility that magneticspins or electric polarization vectors mightorder not rectilinearly but in circles ortoroids. These vortex domain structures arewell known in nanoferromagnets, such asFe/Ti ilmenite. They have recently beenpredicted in ferroelectrics, but have not beenreported experimentally.

Gruverman (University of Nebraska-Lincoln, USA) and co-workers reportobservation of the instantaneous Pz domainconfigurations during switching in micron-sized ferroelectric capacitors oflead–zirconate–titanate on a 100 nstimescale. Formation of the doughnut-shapePz domain pattern with an unswitchedcircular domain at its center is attributed to avortex domain structure developing duringdynamic switching. This conclusion is basedon numerical switching simulations usingthe Landau–Lifshitz–Gilbert equationswhich agree very closely with experimentaldata for both circular and square-shapedcapacitors. The simulations predict asignificant in-plane polarization duringswitching in circular capacitors, whichagrees with the model of Naumov and Fu butis not directly measurable by the presentquasi-static measurements.

Vortex ferroelectric domainsA Gruverman, D Wu, H-J Fan, I Vrejoiu, MAlexe, R J Harrison and J F Scott2008 J. Phys.: Condens. Matter 20 342201 (5pp)

In another fast-track communication, Scottand colleagues interpret their recentdiscovery of two new magnetic phasetransitions near 140 and 200 K in bismuthferrite as spin-reorientation transitions.They have measured the magnon Ramancross-sections for bismuth ferrite as afunction of temperature near these phasetransitions and evaluated the criticalexponents characterizing them. Thesecritical exponents are much less than 1, andhence the data fit a logarithmic divergenceabout as well. They apply the basic Pippardrelationship between susceptibilities and

specific heat to a magnetoelastic system. Theobservations are related to the mechanicalloss anomalies observed at the sametemperatures. Their results support thedistorted spin cycloid model of Zalesskii et aland not the earlier model of Sosnowska et al.

Critical phenomena at the 140 and 200 Kmagnetic phase transitions in BiFeO3

J F Scott, M K Singh and R S Katiyar2008 J. Phys.: Condens. Matter 20 322203 (4pp)

Two-dimensional maps of polarization components Pz , Py and Px and in-plane polarization Pxy corresponding to an instantaneous domainpattern in the 1-µm-diameter circular capacitor during switching; the in-plane polarization map Pxy illustrates a vortex structure.

Vortex domains are observed during switching of ferroelectric capacitors and modeled theoretically



Quantum oscillations in SrFe2As2

Knowing the nature of elementaryexcitations and their interplay with the Fermisurface is essential for understanding theorigin of unconventional pairing in high-temperature superconductors. These areinaccessible in the parent antiferromagneticphases of layered cuprate superconductors,whose strongly coupled nature precludes theexistence of a Fermi surface. In contrast, theparent phase of the recently discoveredFeAs-layered high-Tc superconductors iselectrically conducting, leaving open thepossibility of fermionic excitations.

Sebastian (University of Cambridge, UK)and co-workers report measurements ofquantum oscillations in SrFe2As2, whichbecomes superconducting under doping andthe application of pressure. The magneticfield and temperature dependences of theoscillations between 20 and 55 T in the liquidhelium temperature range suggest that theelectronic excitations are those of a Fermiliquid. They show that the observed Fermisurface comprising small pockets isconsistent with the formation of a spin-density wave, showing that high-Tc

superconductivity can occur on doping orpressurizing a conventional metallic spin-density-wave state.

While the ultimate relevance of theseresults on the pnictide family ofsuperconductors to high-Tc cuprates willdepend on the nature of the pairingmechanism and Cooper pair wavefunctionsymmetry in the two systems, a comparativestudy of the elementary excitations isinformative. Mott physics, implying strongcorrelations and non-fermionic excitations,remains a central point of discussion forunconventional pairing in high Tc. Theseresults indicate that strong correlations inthe parent high- Tc antiferromagnetic phaseSrFe2As2 are no obstacle to the establishmentof conventional metallic behaviour.Importantly, Mott physics is not a requisitecharacteristic of the parent phase for high-Tc

superconductivity to develop.

Quantum oscillations in the parentmagnetic phase of an iron arsenide high-temperature superconductorSuchitra E Sebastian, J Gillett, N Harrison, P H C Lau, D J Singh, C H Mielke and G G Lonzarich2008 J. Phys.: Condens. Matter 20 422203 (5pp)

(a) The paramagnetic (unreconstructed) Fermi surface of SrFe2As2

calculated using the local-density approximation, together with atop–down view showing the relative sizes of the sections within theBrillouin zone. (b) The reconstructed Fermi surface resulting fromfolding together with a top–down view of the antiferromagneticBrillouin zone.

Study of quantum oscillations in their parent compound helps understanding of FeAs superconductors

The 0.7 anomaly

One-dimensional (1D) electron quantumwires and quantum point contacts have beenextensively studied for the past 20 years.While the quantization of the 1D ballisticconductance g = i x 2e2/h (where i is aninteger) is well established, the origin of anunexpected feature at ~0.7 x 2e2/h remains anoutstanding anomaly. The so-called ‘0.7structure’ is the subject of a special issueJ. Phys: Condensed Matter 20 No 16.

Hamilton (University of New SouthWales, Australia) and co-workers havestudied the anomalous 0.7 structure andzero-bias anomaly (ZBA) in high-qualityballistic one-dimensional hole systems.Interaction effects should be much strongerin these systems, because of the enhancedhole effective mass. Yet the zero magneticfield data for electron and hole quantumwires are strikingly similar, with a clear 0.7structure that becomes stronger withincreasing temperature. However unlike 1Delectron systems the Zeeman splittingcaused by an in-plane magnetic field ishighly anisotropic for 1D holes, due to thestrong spin–orbit coupling. They have

shown that both the 0.7 structure and theZBA share this anisotropic response to an in-plane magnetic field. These results suggestthat the 0.7 structure and the ZBA aredirectly linked, and that they are both relatedto spin.

Their data also provide new constraints totest several of the proposed models for the0.7 structure. Firstly, since the phononcoupling is very different for holes andelectrons, it is unclear whether modelsinvolving acoustic phonon scattering can

explain the data both in electrons and holes.Secondly, it is not obvious that the Kondomodel can be applied to hole systems whenonly the mJ = 3/2 band is occupied. Morework is needed to understand the electronicand spin properties of holes confined to 1Dsystems, but already it is clear that 1D holeshave unusual spin properties with nocounterpart in spin-1/2 electron systems.

The 0.7 anomaly in one-dimensionalhole quantum wiresA R Hamilton, R Danneau, O Klochan, W R Clarke, A P Micolich, L H Ho, M Y Simmons, D A Ritchie, M Pepper, K Muraki and Y Hirayama2008 J. Phys.: Condens. Matter 20 164205 (8pp)

(a) Conductance of a quantum wire as a function of side-gate voltageVSG, for different values of the magnetic field applied parallel to thewire (along [-233]). Traces have been offset for clarity, and B|| isincreased in 0.2 T steps from 0 to 3.6 T. (b) Greyscale image of thedata, showing the evolution of the conductance plateaus as a functionof the applied magnetic field. The colour axis is the transconductance∂g/∂VSG, with dark regions denoting the conductance plateaus.

An unusual feature in the conductance of 1D conductors is the subject of intense study



Capillary adhesion

When two solids are in close contactcapillary bridges may form at the interface,either from liquid-like contamination layersor from lubrication. The influence of thesecapillary bridges on adhesion is well known(one can build sand castles from slightly wetsand but not from dry sand or sand floodedwith water). The capillary bridge mediatedwall–wall interaction has many importantapplications, e.g. for granular materials,animal adhesion and MEMS.

Persson (Jülich, Germany) presents ageneral theory for how the contact area andthe work of adhesion between two elasticsolids with randomly rough surfaces dependon the relative humidity. The surfaces areassumed to be hydrophilic, and capillarybridges form at the interface between thesolids. For elastically hard solids withrelatively smooth surfaces, the area of realcontact and therefore also the sliding frictionare maximal when there is just enough liquidto fill out the interfacial space between thesolids, which typically occurs when dK, theheight of the capillary bridge, is about threetimes hrms, the root-mean-square (rms)roughness of the (combined) surface

roughness profile. For elastically soft solids,the area of real contact is maximal for verylow humidity where the capillary bridges areable to pull the solids into nearly completecontact. In both cases, the work of adhesionis maximal when dK >> hrms, corresponding tohigh relative humidity.

The theory is compared withexperimental data for microcantileverstructures and shows good agreement withthe data, while the classicalGreenwood–Williamson theory fails

qualitatively. Persson also presentsapplications to rubber wiper blades, wherethe theory can explain the large frictionobserved in the so-called ‘tacky region’ fornearly dry contacts, where the capillarybridges pull the rubber into intimate contactwith the glass substrate over a large fractionof the nominal contact area.

Capillary adhesion between elasticsolids with randomly rough surfacesB N J Persson2008 J. Phys.: Condens. Matter 20 315007(11pp)

See also

On the elastic energy and stresscorrelation in the contact betweenelastic solids with randomly roughsurfacesB N J Persson2008 J. Phys.: Condens. Matter 20 312001 (3pp)

Capillary bridges formed at two asperity contact areas exert anattractive force on the block. The sum of the capillary force and theexternal load must equal the repulsive force arising from the area ofreal contact between the solids.

Humidity has a strong effect on the adhesion between randomly rough surfaces

Image: Artistic impression of coordination of atoms on a chiral surface of copper A Kara and T Rahman 2006 Journal of Physics: Condensed Matter 18 8883–8890

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Optical properties of quantum dots

The optical properties of semiconductorquantum dots (QDs) are in many respectssimilar to those of atoms. Since QDs can bedefined by state-of-the-art semiconductortechnologies, they exhibit long-termstability and allow for well-controlled andefficient interactions with both optical andelectrical fields. Resonant picosecondexcitation of single QD photodiodes leads tonew classes of coherent optoelectronicfunctions and devices, which exhibit precisestate preparation, phase-sensitive opticalmanipulations and the control of quantumstates by electrical fields.

Zrenner (Paderborn University, Germany)and co-workers have reported on thecoherent optical and optoelectronicproperties of a single QD in a photodiode,which can be described as an exciton two-level system with electrical contacts.Compared with conventional photodiodes,single QD photodiodes exhibit resonantspectral sensitivity, which can be tuned overa wide range by the quantum-confined Starkeffect. With resonant ps laser excitation newand so far unattained coherentfunctionalities also become available. Ofparticular interest for future applications are(i) the phase-sensitive response on multiplelaser pulses, (ii) the availability of coherentstate preparation and manipulation in anoptoelectronic device and (iii) the availabilityof quantitative (photocurrent), projectivequantum measurements.

Coherent optoelectronics with singlequantum dotsA Zrenner, P Ester, S Michaelis deVasconcellos, M C Hübner, L Lackmann, S Stufler and M Bichler2008 J. Phys.: Condens. Matter 20 454210 (6pp)

Winkelnkemper et al (Berlin TechnicalUniversity, Germany) have theoreticallyinvestigated the spectroscopic properties ofc-plane GaN/AlN QDs with a specialemphasis on their suitability as sources ofsingle polarized photons. They have shownthat the linear polarization of the interbandtransitions is effectively controlled by ananisotropic strain field within the QDs.Transitions involving either A- or B-typehole states are polarized in orthogonaldirections. The separation of the A-typeground state and the orthogonally polarizedB-type first excited state is ~~10 meV andlargely independent from the QD size andshape.

A sufficient anisotropy can be inducedeither by a structural elongation of the QDsor by an externally applied uniaxial stress.An in-plane elongation of the QDs of only10% leads to a polarization degree of theexcitonic ground-state transition of up to6:1, depending on the other structuralparameters of the QDs. An externallyapplied uniaxial stress of about 300 MPaleads to a polarization degree of more than

5:1; larger stress results in a completepolarization of the emission. Moreover, apolarization resulting from structuralelongation of the QDs can be compensatedby external stress. This effect could beexploited for future devices, in particular toachieve a well-defined polarization in QD-based single-photon emitters.

GaN/AlN quantum dots for single qubitemittersM Winkelnkemper, R Seguin, S Rodt, A Hoffmann and D Bimberg2008 J. Phys.: Condens. Matter 20 454211 (6pp)

(a) Schematic view of a single QD photodiode. Optical access to a single QD is provided via a shadow mask and a semitransparentSchottky gate. (b) Photocurrent spectrum of the single exciton ground state with fully resolved fine structure splitting.

The journal published a special section containing papers from the Proceedings of the 15th International Winterschool on NewDevelopments in Solid State Physics (Mauterndorf (Bad Hofgastein), 18–22 February 2008) (J. Phys.: Condens. Matter 20 No 45). It published papers on transport in semiconductor nanostructures, quantum dots—optics, growth and characterization ofnanostructures, photonic crystals, magnetism in semiconductors, graphite and graphene and nanowires. Here are summaries of two of the papers on quantum dots

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