Condensed Matter - IOPscience...journal of Physics: Condensed Matter Highlights 2011 7 Electronic structure of optimally doped pnictide Ba 0.6 K 0.4 Fe 2 As 2: a comprehensive angle-resolved

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Highlights 2011

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Image: Reciprocal space map of a 270 nm thick NBFO film on a MgO substrate. Leontyev et al 2011 J. Phys. Condens. Matter 23 332201.

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Dear Colleagues,

2011 was a great year for Journal of Physics: Condensed Matter (jPCM), with the journal’s impact strengthening in the community and our ever-increasing readership enjoying high-quality science. In january, I took over as Editor-in-Chief from David Ferry who has steered this journal through some significant changes. I hope that throughout, and after, my tenure as Editor-in-Chief the journal will remain a place that the condensed matter community chooses to publish exciting research.

jPCM remains a perfect venue for papers reporting work in fast-moving areas of condensed matter physics. Very fast publication times ensure that papers reach the community as quickly as possible, with no compromise on quality. For your most timely and high impact work Fast Track Communications (FTCs) offer average receipt-to-publication times of under 50 days. Look out for our FTC brochure, which highlights some of the best FTCs from 2011.

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This highlights brochure presents some of the leading research published in the journal during 2011. The articles have been selected based on downloads from our readers. Please visit our website at www.iopscience.org/jpcm to read about the latest developments in condensed matter physics and remember that all articles are free to read for 30 days following publication.

Editor-in-Chief Jason S Gardner

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Journal of PhysicsCondensed MatterJournal of Physics

Featured in this issueSurface, Interface and Atomic-Scale Science

Volume 23 Number 1 12 January 2011


Topical reviewControl of molecule-based transport for future molecular devicesSilvia Karthäuser

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Front cover image: Artistic impression of traction stress maps of neutrophils migrating on hydrogels of varying stiffness in response to a gradient of the bacterial chemoattractant fMLP, R A Jannat et al 2010 J. Phys.: Condens. Matter 22 194117.

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H i g h l i g h t s 2 0 1 1 5

Contents

Direct graphene growth on MgO: origin of the band gap page 8Sneha Gaddam, Cameron Bjelkevig, Siping Ge, Keisuke Fukutani, Peter A Dowben and Jeffry A Kelber

Plasmon electron–hole resonance in epitaxial graphene 8C Tegenkamp, H Pfnür, T Langer, J Baringhaus and H W Schumacher

Phononic heat transfer across an interface: thermal boundary resistance 9B N J Persson, A I Volokitin and H Ueba

Atomistic simulation of a graphene-nanoribbon–metal interconnect 9A Smolyanitsky and V K Tewary

A density functional theory study of Mn nanowires on the Si(001) surface 9Alex M P Sena and David R Bowler

Nanobubbles and micropancakes: gaseous domains on immersed substrates 10James R T Seddon and Detlef Lohse

Structure and flow of droplets on solid surfaces 10P Müller-Buschbaum, D Magerl, R Hengstler, J-F Moulin, V Körstgens, A Diethert, J Perlich, S V Roth, M Burghammer, C Riekel, M Gross, F Varnik,

P Uhlmann, M Stamm, J M Feldkamp and C G Schroer

Histone-based self-assembly into DNA-wrapped meso-clusters 10M Inoue, S Tanaka and H Frusawa

Theory and simulations of water flow through carbon nanotubes: prospects and pitfalls 11Douwe Jan Bonthuis, Klaus F Rinne, Kerstin Falk, C Nadir Kaplan, Dominik Horinek, A Nihat Berker, Lydéric Bocquet and Roland R Netz

Yielding in dense suspensions: cage, bond, and rotational confinements 11Ryan C Kramb and Charles F Zukoski

Loop formation of microtubules during gliding at high density 11Lynn Liu, Erkan Tüzel and Jennifer L Ross

The origin of the attraction between like charged hydrophobic and hydrophilic walls confining a near-critical binary 12aqueous mixture with ions Faezeh Pousaneh and Alina Ciach

Electronic properties of corrugated graphene: the Heisenberg principle and wormhole geometry in the solid state 12Victor Atanasov and Avadh Saxena

Melting of graphene: from two to one dimension 13K V Zakharchenko, Annalisa Fasolino, J H Los and M I Katsnelson

Graphene as a non-magnetic spin current lens 13F S M Guimarães, A T Costa, R B Muniz and M S Ferreira

Thermal transport by phonons in zigzag graphene nanoribbons with structural defects 13Zhong-Xiang Xie, Ke-Qiu Chen and Wenhui Duan

Surface, interface and atomic-scale science

Liquids, soft matter and biological physics

Nanostructures and nanoelectronics


6 H i g h l i g h t s 2 0 1 1

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Electronic properties of graphene nanostructures 14F Molitor, J Güttinger, C Stampfer, S Dröscher, A Jacobsen, T Ihn and K Ensslin

Correlated conformation and charge transport in multiwall carbon nanotube-conducting polymer nanocomposites 14Paramita Kar Choudhury, S Ramaprabhu, K P Ramesh and Reghu Menon

Phonons in graphene with point defects 14Vadym Adamyan and Vladimir Zavalniuk

Probing the continuous radio frequency spectrum of water relaxation using a carbon nanotube 15H T Kim, D W Kim, J S Hwang, J S Shin, D Whang, D Ahn and S W Hwang

First-principles study on thermodynamic properties and phase transitions in TiS2 15Yonggang G Yu and Nancy L Ross

Electron transport and Goos–Hänchen shift in graphene with electric and magnetic barriers: optical analogy and band structure 16Manish Sharma and Sankalpa Ghosh

Spin–orbit splitting in graphene on metallic substrates 16Z Y Li, Z Q Yang, S Qiao, J Hu and R Q Wu

Zigzag graphene nanoribbons: bandgap and midgap state modulation 17Hassan Raza

Effect of strain on the thermoelectric properties of silicon: an ab initio study 17N F Hinsche, I Mertig and P Zahn

Ab initio random structure searching 17Chris J Pickard

Electronic structure and jahn–Teller effect in GaN:Mn and ZnS:Cr 18F Virot, R Hayn and A Boukortt and R J Needs

Synthesis of cubic SrCoO3 single crystal and its anisotropic magnetic and transport properties 18Youwen Long, Yoshio Kaneko, Shintaro Ishiwata, Yasujiro Taguchi and Yoshinori Tokura

General DFT++ method implemented with projector augmented waves: electronic structure of SrVO3 and the Mott 18transition in Ca2 − xSrxRuO4 M Karolak, T O Wehling, F Lechermann and A I Lichtenstein

Eliashberg theory of excitonic insulating transition in graphene 19Jing-Rong Wang and Guo-Zhu Liu

Effects of electronic correlation on x-ray absorption and dichroic spectra at L2, 3 edge 19L Pardini, V Bellini and F Manghi

Synthesis and crystal growth of Cs0.8(FeSe0.98)2: a new iron-based superconductor with Tc = 27 K 19A Krzton-Maziopa, Z Shermadini, E Pomjakushina, V Pomjakushin, M Bendele, A Amato, R Khasanov, H Luetkens and K Conder

Theory of high-TC superconductivity: transition temperature 20Dale R Harshman, Anthony T Fiory and John D Dow

Solid structure and lattice dynamics

Electronic structure

Correlated electrons

Superconductors and metals


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Electronic structure of optimally doped pnictide Ba0.6K0.4Fe2As2: a comprehensive angle-resolved photoemission 20spectroscopy investigation H Ding, K Nakayama, P Richard, S Souma, T Sato, T Takahashi, M Neupane, Y-M Xu, Z-H Pan, A V Fedorov, Z Wang, X Dai, Z Fang, G F Chen, J L Luo and N L Wang

Temperature-dependent local structure of NdFeAsO1 − xFx system using arsenic K-edge extended x-ray absorption fine 21structure B Joseph, A Iadecola, L Malavasi and N L Saini

Local structural investigation of SmFeAsO1 − xFx high temperature superconductors 21Lorenzo Malavasi, Gianluca A Artioli, Hyunjeong Kim, Beatrice Maroni, Boby Joseph, Yang Ren, Thomas Proffen and Simon J L Billinge

First-principles study of nitrogen doping in cubic and amorphous Ge2Sb2Te5 22S Caravati, D Colleoni, R Mazzarello, T D Kühne, M Krack, M Bernasconi and M Parrinello

Wavepacket scattering of Dirac and Schrödinger particles on potential and magnetic barriers 22Kh Yu Rakhimov, Andrey Chaves, G A Farias and F M Peeters

Hydrogenated cation vacancies in semiconducting oxides 22J B Varley, H Peelaers, A Janotti and C G Van de Walle

Electron and hole stability in GaN and ZnOs 23Aron Walsh, C Richard A Catlow, Martina Miskufova and Alexey A Sokol

Electronic and local structures of BiFeO3 films 23Y Yoneda and W Sakamoto

Nanoscale polarization switching mechanisms in multiferroic BiFeO3 thin films 23H Béa, B Ziegler, M Bibes, A Barthélémy and P Paruch

Multiferroic magnetoelectric fluorides: why are there so many magnetic ferroelectrics? 24J F Scott and R Blinc

Evidence of orbital excitations in CaCu3Ti4O12 probed by Raman spectroscopy 24Dileep K Mishra and V G Sathe

The giant anomalous Hall effect in the ferromagnet Fe3Sn2—a frustrated kagome metal 24T Kida, L A Fenner, A A Dee, I Terasaki, M Hagiwara and A S Wills

Magneto-structural properties and magnetic anisotropy of small transition-metal clusters: a first-principles study 25Piotr Błonski and Jürgen Hafner

Absence of long-range magnetic ordering in the pyrochlore compound Er2Sn2O7 25P M Sarte, H J Silverstein, B T K Van Wyk, J S Gardner, Y Qiu, H D Zhou and C R Wiebe

Magnetic order in orbital models of the iron pnictides 26P M R Brydon, Maria Daghofer and Carsten Timm

Exchange bias effect in alloys and compounds 26S Giri, M Patra and S Majumdar

A critical re-examination and a revised phase diagram of La1 − xSrxCoO3 26D Samal and P S Anil Kumar

Superconductors and metals (continued)

Semiconductors

Dielectrics and ferroelectrics

Magnetism and magnetic materials


8 H i g h l i g h t s 2 0 1 1

Featured in

LabTalk

Direct graphene growth on MgO: origin of the band gap Sneha Gaddam1, Cameron Bjelkevig1,3, Siping Ge1,4, Keisuke Fukutani2, Peter A Dowben2 and Jeffry A Kelber1

1 Department of Chemistry and Center for Electronic Materials Processing and Integration, University of North Texas, Denton, TX 76203-5017, USA

2 Department of Physics and Astronomy, Nebraska Center for Nanostructures and Materials, University of Nebraska-Lincoln, Lincoln, Theodore jorgensen Hall, 855 North 16th Street, NE 68588-0111, USA

3 Permanent address: Intel Corporation, 4100 Sara Road SE, Rio Rancho, NM 87124, USA

4 Permanent address: Department of Physics, China Agricultural University, Beijing, People’s Republic of China

2011 J. Phys.: Condens. Matter 23 072204

A 2.5 monolayer (ML) thick graphene film grown by chemical vapor deposition of thermally dissociated C

2H4 on MgO(111), displays a significant band gap. The apparent six-fold low energy electron diffraction (LEED) pattern actually consists of two three-fold patterns with different ‘A’ and ‘B’ site diffraction intensities. Similar effects are observed for the LEED patterns of a 1 ML carbon film derived from annealing adventitious carbon on MgO(111), and for a 1.5 ML thick graphene film grown by sputter deposition on the 1 ML film. The LEED data indicate different electron densities at the A and B sites of the graphene lattice, suggesting that the observed band gap results from lifting the graphene HOMO/LUMO degeneracy at the Dirac point. The data also indicate that disparities in A site/B site LEED intensities decrease with increasing carbon overlayer thickness, suggesting that the graphene band gap size decreases with increasing number of graphene layers on MgO(111).

Plasmon electron–hole resonance in epitaxial graphene C Tegenkamp1, H Pfnür1, T Langer1,2, J Baringhaus1 and H W Schumacher2

1 Institut für Festkörperphysik, Leibniz-Universität Hannover, Appelstrasse 2, 30167 Hannover, Germany

2 Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany


The quasiparticle dynamics of the sheet plasmons in epitaxially grown graphene layers on SiC(0001) has been studied systematically as a function of temperature, intrinsic defects, influence of multilayers and carrier density using electron energy loss spectroscopy with high energy and momentum resolution. The opening of an inter-band decay channel appears as an anomalous kink in the plasmon dispersion which we describe as a resonance effect in the formation of electron–hole pairs. Due to the inevitable strong coupling of plasmons with single particle excitations in reduced dimensions, such signatures are generally expected.

LEED pattern, acquired at 80 eV beam energy, for graphene formed by physical vapour deposition on an ordered carbon monolayer.

Surface, interface and atomic-scale science

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Graphene plasmon dispersion before () and after () F4-TCNQ doping. Inset: corresponding changes of k-dependent FWHM. Upper inset: stereographic model of F4-TCNQ. Solid line: fit with the NFEG model without resonant damping.

jPCM articles received 1.4 million downloads in 2011.

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H i g h l i g h t s 2 0 1 1 9

Photonic heat transfer across an interface: thermal boundary resistance B N J Persson1,2, A I Volokitin2,3 and H Ueba1

1 Division of Nanotechnology and New Functional Material Science, Graduate School of Science and Engineering, University of Toyama, Toyama, japan

2 IFF, FZ-jülich, 52425 jülich, Germany 3 Samara State Technical University, 443100 Samara, Russia


We present a general theory of phononic heat transfer between two solids (or a solid and a fluid) in contact at a flat interface. We present simple analytical results which can be used to estimate the heat transfer coefficient (the inverse of which is usually called the ‘thermal boundary resistance’ or ‘Kapitza resistance’). We present numerical results for the heat transfer across solid–solid and solid–liquid He contacts, and between a membrane (graphene) and a solid substrate (amorphous SiO

2). The latter system involves the heat transfer between weakly coupled systems, and the calculated value of the heat transfer coefficient is in good agreement with the value deduced from experimental data.

Heat flow in the contact region between a rigid block with a flat surface. The orange lines denote the heat current flux lines in the upper solid.

Atomistic simulation of a graphene- nanoribbon–metal interconnect A Smolyanitsky and V K Tewary

Materials Reliability Division, National Institute of Standards and Technology, Boulder, CO 80305, USA


We report a molecular statics simulation of the physical processes responsible for binding and lattice distortions in a nanoscale electrical interconnect with realistic boundary conditions. The interconnect consists of a graphene ribbon interfaced with the (111) crystallographic surfaces (over 11000 atoms overall) of two nickel electrodes. We quantify the graphene lattice distortions by mapping strains, as well as out-of-plane atomic displacements on a grid, throughout the simulated interconnect. The results suggest strongly localized graphene lattice distortions at the edges and strains that do not exceed 0.5% elsewhere. Such strains are not expected to affect the electrical properties of the graphene nanoribbon interconnect. A stand-alone graphene nanoribbon is simulated in order to identify the effect of electrodes partially supporting the graphene nanoribbon. Our results indicate that the electrodes reduce the in-plane strains induced by the nanoribbon edges, while causing rippling of the graphene lattice. The average graphene–nickel intersurface separation and the cohesive energy for the top-fcc configuration are calculated at ~2.13 Å and 68.22 meV Å−2. In order to describe the interatomic interactions in the simulation, we utilize a set of accurate atomistic potentials for graphene on a nickel surface. The approach is based on the modified embedded atom method (MEAM) for the C–C and Ni–Ni interactions, and a Morse-type potential, which takes the surface configuration into account, for the Ni–C interactions. Our focus is on

In-plane graphene strains (εxx and ε

yy) and out-of-plane displacements mapped on a 2D grid. The black rectangles show the approximate positions of the nickel electrodes.

the Ni-(111) crystallographic surface interfaced with graphene in top-fcc, top-hcp, and hcp–fcc initial configurations. The interactions were validated by calculating the equilibrium binding energy and intersurface distance. The resulting binding energies and equilibrium intersurface separations obtained are in very good agreement with previous experimental and ab initio data obtained by use of density functional theory (DFT).

A density functional theory study of Mn nanowires on the Si(001) surface Alex M P Sena1,2,3 and David R Bowler1,2,3,4

1 Thomas Young Centre, UCL, Gower St, London WC1E 6BT, UK2 London Centre for Nanotechnology, UCL, 17–19 Gordon St, London WC1H 0AH, UK3 Department of Physics and Astronomy, UCL, Gower St, London WC1E 6BT, UK4 International Centre for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, japan


The structure of experimentally observed Mn nanolines on the Si(001) surface is investigated using density functional theory (DFT) and the DFT + U method. A candidate line structure consisting of a two-atom sub-unit is proposed, based on total energy and appearance in simulated scanning tunnelling microscopy images. The electronic and magnetic properties of this structure are investigated. The atoms in the line are strongly antiferromagnetically coupled with individual Mn atoms having moments of 4 µB. The atoms in the sub-unit are seen to move further apart by 0.57 Å upon forcing ferromagnetic alignment.

Simulated STM images for structure line HM with binding energy 2.00 eV. Images are for filled (left panels) and empty (right panels) states at ±1.5 V.


10 H i g h l i g h t s 2 0 1 1

Nanobubbles and micropancakes: gaseous domains on immersed substrates James R T Seddon and Detlef Lohse

Physics of Fluids, Faculty of Science and Technology, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands


Surface nanobubbles and micropancakes are two recent discoveries in interfacial physics. They are nanoscopic gaseous domains that form at the solid/liquid interface. The fundamental interest focuses on the fact that they are surprisingly stable to dissolution, lasting for at least 10–11 orders of magnitude longer than the classical expectation. So far, many articles have been published that describe various different nucleation methods and ‘ideal’ systems and experimental techniques for nanobubble research, and we are now at the stage where we can begin to investigate the fundamental questions in detail. In this topical review, we summarize the current state of research in the field and give an overview of the partial answers that have been proposed or that can be inferred to date. We relate nanobubbles and micropancakes, and we try to build a framework within which nucleation may be understood. We also discuss evidence for and against different aspects of nanobubble stability, as well as suggesting what still needs to be done to obtain a full understanding.

Topical Review

Liquids, soft matter and biological physicsq

Structure and flow of droplets on solid surfaces P Müller-Buschbaum1, D Magerl1, R Hengstler1, J-F Moulin1, V Körstgens1, A Diethert1, J Perlich1,2, S V Roth2, M Burghammer3, C Riekel3, M Gross4, F Varnik4, P Uhlmann5, M Stamm5, J M Feldkamp6 and C G Schroer6

1 Physik-Department E13, Technische Universität München, Lehrstuhl für Funktionelle Materialien, james-Franck-Straße 1, D-85748 Garching, Germany

2 Deutsches Elektronen Synchrotron, Notkestrasse 85, D-22607 Hamburg, Germany 3 ESRF, BP 220, F-38043 Grenoble Cedex 09, France 4 Ruhr Universität Bochum, Stiepeler Strasse 129, D-44801 Bochum, Germany 5 Leibniz Institut für Polymerforschung Dresden e.V., Hohe Straße 6, D-01069 Dresden, Germany

6 Institute of Structural Physics, Technische Universität Dresden, D-01062 Dresden, Germany


The structure and flow of droplets on solid surfaces is investigated with imaging and scattering techniques and compared to simulations. To access nanostructures at the liquid–solid interface advanced scattering techniques such as grazing incidence small-angle x-ray scattering (GISAXS) with micro- and nanometer-sized beams, GISAXS and in situ imaging ellipsometry and GISAXS tomography are used. Using gold nanoparticle suspensions, structures observed in the wetting area due to deposition are probed in situ during the drying of the droplets. After drying, nanostructures in the wetting area and inside the dried droplets are monitored. In addition to drying, a macroscopic movement of droplets is caused by body forces acting on

an inclined substrate. The complexity of the solid surfaces is increased from simple silicon substrates to binary polymer brushes, which undergo a switching due to the liquid in the droplet. Nanostructures introduced in the polymer brush due to the movement of droplets are observed.

Composite image of GISAXS data measured with a 300 nm-sized x-ray beam scanning the edge of a droplet: a y range of 100 μm is scanned with a step size of Δy = 1 µm.

Featured in

LabTalk

Histone-based self-assembly into DNA-wrapped meso-clusters M Inoue1, S Tanaka2 and H Frusawa1

1 Center for Nanoscience and Nanotechnology, Kochi University of Technology, Tosa-Yamada, Kochi 782-8502, japan

2 Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, japan


The recent discovery of meso-cluster phase includes not only colloidal molecules of synthetic polymer particles but also equilibrium protein clusters. Here we report self-assembly of histone protein into stable submicron clusters that can be generated even in centrifuged supernatants containing no initial aggregates. Furthermore, dark-field microscopy of the electrophoresis has verified charge reversal of individual histone clusters by adding DNA. We have determined the critical nucleotide concentration at which the electrophoretic mobility vanishes in three types of DNA, revealing the coexistence of nucleosomes with DNA-wrapped meso-clusters.

A representative field emission scanning electron microscopy image of dried sample-II, suggesting the coexistence of several submicron particles (marked by red circles) with a large amount of nanoparticles, histone octamers.


JPCM is indexed in PubMed.

Biomedical papers are also indexed and included in Medline.

Special issue on nano- and microfluidics


H i g h l i g h t s 2 0 1 1 11

Theory and simulations of water flow through carbon nanotubes: prospects and pitfalls Douwe Jan Bonthuis1, Klaus F Rinne1, Kerstin Falk2, C Nadir Kaplan1,3, Dominik Horinek1, A Nihat Berker1,4, Lydéric Bocquet1,2 and Roland R Netz1

1 Physik Department, Technische Universität München, Garching 85748, Germany 2 Laboratoire PMCN, Université Lyon 1, CNRS, UMR 5586, 69622 Villeurbanne, France

3 Martin Fisher School of Physics, Brandeis University, Waltham, MA 02454, USA 4 Sabanci Üniversitesi, Orhanli-Tuzla, Istanbul 34956, Turkey


We study water flow through carbon nanotubes using continuum theory and molecular dynamics simulations. The large slip length in carbon nanotubes greatly enhances the pumping and electrokinetic energy conversion efficiency. In the absence of mobile charges, however, the electro-osmotic flow vanishes. Uncharged nanotubes filled with pure water can therefore not be used as electric field-driven pumps, contrary to some recently ventured ideas. This is in agreement with results from a generalized hydrodynamic theory that includes the angular momentum of rotating dipolar molecules. The electro-osmotic flow observed in simulations of such carbon nanotubes is caused by an imprudent implementation of the Lennard-jones cutoff. We also discuss the influence of other simulation parameters on the spurious electro-osmotic flow.

Snapshot of one carbon nanotube used in the simulations: (16, 16) with d = 2.17 nm.

Yielding in dense suspensions: cage, bond, and rotational confinements Ryan C Kramb1 and Charles F Zukoski

Department of Chemical and Biomolecular Engineering, University of Illinois, Urbana, IL 60801, USA 1 Present address: Air Force Research Laboratory, Wright-Patterson Air Force Base, OH, USA


The effect of weak particle anisotropy on the onset of fluidity in dense suspensions of glasses of repulsive, weakly attractive and strongly attractive spherical and dumbbell shaped particles is explored. Yield stresses are found to scale with volume fraction showing a divergence at random close packing for all systems. However the onsets of yielding in suspensions of spherical and dumbbell shaped particles are shown to display qualitatively different behaviors. Suspensions of hard spheres exhibit a single yield stress (strain) while suspensions of spheres experiencing short range attractions in dense gels display two yielding events. Double yielding occurs when attractions between particles are only a few kT and the suspensions are sufficiently dense. For dumbbell suspensions, single yielding is

observed for hard dumbbell glasses in a region where the glasses are expected to be plastic while double yielding is observed when the particles are expected to have localized centers of mass and localized orientations. Double yielding is also observed for dense dumbbell suspensions that experience attractions while only single yielding events are observed in strongly attractive gels for both dumbbells and spheres. These results are discussed in the light of recent theories and simulations of mechanisms of localization in suspensions of spherical and weakly anisotropic particles.

SEM images of the dicolloids used in the experiments.

Featured in

LabTalk

Loop formation of microtubules during gliding at high density Lynn Liu1, Erkan Tüzel2 and Jennifer L Ross1

1 Department of Physics, University of Massachusetts Amherst, Amherst, MA 01003, USA

2 Department of Physics, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609, USA


The microtubule cytoskeleton, including the associated proteins, forms a complex network essential to multiple cellular processes. Microtubule-associated motor proteins, such as kinesin-1, travel on microtubules to transport membrane bound vesicles across the crowded cell. Other motors, such as cytoplasmic dynein and kinesin-5, are used to organize the cytoskeleton during mitosis. In order to understand the self-organization processes of motors on microtubules, we performed filament-gliding assays with kinesin-1 motors bound to the cover glass with a high density of microtubules on the surface. To observe microtubule organization, 3% of the microtubules were fluorescently labeled to serve as tracers. We find that microtubules in these assays are not confined to two dimensions and can cross one other. This causes microtubules to align locally with a relatively short correlation length. At high density, this local alignment is enough to create ‘intersections’ of perpendicularly oriented groups of microtubules. These intersections create vortices that cause microtubules to form loops. We characterize the radius of curvature and time duration of the loops. These different behaviors give insight into how crowded conditions, such as those in the cell, might affect motor behavior and cytoskeleton organization.

Imagej measurements of: (i) the radius, (ii) the filament contour length, and (iii) the velocity.

Special issue on nano- and microfluidics

Special section on cooperative dynamics


12 H i g h l i g h t s 2 0 1 1

Featured in

LabTalk

The origin of the attraction between like charged hydrophobic and hydrophilic walls confining a near-critical binary aqueous mixture with ions Faezeh Pousaneh and Alina Ciach

Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warszawa, Poland


The effect of ionic solute on a near-critical binary aqueous mixture confined between charged walls with different adsorption preferences is considered within a simple density functional theory. For the near-critical system containing small amounts of ions, a Landau-type functional is derived on the basis of the assumption that the correlation, ξ, and the Debye screening length, k–1, are both much larger than the molecular size. The corresponding approximate Euler–Lagrange equations are solved analytically for ions insoluble in the organic solvent. A nontrivial concentration profile of the solvent is found near the charged hydrophobic wall as a result of the competition between the short-range attraction of the organic solvent and the electrostatic attraction of the hydrated ions. An excess of water may be present near the hydrophobic surface for some range of the surface charge and ξk. As a result, the effective potential between the hydrophilic and the hydrophobic surface can be repulsive far from the critical point, then attractive and again repulsive when the critical temperature is approached, in agreement with a recent experiment (Nellen et al 2011 Soft Matter 7 5360).

Model system consisting of water, organic liquid (for example lutidine) and ions between negatively charged hydrophilic (dark, blue) and hydrophobic (light, red) walls.


Nanostructures and nanoelectronicsq

Electronic properties of corrugated graphene: the Heisenberg principle and wormhole geometry in the solid state Victor Atanasov1 and Avadh Saxena2

1 Department of Condensed Matter Physics, Sofia University, 5 Boulevard j Boucher, 1164 Sofia, Bulgaria

2 Theoretical Division and Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, NM 87545, USA


Adopting a purely two-dimensional relativistic equation for graphene’s carriers contradicts the Heisenberg uncertainty principle since it requires setting the off-the-surface coordinate of a three-dimensional wavefunction to zero. Here we present a theoretical framework for describing graphene’s massless relativistic carriers in accordance with this most fundamental of all quantum principles. A gradual confining procedure is used to restrict the dynamics onto a surface and normal to the surface parts, and in the process the embedding of this surface into the three-dimensional world is accounted for. As a result an invariant geometric potential arises in the surface part which scales linearly with the mean curvature and shifts the Fermi energy of the material proportional to bending. Strain induced modification of the electronic properties or ‘straintronics’ is clearly an important field of study in graphene. This opens an avenue to producing electronic devices: micro- and nano-electromechanical systems (MEMS and NEMS), where the electronic properties are controlled by geometric means and no additional alteration of graphene is necessary. The appearance of this geometric potential also provides us with clues as to how quantum dynamics looks in the curved space–time of general relativity. In this context we explore a two-dimensional cross-section of the wormhole geometry, realized with graphene as a solid state thought experiment.

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H i g h l i g h t s 2 0 1 1 13

Melting of graphene: from two to one dimension K V Zakharchenko, Annalisa Fasolino, J H Los and M I Katsnelson

Radboud University of Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 Aj Nijmegen, The Netherlands


The high temperature behaviour of graphene is studied by atomistic simulations based on an accurate interatomic potential for carbon. We find that clustering of Stone–Wales defects and formation of octagons are the first steps in the process of melting which proceeds via the formation of carbon chains. The molten state forms a three-dimensional network of entangled chains rather than a simple liquid. The melting temperature estimated from the two-dimensional Lindemann criterion and from extrapolation of our simulation for different heating rates is about 4900 K.


Snapshot of graphene at T = 5000 K during the melting process in an NPT simulation (N = 1008, P = 0). All pentagons and heptagons are marked in red.

Graphene as a non-magnetic spin current lens F S M Guimarães1, A T Costa1, R B Muniz1 and M S Ferreira2

1 Instituto de Física, Universidade Federal Fluminense, Niterói, Brazil 2 School of Physics, Trinity College Dublin, Dublin 2, Ireland


In spintronics, the ability to transport magnetic information often depends on the existence of a spin current traveling between two different magnetic objects acting as the source and probe. A large fraction of this information never reaches the probe and is lost because the spin current tends to travel omnidirectionally. We propose that a curved boundary between a gated and a non-gated region within graphene acts as an ideal lens for spin currents despite being entirely of non-magnetic nature. We show as a proof of concept that such lenses can be utilized to redirect the spin current that travels away from a source onto a focus region where a magnetic probe is located, saving a considerable fraction of the magnetic information that would be otherwise lost.

Schematic diagram representing an infinitely large graphene sheet under the effect of a gate voltage that acts only in a limited region of space (represented by the grey area). The gated region has a boundary with a curvature defined by the radius R that lies at a distance L

1 from the magnetic atoms. The section delimited by the two horizontal dotted lines is the unit cell used in representing the system with lateral periodic boundary conditions.

Thermal transport by phonons in zigzag graphene nanoribbons with structural defects Zhong-Xiang Xie1, Ke-Qiu Chen1 and Wenhui Duan2

1 Department of Applied Physics, Hunan University and Key Lab for Micro-Nano Physics and Technology of Hunan Province, Changsha 410082, People’s Republic of China

2 Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China


The thermal transport properties by phonons in zigzag graphene nanoribbons with structural defects are investigated by using nonequilibrium phonon Green’s function formalism. We find that the combined effect of the edge and local defect plays an important role in determining the thermal transport properties. In the limit T → 0, the thermal conductance approaches the universal quantum value 3k0(k0 = π 2kB

2T/3h) even when structural defects are presented in graphene nanoribbons. The thermal transport shows a noticeable transformation from quantum to classical features with increasing temperature in the system. A suggestion to tune the thermal conductance by modulating structural defects and the ribbon width in graphene nanoribbons is presented.

The PDOS associated with defects at the position of particular frequency. (a) and (b) depict the PDOS around the vacancy and the SW defect at ω = 28 cm−1 respectively. The radius of the carbon atoms indicates the PDOSs, which are linearly normalized by the maximum value.

(a) (b)

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14 H i g h l i g h t s 2 0 1 1

Electronic properties of graphene nanostructures F Molitor, J Güttinger, C Stampfer1, S Dröscher, A Jacobsen, T Ihn and K Ensslin

Solid State Physics Laboratory, ETH Zurich, 8093 Zurich, Switzerland 1 Present address: jARA-FIT and II Institute of Physics, RWTH Aachen, 52074 Aachen, Germany


In this review, recent developments in the fabrication and understanding of the electronic properties of graphene nanostructures are discussed. After a brief overview of the structure of graphene and the two-dimensional transport properties, the focus is put on graphene constrictions, quantum dots and double quantum dots. For constrictions with a width below 100 nm, the current through the constriction is strongly suppressed for a certain back gate voltage range, related to the so-called transport gap. This transport gap is due to the formation of localized puddles in the constriction, and its size depends strongly on the constriction width. Such constrictions can be used to confine charge carriers in quantum dots, leading to Coulomb blockade effects.

Topical Review

Band structure of graphene.

Correlated conformation and charge transport in multiwall carbon nanotube- conducting polymer nanocomposites Paramita Kar Choudhury1, S Ramaprabhu2, K P Ramesh1 and Reghu Menon1

1 Department of Physics, Indian Institute of Science, Bangalore 560012, India 2 Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India


The strikingly different charge transport behaviours in nanocomposites of multiwall carbon nanotubes (MWNTs) and conducting polymer polyethylenedioxythiophene–polystyrene-sulfonic-acid (PEDOT–PSS) at low temperatures are explained by probing their conformational properties using small-angle x-ray scattering (SAXS). The SAXS studies indicate the assembly of elongated PEDOT–PSS globules on the walls of nanotubes, coating them partially, thereby limiting the interaction between the nanotubes in the polymer matrix. This results in a charge transport governed mainly by small polarons in the conducting polymer despite the presence of metallic MWNTs. At T > 4 K, hopping of the charge carriers following one-dimensional variable range hopping is evident which also gives rise to a positive magnetoresistance (MR) with an enhanced localization length (~5 nm) due to the presence of MWNTs. However, at T < 4 K, the observation of an unconventional positive temperature coefficient of resistivity is attributed to small polaron tunnelling. The

exceptionally large negative MR observed in this temperature regime is conjectured to be due to the presence of quasi-1D MWNTs that can aid in lowering the tunnelling barrier across the nanotube–polymer boundary resulting in large delocalization.

Transmission electron microscopy image of 3 wt% CNT–PEDOT–PSS nanocomposite.

Solid structure and lattice dynamicsq

Phonons in graphene with point defects Vadym Adamyan and Vladimir Zavalniuk

Department of Theoretical Physics, Odessa I I Mechnikov National University, Ukraine


The phonon density of states (DOS) of graphene with different types of point defects (carbon isotopes, substitution atoms, vacancies) is considered. Using a solvable model which is based on the harmonic approximation and the assumption that the elastic forces act only between nearest neighboring ions we calculate corrections to the graphene DOS dependent on the type and concentration of defects. In particular the correction due to isotopic dimers is determined. It is shown that a relatively small concentration of defects may lead to significant and specific changes in the DOS, especially at low frequencies, near the Van Hove points and in the vicinity of the K points of the Brillouin zone. In some cases defects generate one or several narrow gaps near the critical points of the phonon DOS as well as resonance states in the Brillouin zone regular points. All types of defects are characterized by the appearance of one or more additional Van Hove peaks near the (Dirac) K points and their singular contribution may be comparable with the effect of electron–phonon interaction. Besides, for low frequencies and near the critical points the relative change in density of states may be many times higher than the concentration of defects.

The graphitic plane structure. A (full circles) and B (hollow circles) represent two sublattices, a

1 and a2 are two primitive translation vectors, |a

1,2| = a =√3b, where b = 0.142 nm is the interatomic distance or the length of the carbon–carbon σ -bond.


H i g h l i g h t s 2 0 1 1 15

Featured in

LabTalk

Probing the continuous radio frequency spectrum of water relaxation using a carbon nanotube H T Kim1, D W Kim1, J S Hwang1, J S Shin1,2, D Whang1,3, D Ahn4 and S W Hwang1

1 Research Center for Time-domain Nano-functional Devices and School of Electrical Engineering, Korea University, 5-1 Anam, Sungbuk, Seoul 136-701, Korea

2 Communication Laboratory, Samsung Advanced Institute of Technology San 14-1, Giheung, Yongin, Gyeonggi 446-712, Republic of Korea

3 School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon, jangan, Suwon, Gyeonggi 440-746, Korea

4 School of Electrical and Computer Engineering, University of Seoul, 90 jeonnong, Dongdaemun, Seoul 130-743, Korea


We have obtained the continuous radio frequency spectrum of water molecule relaxation using carbon nanotubes (CNT) as a high-speed nanoprobe. Three sets of characteristic time scales are clearly identified. Two sets are attributed to the electric-field-driven polarization of water molecules bound to CNTs and the collective relaxation of water layers in the vicinity of CNTs, respectively. The third set is appreciable only in air, and can be related to triplet oxygen relaxation.


(a) IDS –VGS characteristics obtained from our CNT field-effect transistor (shown in the inset) in air (solid lines) and in vacuum (dotted lines). V

DS was fixed at 1, 3 and 5 V. (b) Schematic of the coplanar waveguide, on top of which a single CNT bridges Au electrodes. (c) Schematic of the real-time measurement circuit.

First-principles study on thermodynamic properties and phase transitions in TiS2 Yonggang G Yu and Nancy L Ross

Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060, USA


Structural and vibrational properties of TiS2 with the CdI2 structure have been studied to high pressures from density functional calculations with the local density approximation (LDA). The calculated axial compressibility of the CdI

2-type phase agrees well with experimental data and is typical of layered transition-metal dichalcogenides. The obtained phonon dispersions show a good correspondence with available experiments. A phonon anomaly is revealed at 0 GPa, but is much reduced at 20 GPa. The thermodynamic properties of this phase were also calculated at high pressures and high temperatures using the quasi-harmonic approximation. Our LDA study on the pressure-induced phase transition sequence predicts that the CdI

2-type TiS2, the phase stable at ambient conditions, should transform to the cotunnite phase at 15.1 GPa, then to a tetragonal phase (I4/mmm) at 45.0 GPa. The tetragonal phase remains stable to at least 500 GPa. The existence of the tetragonal phase at high pressures is consistent with our previous findings in NiS

2 (Yu and Ross 2010 J. Phys.: Condens. Matter 22 235401). The cotunnite phase, although only stable in a narrow pressure range between 15.1 and 45.0 GPa, displays the formation of a compact S network between 100 and 200 GPa, which is evidenced by a kink in the variation of unit cell lengths with pressure. The electron density analysis in cotunnite shows that valence electrons are delocalized from Ti atoms and concentrated near the S network.

The TiS2 cotunnite phase at 200 GPa calculated by the LDA: valence electron density (left), and electron localization function (right).

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16 H i g h l i g h t s 2 0 1 1

Electronic structureq

Featured in

LabTalk

Electron transport and Goos–Hänchen shift in graphene with electric and magnetic barriers: optical analogy and band structure Manish Sharma1 and Sankalpa Ghosh2,3

1 Centre for Applied Research in Electronics, Indian Institute of Technology Delhi, New Delhi-110016, India

2 Department of Physics, Indian Institute of Technology Delhi, New Delhi-110016, India

3 Max-Planck Institut für Physik Komplexer Systeme, Nöthnitzer Straße 38, 01187 Dresden, Germany


Transport of massless Dirac fermions in graphene monolayers is analysed in the presence of a combination of singular magnetic barriers and applied electrostatic potential. Extending a recently proposed (Ghosh and Sharma 2009 J. Phys.: Condens. Matter 21 292204) analogy between the transmission of light through a medium with modulated refractive index and electron transmission in graphene through singular magnetic barriers to the present case, we find the addition of a scalar potential profoundly changes the transmission. We calculate the quantum version of the Goos–Hänchen shift that the electron wave suffers upon being totally reflected by such barriers. The combined electric and magnetic barriers substantially modify the band structure near the Dirac point. This affects transport near the Dirac point significantly and has important consequences for graphene-based electronics.These intersections create vortices that cause microtubules to form loops. We characterize the radius of curvature and time duration of the loops. These different behaviors give insight into how crowded conditions, such as those in the cell, might affect motor behavior and cytoskeleton organization.

Goos–Hänchen shift for an electrostatic and magnetic vector potential barrier at 3 T. The y-axis corresponds to the incident angle ϕ. The green regions have propagating solutions with no shift.

Spin–orbit splitting in graphene on metallic substrates Z Y Li1,2, Z Q Yang1,3, S Qiao1, J Hu4 and R Q Wu4

1 State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, People’s Republic of China

2 College of Science, University of Shanghai for Science and Technology, Shanghai 200093, People’s Republic of China

3 Department of Chemistry, Northwestern University, Evanston, IL 60208, USA 4 Department of Physics and Astronomy, University of California, Irvine, CA 92697-4575, USA


Substrate-induced spin–orbit splitting in graphene on Ni, Au and Ag(111) is examined on the basis of density-functional theory. The Rashba splitting of π bands along the M direction of the graphene surface Brillouin zone in graphene on Ni(111) is found to be very small (a few millielectronvolts), consistent with the experimental report of Rader M. Instead, very strong Rashba splitting (near 100 meV) can be obtained for graphene with a certain stretch distortion on a Au substrate. It can be ascribed to the effective match in energy between the C 2p and Au 5d bands, obtained from the analysis of densities of states. The net charge transfer between the graphene and the substrates just affects the spin–orbit effect indirectly. The small spin–orbit splitting induced by the Ag substrates indicates that heavy metals do not always produce large SO splitting. Our findings provide important insights that are useful for understanding the metal-induced Rashba effect in graphene.

The difference in the charge density between Gr/Au(II) and isolated graphene plus Au. The red colour indicates a gaining of electrons, while the blue indicates losing them.

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H i g h l i g h t s 2 0 1 1 17

Zigzag graphene nanoribbons: bandgap and midgap state modulation Hassan Raza

Department of Electrical and Computer Engineering, University of Iowa, Iowa City, IA 52242, USA


We study zigzag graphene nanoribbons with periodic edge roughness and report significant band gap opening. Interestingly, such nanoribbons have a near-midgap state with a small band width. We extensively study the electronic structure and the electric-field modulation of the conduction/valence bands and the near-midgap state. We summarize the important electronic-structure features like the band gap, the band width and the effective mass. We show that by applying an external electric field in the width direction, the band width of the near-midgap state varies linearly due to the edge localization, whereas the band gap remains almost constant. Additionally, the effective mass of these states can switch polarity from negative (hole-like) to positive (carrier-like) at the -point with the field modulation.


Electronic structure of passivated zigzag graphene nanoribbons with periodic edge roughness.

Effect of strain on the thermoelectric properties of silicon: an ab initio study N F Hinsche1, I Mertig1,2 and P Zahn1

1 Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, D-06099 Halle, Germany

2 Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany


On the basis of detailed first-principles calculations the anisotropic thermoelectric transport properties of biaxially strained silicon were studied with the focus on a possible enhancement of the power factor. Electron as well as hole doping was examined in a broad doping and temperature range. In the low temperature and low doping regime an enhancement of the power factor was obtained for compressive and tensile strain in the electron-doped case, and for compressive strain in the hole-doped case. In the thermoelectrically more important high temperature and high doping regime a slight enhancement of the power factor was only found for the hole-doped case under small biaxial tensile strain. The results are discussed

Fermi surfaces of electron-doped silicon under compressive strain (left), no strain (middle) and tensile strain (right).

Ab initio random structure searching Chris J Pickard1 and R J Needs2

1 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

2 Theory of Condensed Matter Group, Cavendish Laboratory, j j Thomson Avenue, Cambridge CB3 0HE, UK


It is essential to know the arrangement of the atoms in a material in order to compute and understand its properties. Searching for stable structures of materials using first-principles electronic structure methods, such as density-functional-theory (DFT), is a rapidly growing field. Here we describe our simple, elegant and powerful approach to searching for structures with DFT, which we call ab initio random structure searching (AIRSS). Applications to discovering the structures of solids, point defects, surfaces, and clusters are reviewed. New results for iron clusters on graphene, silicon clusters, polymeric nitrogen, hydrogen-rich lithium hydrides, and boron are presented.

Topical Review

Left: a structure built by placing carbon atoms randomly within a small sub-box, subject to symmetry constraints. Right: relaxation of this structure within DFT gave the well-known C60 ‘buckyball’.

in terms of band structure effects. An analytical model is presented to understand the fact that the thermopower decreases if degenerate bands are energetically lifted due to a strain-induced redistribution of states.

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18 H i g h l i g h t s 2 0 1 1

Featured in

LabTalk

Electronic structure and jahn– Teller effect in GaN:Mn and ZnS:CrF Virot1, R Hayn1 and A Boukortt1,2

1 Institut Matériaux Microélectronique Nanosciences de Provence, Faculté St jérôme, Case 142, F-13397 Marseille Cedex 20, France

2 Faculty of Science and Technology, Genie Electric Department 27000, Mostaganem University, Mostaganem, Algeria


We present an ab initio and analytical study of the jahn–Teller effect in two diluted magnetic semiconductors with d4 impurities, namely Mn-doped GaN and Cr-doped ZnS. We show that the correct insulating electronic structure may be obtained by a proper treatment of the strong electron correlation in the 3d shell in combination with the jahn–Teller distortion which breaks the local symmetry. Using the LSDA + U approach, we treat the zinc-blende and the wurtzite crystal structures of GaN:Mn, as well as zinc-blende ZnS:Cr. We show that the trigonal distortion due to the wurtzite structure is less important than the jahn–Teller deformation. This observation allows us to construct a simplified phenomenological ligand field theory (trigonal influence is neglected) which completes the ab initio part. Our work corrects previous studies and the obtained energy gain due to the jahn–Teller effect (from both the LSDA + U calculation and the ligand field theory) is in good agreement with the experimental data. The same is true for the complete set of crystal field parameters obtained from the phenomenological model which agrees well with previous optical measurements.

Density of states of zinc-blende GaN:Mn resulting from the LSDA + U calculation (U = 4 eV).

Correlated electronsq

Synthesis of cubic SrCoO3 single crystal and its anisotropic magnetic and transport propertiesYouwen Long1, Yoshio Kaneko1, Shintaro Ishiwata2,3, Yasujiro Taguchi2 and Yoshinori Tokura1,2,3

1 Multiferroics Project, Exploratory Research for Advanced Technology, japan Science and Technology Agency, c/o RIKEN Advanced Science Institute, Wako 351-0198, japan

2 Cross-Correlated Materials Research Group (CMRG) and Correlated Electron Research Group (CERG), RIKEN Advanced Science Institute, Wako 351-0198, japan

3 Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Hongo, Tokyo 113-8656, japan


A large-size single crystal of nearly stoichiometric SrCoO3 was prepared with a two-step method combining the floating-zone technique and subsequent

high oxygen pressure treatment. SrCoO3 crystallizes in a cubic perovskite structure with space group Pm3-m, and displays an itinerant ferromagnetic behavior with the Curie temperature of 305 K. The easy magnetization axis is found to be along the [111] direction, and the saturation moment is 2.5 µB/f.u., in accord with the picture of the intermediate spin state. The resistivity at low temperatures (T) is proportional to T2, indicative of the possible effect of orbital fluctuation in the intermediate spin ferromagnetic metallic state. Unusual anisotropic magnetoresistance is also observed and its possible origin is discussed.

Crystal structure of (δ) orthorhombic brownmillerite-type SrCoO

2.5 (left), and (b) cubic perovskite-type SrCoO

3 (right).

General DFT++ method implemented with projector augmented waves: electronic structure of SrVO3 and the Mott transition in Ca2 − xSrxRuO4M Karolak, T O Wehling, F Lechermann and A I Lichtenstein

I. Institut für Theoretische Physik, Universität Hamburg, jungiusstraße 9, D-20355 Hamburg, Germany


The realistic description of correlated electron systems took an important step forward a few years ago as the combination of density functional methods and dynamical mean-field theory was conceived. This framework allows access to both high and low energy physics and is capable of the description of the specific physics of strongly correlated materials, like the Mott metal–insulator transition. A very important step in the procedure is the interface between the band structure method and the dynamical mean-field theory and its impurity solver. We present a general interface between a projector augmented-wave-based density functional code and many-body methods based on Wannier functions obtained from a projection on local orbitals. The implementation is very flexible and allows for various applications. Quantities like the momentum-resolved spectral function are accessible. We present applications to SrVO

3 and the metal–insulator transition in Ca

2 − xSrxRuO4.

Momentum-resolved impurity spectral function obtained by DFT + DMFT (QMC) and PLO(V) for U = 6 eV. The V t

2g and O 2p states were used in the projection (12 bands). The LDA band structure of the V t

2g and O 2p Bloch states is shown for comparison.


H i g h l i g h t s 2 0 1 1 19

Eliashberg theory of excitonic insulating transition in grapheneJing-Rong Wang1 and Guo-Zhu Liu1,2

1 Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China

2 Institut für Theoretische Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany


A sufficiently strong Coulomb interaction may open an excitonic fermion gap and thus drive a semimetal–insulator transition in graphene. In this paper, we study the Eliashberg theory of excitonic transition by coupling the fermion gap equation self-consistently to the equation of the vacuum polarization function. Including the fermion gap into the polarization function increases the effective strength of the Coulomb interaction because it reduces the screening effects due to the collective particle–hole excitations. Although this procedure does not change the critical point, it leads to a significant enhancement of the dynamical fermion gap in the excitonic insulating phase. The validity of the Eliashberg theory is justified by showing that the vertex corrections are suppressed at the large N limit.

The λ dependence of dynamical fermion gap at zero momentum and zero temperature. At physical flavor N = 2, the critical strength is given by 1/λ

c = 0.356. When λ <λc, the excitonic insulating transition cannot happen.

Effects of electronic correlation on x-ray absorption and dichroic spectra at L2, 3 edge L Pardini, V Bellini and F Manghi

CNR—Institute of Nanosciences—S3, Via Campi 213/A, I-41125 Modena, Italy

Dipartimento di Fisica, Università di Modena e Reggio Emilia, Via Campi 213/A, I-41125 Modena, Italy


We present a new theoretical approach to describe x-ray absorption and magnetic circular dichroism spectra in the presence of electron–electron correlation. Our approach provides an unified picture to include correlations in both charged and neutral excitations, namely in direct/inversion photoemission where electrons are removed/added, and photoabsorption where electrons are promoted from core levels to empty states. We apply this approach to the prototypical case of the L

2, 3 edge of 3d transition metals and we show that the inclusion of many-body effects in the core level excitations is essential to reproduce, together with satellite structures in core level photoemission, the observed asymmetric lineshapes in x-ray absorption and dichroic spectra.

Iron empty states: interacting spectral functions are shown as a color map and compared with single-particle eigenstates reported as red (spin down) and blue (spin up) arrows.

Featured in

LabTalk

Synthesis and crystal growth of Cs0.8(FeSe0.98)2: a new iron-based superconductor with Tc = 27 K A Krzton-Maziopa1, Z Shermadini2, E Pomjakushina1, V Pomjakushin3, M Bendele2,4, A Amato2, R Khasanov2, H Luetkens2 and K Conder1

1 Laboratory for Developments and Methods, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

2 Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

3 Laboratory for Neutron Scattering, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

4 Physik-Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland


We report on the synthesis of large single crystals of a new FeSe layer superconductor Cs

0.8(FeSe0.98)2. X-ray powder diffraction, neutron powder diffraction and magnetization measurements have been used to compare the crystal structure and the magnetic properties of Cs

0.8(FeSe0.98)2 with those of the recently discovered potassium intercalated system KxFe2Se2. The new compound, Cs

0.8(FeSe0.98)2, shows a slightly lower superconducting transition temperature (T

c = 27.4 K) in comparison to 29.5 in (K

0.8(FeSe0.98)2). The volume of the crystal unit cell increases by replacing K by Cs—the c parameter grows from 14.1353(13) to 15.2846(11) Å. For the alkali metal intercalated layered compounds known so far, (K

0.8Fe2Se2 and Cs

0.8(FeSe0.98)2), the Tc dependence on the anion height (distance between Fe layers and Se layers) was found to be analogous to those reported for As-containing Fe superconductors and Fe(Se

1 − xChx), where Ch = Te, S.


Superconductors and metalsq

Rietveld refinement pattern (upper—red) and difference plot (lower—black) of the x-ray diffraction data for the crystal with the nominal composition of K

0.8(FeSe0.98)2. The rows of ticks show the Bragg peak positions for the

I4/mmm phase. The left insert shows the difference Fourier density map at z = 1/2 slice obtained from NPD data showing the presence of K at (0.5, 0.5, 0.5), while the colored scale shows scattering density in fm. The right insert shows a picture of the cleaved K

0.8(FeSe0.98)2 crystal.


20 H i g h l i g h t s 2 0 1 1

Featured in

LabTalk

Theory of high-TC superconductivity: transition temperatureDale R Harshman1,2,3, Anthony T Fiory4 and John D Dow3,5

1 Physikon Research Corporation, Lynden, WA 98264, USA 2 Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA 3 Department of Physics, Arizona State University, Tempe, AZ 85287, USA 4 Department of Physics, New jersey Institute of Technology, Newark, Nj 07102, USA 5 Institute for Postdoctoral Studies, Scottsdale, AZ 85253, USA


It is demonstrated that the transition temperature (TC) of high-TC superconductors is determined by their layered crystal structure, bond lengths, valency properties of the ions, and Coulomb coupling between electronic bands in adjacent, spatially separated layers. Analysis of 31 high-T

C materials (cuprates, ruthenates, ruthenocuprates, iron pnictides, organics) yields the universal relationship for optimal compounds, k

BTC0 = β/ℓζ, where ℓ is related to the mean spacing between interacting charges in the layers, ζ is the distance between interacting electronic layers, β is a universal constant and TC0 is the optimal transition temperature (determined to within an uncertainty of ± 1.4 K by this relationship). Non-optimum compounds, in which sample degradation is evident, e.g. by broadened superconducting transitions and diminished Meissner fractions, typically exhibit reduced T

C < TC0. It is shown that TC0 may be obtained from an average of the Coulomb interaction forces between the two layers.

Representative model structure of high-TC superconductors. Cross section view perpendicular to the basal plane of periodic electronic layers of types I (red, depicted here with v = 2) and II (blue).

Electronic structure of optimally doped pnictide Ba0.6K0.4Fe2As2: a comprehensive angle-resolved photoemission spectroscopy investigationH Ding1, K Nakayama2, P Richard1,3, S Souma3, T Sato2,4, T Takahashi2,3, M Neupane5, Y-M Xu5,6, Z-H Pan7, A V Fedorov8, Z Wang5, X Dai1, Z Fang1, G F Chen9, J L Luo1 and N L Wang1

1 Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

2 Department of Physics, Tohoku University, Sendai 980-8578, japan 3 WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, japan

4 TRIP, japan Science and Technology Agency (jST), Kawaguchi 332-0012, japan 5 Department of Physics, Boston College, Chestnut Hill, MA 02467, USA 6 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

7 Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA

8 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

9 Department of Physics, Renmin University, Beijing 100872, People’s Republic of China


The electronic structure of the Fe-based superconductor Ba0.6K0.4Fe2As2 is studied by means of angle-resolved photoemission. We identify dispersive bands crossing the Fermi level forming hole-like (electron-like) Fermi surfaces (FSs) around Γ (M) with nearly nested FS pockets connected by the antiferromagnetic wavevector. Compared to band structure calculation findings, the overall bandwidth is reduced by a factor of 2 and the low energy dispersions display even stronger mass renormalization. Using an effective tight banding model, we fitted the band structure and the FSs to obtain band parameters reliable for theoretical modeling and calculation of physical quantities.

Intensity plot of second derivatives of spectra along –X and X–M. LDA bands (red lines) are plotted for comparison.


H i g h l i g h t s 2 0 1 1 21

Temperature-dependent local structure of NdFeAsO1 − xFx system using arsenic K-edge extended x-ray absorption fine structure B Joseph1, A Iadecola2, L Malavasi1 and N L Saini2

1 Dipartimento di Chimica, Sezione di Chimica Fisica, INSTM (UdR Pavia), Università di Pavia, Viale Taramelli 16, 27100 Pavia, Italy

2 Dipartimento di Fisica, Università di Roma ‘La Sapienza’, Piazzale Aldo Moro 2, I-00185 Roma, Italy


Local structure of NdFeAsO1 − xFx (x = 0.0, 0.05, 0.15 and 0.18) high temperature iron-pnictide superconductor system is studied using arsenic K-edge extended x-ray absorption fine structure (EXAFS) measurements as a function of temperature. Fe–As bond length shows only a weak temperature and F-substitution dependence, consistent with the strong covalent nature of this bond. The temperature dependence of the mean square relative displacements of the Fe–As bond length are well described by the correlated Einstein model for all the samples, but with different Einstein temperatures for the superconducting and non-superconducting samples. The results indicate distinct local Fe–As lattice dynamics in the superconducting and non-superconducting iron-pnictide systems.

Fourier transforms of the arsenic K-edge EXAFS oscillations measured (symbols) on NdFeAsO

1−xFx (x = 0.0, 0.05, 0.15 and 0.18) at low temperature (12 K), together with a single shell fit (solid line).


Refinement result of the PDF data at 4 K using a structural model in the P.1 space group for the SmFeAsO (top panel) and SmFeAsO

0.85F0.15 (bottom panel) samples.

Local structural investigation of SmFeAsO1 − xFx high temperature superconductors Lorenzo Malavasi1, Gianluca A Artioli1, Hyunjeong Kim2,3, Beatrice Maroni1, Boby Joseph1, Yang Ren4, Thomas Proffen2 and Simon J L Billinge5,6

1 Dipartimento di Chimica—Sezione di Chimica Fisica, INSTM (UdR Pavia), Università di Pavia, Viale Taramelli 16, 27100 Pavia, Italy

2 Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

3 Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Central 5-2, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, japan

4 Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA 5 Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA

6 Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA


A strong revitalization of the field of high temperature superconductivity (HTSC) has been induced recently by the discovery of T

C around 26 K in F-doped LaFeAsO iron pnictides. Starting from this discovery, a huge amount of experimental data have been accumulated. This important corpus of results will allow the development of suitable theoretical models aimed at describing the basic electronic structure properties and nature of superconducting states in these fascinating new systems. A close correlation between structural features and physical properties of the normal and superconducting states has already been demonstrated in the current literature. Advanced theoretical models are also based on the close correlation with structural properties and in particular with the Fe–As tetrahedral array. As for other complex materials, a deeper understanding of their structure–properties correlation requires a full knowledge of the atomic arrangement within the structure. Here we report an investigation of the local structure in the SmFeAsO

1 − xFx system carried out by means of x-ray total scattering measurements and pair distribution function (PDF) analysis. The results presented indicate that the local structure of these HTSC significantly differs from the average structure determined by means of traditional diffraction techniques, in particular the distribution of Fe–As bond lengths. In addition, a model for describing the observed discrepancies is presented.

• Domain wall dynamics in nanostructures

• Non-contact AFM

• Molecular switches at surfaces

• Liquid-solid interfaces

• Van der Waals interactions

• Ultrathin layers of graphene, h-BN and other honeycomb structures

Some forthcoming 2012 JPCM special issues:


22 H i g h l i g h t s 2 0 1 1

Semiconductorsq

First-principles study of nitrogen doping in cubic and amorphous Ge2Sb2Te5 S Caravati1, D Colleoni2, R Mazzarello1,3, T D Kühne1,4, M Krack5, M Bernasconi2 and M Parrinello1

1 Computational Science, Department of Chemistry and Applied Biosciences, ETH Zurich, USI Campus, Via Giuseppe Buffi 13, CH-6900 Lugano, Switzerland

2 Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Via R Cozzi 53, I-20125 Milano, Italy

3 Institute for Theoretical Solid State Physics and jARA—Fundamentals of Future Information Technology, RWTH Aachen, D-52056 Aachen, Germany

4 Institute of Physical Chemistry and Center for Computational Sciences, johannes Gutenberg University Mainz, Staudinger Weg 9, D-55128 Mainz, Germany

5 Paul Scherrer Institut, CH-5232 Villigen, Switzerland


We investigated the structural, electronic and vibrational properties of amorphous and cubic Ge2Sb2Te5 doped with N at 4.2 at.% by means of large scale ab initio simulations. Nitrogen can be incorporated in molecular form in both the crystalline and amorphous phases at a moderate energy cost. In contrast, insertion of N in the atomic form is very energetically costly in the crystalline phase, though it is still possible in the amorphous phase. These results support the suggestion that N segregates at the grain boundaries during the crystallization of the amorphous phase, resulting in a reduction in size of the crystalline grains and an increased crystallization temperature.

Representative relaxed configurations of N in interstitial sites.

Wavepacket scattering of Dirac and Schrödinger particles on potential and magnetic barriers Kh Yu Rakhimov1,2,3, Andrey Chaves1,4, G A Farias4 and F M Peeters1,4

1 Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium

2 Physics Department, National University of Uzbekistan, 700174 Tashkent, Uzbekistan

3 Turin Polytechnic University in Tashkent, 700174 Tashkent, Uzbekistan 4 Departamento de Física, Universidade Federal do Ceará, Caixa Postal 6030, Campus do Pici, 60455-900 Fortaleza, Ceará, Brazil


We investigate the dynamics of a charged particle moving in a graphene layer and in a two-dimensional electron gas, where it obeys the Dirac and the Schrödinger equations, respectively. The charge carriers are described

Contour plots of the cross section of the squared modulus (top) and the real part (bottom) of the wavefunction at x = 0 as a function of time, for a wavepacket propagating in the y direction (φ = 0).

Hydrogenated cation vacancies in semiconducting oxides J B Varley1, H Peelaers2,3, A Janotti2 and C G Van de Walle2

1 Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA

2 Materials Department, University of California, Santa Barbara, CA 93106-5050, USA

3 Departement Fysica, Universiteit Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium


Using first-principles calculations we have studied the electronic and structural properties of cation vacancies and their complexes with hydrogen impurities in SnO

2, In2O3 and β-Ga2O3. We find that cation vacancies have high formation energies in SnO

2 and In2O3 even in the most favorable conditions. Their formation energies are significantly lower in β-Ga2O3. Cation vacancies, which are compensating acceptors, strongly interact with H impurities resulting in complexes with low formation energies and large binding energies, stable up to temperatures over 730 °C. Our results indicate that hydrogen has beneficial effects on the conductivity of transparent conducting oxides: it increases the carrier concentration by acting as a donor in the form of isolated interstitials, and by passivating compensating acceptors such as cation vacancies; in addition, it potentially enhances carrier mobility by reducing the charge of negatively charged scattering centers. We have also computed vibrational frequencies associated with the isolated and complexed hydrogen, to aid in the microscopic identification of centers observed by vibrational spectroscopy.

Structure of the lowest energy configuration of interstitial hydrogen donors and their complexes with the following cation vacancy in In

2O3: (VIn–H)−2 on the 24d site.

as Gaussian wavepackets. The dynamics of the wavepackets is studied numerically by solving both quantum-mechanical and relativistic equations of motion. The scattering of such wavepackets by step-like magnetic and potential barriers is analysed for different values of wavepacket energy and width. We find: (1) that the average position of the wavepacket does not coincide with the classical trajectory, and (2) that, for slanted incidence, the path of the centre of mass of the wavepacket does not have to penetrate the barrier during the scattering process. Trembling motion of the charged particle in graphene is observed in the absence of an external magnetic field and can be enhanced by a substrate-induced mass term.

Special section on semiconducting oxides


H i g h l i g h t s 2 0 1 1 23

Featured in

LabTalk

Electron and hole stability in GaN and ZnOAron Walsh1,2, C Richard A Catlow1, Martina Miskufova1 and Alexey A Sokol1

1 Kathleen Lonsdale Materials Chemistry, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0Aj, UK

2 Centre for Sustainable Chemical Technologies and Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK


We assess the thermodynamic doping limits of GaN and ZnO on the basis of point defect calculations performed using the embedded cluster approach and employing a hybrid non-local density functional for the quantum mechanical region. Within this approach we have calculated a staggered (type-II) valence band alignment between the two materials, with the N 2p states contributing to the lower ionization potential of GaN. With respect to the stability of free electron and hole carriers, redox reactions resulting in charge compensation by ionic defects are found to be largely endothermic (unfavourable) for electrons and exothermic (favourable) for holes, which is consistent with the efficacy of electron conduction in these materials. Approaches for overcoming these fundamental thermodynamic limits are discussed.

The natural valence band offset between wurtzite-structured ZnO and GaN, as calculated from the quantum mechanical/molecular mechanical method with reference to the vacuum level.

Dielectrics and ferroelectricsq

Electronic and local structures of BiFeO3 films Y Yoneda1 and W Sakamoto2

1 japan Atomic Energy Agency, Reaction Dynamics Research Division, 1-1-1, Kouto, Sayo-cho Sayo-gun, Hyogo 679-5148, japan

2 EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, japan


The electronic structure of BiFeO3 (BFO), BiFeO3–PbTiO3 solid solution (BFO–PT), and Mn-doped BFO–PT (BFM–PT) films fabricated by chemical solution deposition was investigated by x-ray absorption fine structure (XAFS). The BiFeO3 shows a large leakage current owing to the mixed valance state of Fe2+ and Fe3+ . The BFO film has a blunt absorption edge jump indicating the charge fluctuated state of the iron ions. The BFO–PT and BFM–PT films have sharp absorption edges, and the absorption energy of these films shifted to opposite energy. The valence fluctuation of the iron ions was closely connected with the leakage current properties. The charge

SEM micrograph of BFM–PT. The film thickness was confirmed to be500 nm.

fluctuated BFO film showed a leaky feature, and the charge unfluctuated BFO–PT and BFM–PT films had improved leakage current properties. The valence fluctuation of the iron ions can be controlled by Mn substitution and by making solid solutions.

Nanoscale polarization switching mechanisms in multiferroic BiFeO3 thin films H Béa1,4, B Ziegler1, M Bibes2,3, A Barthélémy2,3 and P Paruch1

1 DPMC, University of Geneva, 24 Quai Ernest Ansermet, 1211 Geneva 4, Switzerland 2 Unité Mixte de Physique CNRS/Thales, 1 Avenue A Fresnel, 91767 Palaiseau, France

3 Université Paris-Sud, 91405 Orsay, France 4 Present address: SPINTEC, UMR8191, CEA/CNRS/UjF, CEA Grenoble, INAC, 38054 Grenoble Cedex 9, France


Ferroelectric switching in BiFeO3 multiferroic thin films was studied by piezoresponse force microscopy, as a function of the tip voltage and sweep direction, for samples with two different intrinsic domain structures. In all films, the switched polarization direction follows the in-plane and out-of-plane components of the highly inhomogeneous electric field applied by the microscope tip. In films with ‘bubble-like’ intrinsic domains, we observed in-plane switching assisted by out-of-plane switching for lower voltage values, and independent in-plane and out-of-plane switching for higher voltages, in both cases allowing full control of the ferroelectric polarization depending on the tip voltage polarity and sweep direction. In films with ‘stripe-like’ intrinsic domains, independent in-plane and out-of-plane switching was observed, but unswitched stripe domains prevented full control of the ferroelectric polarization over large areas. We correlate the observed switching behavior with the field-driven onset of a highly distorted tetragonal phase predicted by ab initio calculations, which leads to a very high in-plane susceptibility during the return to the non-distorted monoclinic phase when the field is decreased. Depending on the specific strain and disorder present in the sample, the transition towards the highly distorted phase may be asymmetrized, and easier to reach when an electric field opposite to the out-of-plane polarization direction is applied.

Vertical piezoresponse force microscopy and lateral piezoresponse force microscopy images of domain configurations.


Special section on semiconducting oxides


24 H i g h l i g h t s 2 0 1 1

Multiferroic magnetoelectric fluorides: why are there so many magnetic ferroelectrics? J F Scott1 and R Blinc2

1 Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, UK

2 jozef Stefan Institute, Ljubljana 1000, Slovenia


We review work on multiferroic magnetic fluorides with an aim to correct the popular opinion that magnetic ferroelectrics are rare in nature. After a qualitative summary describing the main families of magnetic fluorides that are piezoelectric and probably ferroelectric, we discuss in detail the most popular recent groups, namely the K

3Fe5F15 and Pb5Cr3F19 families.

Topical Review

Crystal structure of Pb5Cr3F19.

Evidence of orbital excitations in CaCu3Ti4O12 probed by Raman spectroscopy Dileep K Mishra and V G Sathe

UGC-DAE Consortium for Scientific Research, University Campus, Indore 452017, India


Raman scattering studies on CaCu3Ti4O12 and SrCu3Ti4O12 compounds provide evidence of the physics underlying the giant dielectric effect in the CaCu

3Ti4O12 compound. The temperature, polarization, and photon energy dependence of a broad Raman mode observed at high wavenumbers below ~130 K indicates its origin from orbital excitations. The orbital order disorder transition observed around 100 K may be responsible for the conductivity changes required in the internal barrier layer capacitance model, hitherto used to explain the huge dielectric constant above 100 K in these compounds.


Temperature dependence of the Raman spectrum of CaCu

3Ti4O12 (a) and SrCu

3Ti4O12 (b) in the parallel polarization (PP) geometry, using the 488 nm laser line.

Magnetism and magnetic materialsq

Featured in

LabTalk

The giant anomalous Hall effect in the ferromagnet Fe3Sn2—a frustrated kagome metal T Kida1, L A Fenner2,3, A A Dee2, I Terasaki4,5, M Hagiwara1 and A S Wills2,3

1 KYOKUGEN, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, japan

2 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0Aj, UK

3 The London Centre for Nanotechnology, 17–19 Gordon Street, London WC1H 0AH, UK

4 Department of Applied Physics, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, japan

5 Present address: Department of Physics, Nagoya University, Furocho, Chikusa, Nagoya 464-8602, japan


The kagome-bilayer material Fe3Sn2 has recently been shown to be an example of a rare class of magnet—a frustrated ferromagnetic metal. While the magnetism of Fe

3Sn2 appears to be relatively simple at high temperature, with localized moments parallel to the c-axis (T

C = 640 K), upon cooling the competing exchange interactions and spin frustration become apparent as they cause the moments to become non-collinear and to rotate towards the kagome plane, forming firstly a canted ferromagnetic structure and then a re-entrant spin glass (T

f _∼ 80 K). In this work we show

that Fe3Sn2 possesses an unusual anomalous Hall effect. The saturated

Hall resistivity of Fe3Sn2 is 3.2 µΩ cm at 300 K, almost 20 times higher than

that of typical itinerant ferromagnets such as Fe and Ni. The anomalous Hall coefficient R

s is 6.7 × 10 − 9 Ω cm G − 1 at 300 K, which is three orders of magnitude larger than that of pure Fe, and obeys an unconventional scaling with the longitudinal resistivity, ρxx, of Rs ∝ ρ x x

3.15. Such a relationship cannot be explained by either the conventional skew or side-jump mechanisms, indicating that the anomalous Hall effect in Fe

3Sn2 has an extraordinary origin that is presumed to be related to the underlying frustration of the magnetism. These findings demonstrate that frustrated ferromagnets, whether based on bulk materials or on artificial nanoscale structures, can provide new routes to room temperature spin-dependent electron transport properties suited to application in spintronics.


The crystal structure of Fe3Sn2 is made up of Fe/Sn bilayers separated by Sn. The Fe ions form a kagome-bilayer structure where each of the layers is made up of two sizes of equilateral triangles.


H i g h l i g h t s 2 0 1 1 25

Magneto-structural properties and magnetic anisotropy of small transition-metal clusters: a first-principles study Piotr Błonski and Jürgen Hafner

Fakultät für Physik and Center for Computational Materials Science, Universität Wien, Sensengasse 8/12, A-1090 Wien, Austria


Ab initio density-functional calculations including spin–orbit coupling (SOC) have been performed for Ni and Pd clusters with three to six atoms and for 13-atom clusters of Ni, Pd, and Pt, extending earlier calculations for Pt clusters with up to six atoms (2011 J. Chem. Phys. 134 034107). The geometric and magnetic structures have been optimized for different orientations of the magnetization with respect to the crystallographic axes of the cluster. The magnetic anisotropy energies (MAE) and the anisotropies of spin and orbital moments have been determined. Particular attention has been paid to the correlation between the geometric and magnetic structures. The magnetic point group symmetry of the clusters varies with the direction of the magnetization. Even for a 3d metal such as Ni, the change in the magnetic symmetry leads to small geometric distortions of the cluster structure, which are even more pronounced for the 4d metal Pd. For a 5d metal the SOC is strong enough to change the energetic ordering of the structural isomers. SOC leads to a mixing of the spin states corresponding to the low-energy spin isomers identified in the scalar-relativistic calculations. Spin moments are isotropic only for Ni clusters, but anisotropic for Pd and Pt clusters, orbital moments are anisotropic for the clusters of all three elements. The magnetic anisotropy energies have been calculated. The comparison between MAE and orbital anisotropy invalidates a perturbation analysis of magnetic anisotropy for these small clusters.

Magnetic structure of the Pd13 biplanar cluster after rotation of the magnetization by 90° to the hard axis. Spin moments are shown by the red arrows, orbital moments by the blue arrows.

Absence of long-range magnetic ordering in the pyrochlore compound Er2Sn2O7 P M Sarte1, H J Silverstein2, B T K Van Wyk1, J S Gardner3,4, Y Qiu3,5, H D Zhou6 and C R Wiebe1,2

1 Department of Chemistry, University of Winnipeg, Winnipeg, MB, R3B 2E9, Canada 2 Department of Chemistry, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada 3 NIST Center for Neutron Research, Gaithersburg, MD 20899-6102, USA 4 Indiana University, Bloomington, IN 47408, USA 5 Department of Material Science and Engineering, University of Maryland, College Park, MD 20742, USA

6 National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA


The low temperature behaviour of powder Er2Sn2O7 samples has been studied by magnetic susceptibility, heat capacity, and neutron scattering experiments. We report here the absence of magnetic ordering down to 100 mK. Anomalies in the heat capacity can be accounted for through an analysis of the crystal field spectrum observed by inelastic neutron scattering spectroscopy. These new measurements on Er

2Sn2O7 suggest a new lower bound for the frustration index of f = |Θ

CW|/TN = 14/0.1 = 140, placing this compound into a highly frustrated regime.


Inelastic neutron scattering profile of Er

2Sn2O7 at (a) 5 K and (b) 100 mK.

Visit our online collections for dedicated selections of papers in specific subject areas. Current collections include:

Graphene iopscience.org/jpcm/page/GrapheneElectronic structure iopscience.org/jpcm/page/Electronic Structure


26 H i g h l i g h t s 2 0 1 1

Magnetic order in orbital models of the iron pnictides P M R Brydon1, Maria Daghofer2 and Carsten Timm1

1 Institut für Theoretische Physik, Technische Universität Dresden, D-01062 Dresden, Germany

2 IFW Dresden, PO Box 270116, D-01171 Dresden, Germany


We examine the appearance of the experimentally observed stripe spin-density-wave magnetic order in five different orbital models of the iron pnictide parent compounds. A restricted mean-field ansatz is used to determine the magnetic phase diagram of each model. Using the random phase approximation, we then check this phase diagram by evaluating the static spin susceptibility in the paramagnetic state close to the mean-field phase boundaries. The momenta for which the susceptibility is peaked indicate in an unbiased way the actual ordering vector of the nearby mean-field state. The dominant orbitally resolved contributions to the spin susceptibility are also examined to determine the origin of the magnetic instability. We find that the observed stripe magnetic order is possible in four of the models, but it is extremely sensitive to the degree of nesting between the electron and hole Fermi pockets. In the more realistic five-orbital models, this order competes with a strong-coupling incommensurate state which appears to be controlled by details of the electronic structure below the Fermi energy. We conclude by discussing the implications of our work for the origin of the magnetic order in the pnictides.

Ratio of the static transverse susceptibility with and without the Hartree shift included forU = 1.05 eV, T = 1000 K.

Exchange bias effect in alloys and compounds S Giri, M Patra and S Majumdar

Department of Solid State Physics, Indian Association for the Cultivation of Science, jadavpur, Kolkata 700 032, India


The phenomenology of exchange bias effects observed in structurally single-phase alloys and compounds but composed of a variety of coexisting magnetic phases such as ferromagnetic, antiferromagnetic, ferrimagnetic, spin-glass, cluster-glass and disordered magnetic states are reviewed. The investigations on exchange bias effects are discussed in diverse types of alloys and compounds where qualitative and quantitative aspects of magnetism are focused based on macroscopic experimental tools such as magnetization and magnetoresistance measurements. Here, we focus

Topical Review

Schematic representation of exchange bias effect due to field cooling in (a) bilayer ferromagnetic/antiferromagnetic structure and (b) bilayer structure with appearance of a new pinned ferromagnetic layer at the interface; (c) no shift and (d) loop shifts.

on improvement of fundamental issues of the exchange bias effects rather than on their technological importance.

A critical re-examination and a revised phase diagram of La1 − xSrxCoO3 D Samal and P S Anil Kumar

Department of Physics, Indian Institute of Science, Bangalore 560012, India


We report the results of a comprehensive study on dc magnetization, ac susceptibility, and the magnetotransport properties of the La1 − xSrxCoO3

(0 ≤ x ≤ 0.5) system. At higher Sr doping (x ≥ 0.18), the system exhibits Brillouin-like field cooled magnetization (M

FC). However, for x < 0.18, the system exhibits a kink in the M

FC, a peak at the intermediate field in the thermoremnant magnetization and a non-saturating tendency in the M–H plot that all point towards the characteristic of spin glass behavior. More interestingly, dc magnetization studies for x < 0.18 do not suggest the existence of ferromagnetic correlation that can give rise to an irreversible line in the spin glass regime. The ac susceptibility study for x > 0.2 exhibits apparently no frequency dependent peak shift around the ferromagnetic transition region. However, a feeble signature of glassiness is verified by studying the frequency dependent shoulder position in X''(T) and the memory effect below the Curie temperature. But, for x < 0.18, the ac susceptibility study exhibits a considerable frequency dependent peak shift, time dependent memory effect, and the characteristic spin relaxation time scale to ~ 10 −13 s. The reciprocal susceptibility versus temperature plot adheres to Curie–Weiss behavior and does not provide any signature of preformed ferromagnetic clusters well above the Curie temperature. The magnetotransport study reveals a cross over from metallic to semiconducting-like behavior for x ≤ 0.18. On the semiconducting side, the system exhibits a large value of magnetoresistance (upto 75%) towards low temperature and it is strongly connected to the spin dependent part of the random potential distribution in the spin glass phase. Based on the above observations, we have reconstructed a new magnetic phase diagram and characterized each phase with associated properties.

The temperature dependent resistance of La

1−xSrxCoO3 for 0.0 ≤ x ≤ 0.5.

LabTalkJournal of Physics: Condensed Matter presents LabTalk,

a dedicated section of short news items written by our authors about their latest papers.

Pitched at a level intended for non-experts in the field, LabTalk items aim to highlight the key findings of a paper.

They often include striking images and a short biography of the authors, with links to the full article and to related stories of interest.

Visit our websiteto view all of our published stories including the

most recent and most read LabTalk articles

iopscience.iop.org/jpcm/labtalk/1


28 H i g h l i g h t s 2 0 1 1

2011 Special issuesThe journal’s authoritative special issue programme aims to cover the most exciting and most rapidly developing areas of condensed matter, with experts in the field contributing to high-quality issues of original research. Below are some highlights from our 2011 special issue programme.

Strongly correlated electron systems

Guest Editors: Filip Ronning and Cristian Batista2011 J. Phys.: Condens. Matter 23 issue 9

Strongly correlated electrons is an exciting and diverse field in condensed matter physics. This special issue contains a number of contributions on f-electron compounds and also covers recent developments relating to strongly correlated electron systems in d-electron materials, such as Sr

3Ru2O7, graphene, and the new Fe-based superconductors.

Spectral function for a heavy-fermion system. O Bodensiek et al 2010 J. Phys.: Condens. Matter 23 094212.

Geometrically frustrated magnetism

Guest Editor: jason S Gardner2011 J. Phys.: Condens. Matter 23 issue 16

Frustrated magnetism is an area that has grown tremendously over the past 20 years. Geometric frustration is a broad phenomenon that results from an intrinsic incompatibility between some fundamental interactions and the underlying lattice geometry based on triangles and tetrahedral. Most studies have centred around the kagomé and pyrochlore based magnets but recent work has looked at other structures including the delafossite, langasites, hyper-kagomé, garnets and Laves phase materials.

Elastic neutron scattering data for SrHo2O4 SC at T = 1.5 K. S Ghosh et al 2011 J. Phys.: Condens. Matter 23 164203.

Nano- and microfluidics

Guest Editor: Karin jacobs2011 J. Phys.: Condens. Matter 23 issue 18

The field of nano- and microfluidics emerged at the end of the 1990s parallel to the demand for smaller and smaller containers and channels for chemical, biochemical and medical applications such as blood and DNA analysis, gene sequencing or proteomics. The articles is this issue have been divided into four subsections: ‘Probing the boundary condition’, ‘Flow over or in special geometries’, ‘Soft objects in fluid flow’ and ‘Manipulating flow’.

PDMS pillar array. Bernardo Nottebrock et al 2011 J. Phys.: Condens. Matter 23 184121.

Colloidal suspensions

Guest Editors: Andrei Petukhov, Willem Kegel and jeroen van Duijneveldt2011 J. Phys.: Condens. Matter 23 issue 19

This issue contains research on a number of themes relating to the topic of colloidal suspensions. These themes are: electrostatics, colloidal rods and platelets, colloid–polymer mixtures and depletion interactions, and colloidal dynamics and crystallization.

Colour spectra. E van den Pol et al 2011 J. Phys.: Condens. Matter 23 194108.


H i g h l i g h t s 2 0 1 1 29

Complex dynamics of fluids in disordered and crowded environments

Guest Editors: Daniele Coslovich, Gerhard Kahl and Vincent Krakoviack2011 J. Phys.: Condens. Matter 23 issue 23

The dynamics of fluids under nanoscale confinement has been of great interest both for practical and fundamental reasons. Problems in a wide range of scientific topics, such as polymer and colloidal sciences, rheology, geology, or biophysics, benefit from a profound understanding of the dynamical behaviour of confined fluids. This special section helps shed light on a number of important issues in the field.

A 5% diffusion limited cluster aggregation gel with red tracers. Jean-Christophe Gimel and Taco Nicolai 2011 J. Phys.: Condens. Matter 23 234115.

Structure and dynamics determined by neutron and x-ray scattering

Guest Editor: Peter Müller-Buschbaum2011 J. Phys.: Condens. Matter 23 issue 25

Neutron and x-ray scattering have emerged as powerful methods for the determination of structure and dynamics. Driven by emerging new, powerful neutron and synchrotron radiation sources, the continuous development of new instrumentation and novel scattering techniques gives rise to exciting possibilities. This special section covers a broad range of different materials from soft to hard condensed matter.

Contour map of an excitation spectrum as measured in the transverse spin density wave phase of Cr. Peter Böni et al 2011 J. Phys.: Condens. Matter 23 254209.

Semiconducting oxides

Guest Editors: Richard Catlow and Aron Walsh2011 J. Phys.: Condens. Matter 23 issue 33

Semiconducting oxides are amongst the most widely studied and topical materials in contemporary condensed matter science, with interest being driven both by the fundamental challenges posed by their electronic and magnetic structures and properties, and by the wide range of applications, including those in catalysis and electronic devices. This special section aims to highlight recent developments in the physics of these materials, and to show the link between developing fundamental understanding and key application areas of oxide semiconductors.AFM image of the surface of In2O3 epilayers on YSZ(111). K H L

Zhang et al 2011 J. Phys.: Condens. Matter 23 334211.

Vibrations at surfaces

Guest Editor: Talat S Rahman2011 J. Phys.: Condens. Matter 23 issue 48

This special section is dedicated to the phenomenon of vibrations at surfaces—a topic that was indispensible a couple of decades ago, since it was one of the few phenomena capable of revealing the nature of binding at solid surfaces. For clean surfaces, the frequencies of modes with characteristic displacement patterns revealed how surface geometry, as well as the nature of binding between atoms in the surface layers, could be different from that in the bulk solid. Dispersion of the surface phonons provided further measures of interatomic interactions.Geometry of a Xe/Cu simulation cell. A Franchini et al 2011

J. Phys.: Condens. Matter 23 484004.


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journal scopeJournal of Physics: Condensed Matter covers the whole of condensed matter physics including soft condensed matter and nanostructures. Papers may report experimental, theoretical and simulation studies.

Papers are published under the following subject sections:• Surface, interface and atomic-scale science• Liquids, soft matter and biological physics• Nanostructures and nanoelectronics• Solid structure and lattice dynamics• Electronic structure• Correlated electrons• Superconductors and metals• Semiconductors• Dielectrics and ferroelectrics• Magnetism and magnetic materials

More information on each of these areas can be found at iopscience.org/jpcm.

IOP Publishing provides publications through which leading-edge scientific research is distributed worldwide. IOP Publishing is central to the Institute of Physics, which was established in 1874. The Institute of Physics is a not-for-profit society and any surplus from IOP Publishing goes to support science through the activities of the Institute. Physics is an international endeavour and the Institute aims to promote and support physics in furthering scientific knowledge and providing economic and social benefits both in the UK and Ireland and internationally – especially in the developing world.

About IOP Publishing

Our dedicated team at IOP Publishing is here to ensure that the peer-review process runs as smoothly as possible for our authors.

Journal team

Production Editor Anna-Ulla jansson

Publishing Editor Caroline Andrew

Publishing AdministratorSarah Obertelli

Publishing Editor Anna Demming

Publishing AdministratorKayleigh Parsons

Production Editor Oliver Saunders

Surface, interface and atomic-scale scienceLiquids, soft matter and biological physicsNanostructures and nanoelectronicsSolid structure and lattice dynamicsElectronic structureCorrelated electronsSuperconductors and metalsSemiconductorsDielectrics and ferroelectricsMagnetism and magnetic materials

Surface, interface and atomic-scale scienceLiquids, soft matter and biological physicsNanostructures and nanoelectronicsSolid structure and lattice dynamicsElectronic structureCorrelated electronsSuperconductors and metalsSemiconductorsDielectrics and ferroelectricsMagnetism and magnetic materials

Subject distribution for 2011

Marketing Executive Emma Watkins

Publishing Editor Philip Semple

Publisher Lucy Smith


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Editor-in-Chiefjason S Gardner, NIST, Gaithersburg, USA and Indiana University, USA

Deputy Editors john E Inglesfield, Cardiff University, UKHideaki Kasai, Osaka University, Japan

Liquids, Soft Matter and Biological Physics Section EditorFrancesco Sciortino, Universita di Roma ‘La Sapienza’, Italy

Surface, Interface and Atomic-Scale Science Section EditorHarold j W Zandvliet, Twente University, The Netherlands

Executive Board jon P Bird, University at Buffalo, The State University of New York, USAMarek Cieplak, Polish Academy of Science, Warsaw, PolandPeter A Dowben, University of Nebraska at Lincoln, USAMikhail Katsnelson, Radboud University Nijmegen, The NetherlandsTalat S Rahman, University of Central Florida, Orlando, USAjeroen van den Brink, IFW Dresden, Germany

Advisory Board David Bowler, University College London, UKGustau Catalan, ICREA and CIN2, Barcelona, SpainVincent H Crespi, Pennsylvania State University, USAPengcheng Dai, University of Tennessee, USA and Oak Ridge National Laboratory, USAMukunda P Das, Australian National University, Canberra, AustraliaSudesh Kumar Dhar, Tata Institute of Fundamental Research, IndiaRobert A de Groot, Radboud University, Nijmegen, The NetherlandsAnna Fontcuberta i Morral, Ecole Polytechnique Fédérale de Lausanne, SwitzerlandMichel j P Gingras, University of Waterloo, Canadajames Greer, Tyndall National Institute, Irelandjohn M Gregg, Queen’s University Belfast, UKG Michael Kalvius, Technische Universität München, GermanyAkio Kimura, University of Hiroshima, JapanAristide Lemaitre, CNRS-LPN, FranceRenbao Liu, Chinese University of Hong Kong, Hong KongDavid Logan, University of Oxford, UKAdam Micolich, University of New South Wales, Australiajoel Moore, University of California, Berkeley, USAAntonio Pires, Universidade Federal de Minas Gerais, BrazilThomas Pruschke, Goettingen University, GermanyFilip Ronning, Los Alamos National Laboratory, USAKenji Sakurai, National Institute for Materials Science, Tsukuba, JapanZdzislawa Szotek, Science and Technology Facilities Council, UKShin-ichi Uchida, University of Tokyo, JapanYoshiaki Uesu, Waseda University, Japan

Liquids, Soft Matter and Biological Physics BoardPatricia Bassereau, Institut Curie-Section de Recherche, FranceErika Eiser, University of Cambridge, UKMatthias Fuchs, Universitat Konstanz, GermanyMargaret Gardel, University of Chicago, USASteve Granick, University of Illionois – Urbana-Champaign, USALudger Harnau, Max-Planck-Institut fuer Intelligente Systeme, GermanyGerhard Kahl, Vienna Technical University, AustriaAlexei A Kornyshev, Imperial College, London, UKLudwig Leibler, Ecole Supérieure Physique Chimie Industrielles, Paris, FranceB Montgomery Pettitt, University of Houston, USARoberto Piazza, Politecnico di Milano, ItalyVeronique Trappe, University of Fribourg, SwitzerlandHiroshi Yokoyama, Kent State University, USAClaire Wilhelm, Universite Paris Diderot, France

Surface, Interface and Atomic-Scale Science Boardjesper Andersen, Lund University, SwedenScott Chambers, Pacific Northwest National Laboratory, USAPedro L De Andres, Consejo Superior de Investigaciones Cientificas, SpainKatsuyuki Fukutani, University of Tokyo, JapanThomas Greber, Zurich University, SwitzerlandRoberto Gunnella, Universita di Camerino, ItalyMarkus Heyde, Fritz Haber Institut der Max Planck Gesellschaft, GermanyMaya Kiskinova, Sincrotrone Trieste, ItalyNatalia Martsinovich, University of Warwick, UKPhilip Moriarty, University of Nottingham, UKj Enrique Ortega, Universidad del Pais Vasco, SpainMiguel Salmeron, Lawrence Berkeley National Laboratory, USASusan Sinnott, University of Florida, USAYuanbo Zhang, Fudan University, China

Editorial Board

We would like to thank all of our authors, referees, board members and supporters across the world for their vital contribution

to the work and progress of Journal of Physics: Condensed Matter.

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Tel +44 (0)117 929 7481 Fax +44 (0)117 929 4318E-mail [email protected] Web iopscience.org/jpcm

Documents

Condensed Matter - IOPscience...journal of Physics: Condensed Matter Highlights 2011 7 Electronic structure of optimally doped pnictide Ba 0.6 K 0.4 Fe 2 As 2: a comprehensive angle-resolved