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Physical Review Letters

6 November 2009

Volume 103, Number 19 , Articles (19xxxx)

Articles published 31 October - 6 November 2009


A rendition of a nonlinear fluid flowing through a disordered porous (Swiss-cheese) medium.

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LETTERS

General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

Fractional Feynman-Kac Equation for Non-Brownian Functionals
Lior Turgeman, Shai Carmi, and Eli Barkai
Published 6 November 2009
190201  Full Text: PDF (169 kB)  | Buy Article
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We derive backward and forward fractional Feynman-Kac equations for the distribution of functionals of the path of a particle undergoing anomalous diffusion. Fractional substantial derivatives introduced by Friedrich and co-workers [Phys. Rev. Lett. 96, 230601 (2006)] provide the correct fractional framework for the problem. For applications, we calculate the distribution of occupation times in half space and show how the statistics of anomalous functionals is related to weak ergodicity breaking.

Pulsating and Persistent Vector Solitons in a Bose-Einstein Condensate in a Lattice upon Phase Separation Instability
Uttam Shrestha, Juha Javanainen, and Janne Ruostekoski
Published 6 November 2009
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We study numerically the outcome of the phase separation instability of a dual-species Bose-Einstein condensate in an optical lattice. When only one excitation mode is unstable a bound pair of bright and dark solitonlike structures periodically appears and disappears, whereas for more than one unstable mode a persistent soliton-antisoliton pair develops. The oscillating soliton represents a regime where the two-species condensate neither remains phase-separated nor is dynamically stable.

Quantum Thermal Bath for Molecular Dynamics Simulation
Hichem Dammak, Yann Chalopin, Marine Laroche, Marc Hayoun, and Jean-Jacques Greffet
Published 5 November 2009
190601  Full Text: PDF (338 kB)  | Buy Article
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Molecular dynamics (MD) is a numerical simulation technique based on classical mechanics. It has been taken for granted that its use is limited to a large temperature regime where classical statistics is valid. To overcome this limitation, the authors introduce in a universal way a quantum thermal bath that accounts for quantum statistics while using standard MD. The efficiency of the new technique is illustrated by reproducing several experimental data at low temperatures in a regime where quantum statistical effects cannot be neglected.

Gravitation and Astrophysics

Improved Flux Limits for Neutrinos with Energies above 1022 eV from Observations with the Westerbork Synthesis Radio Telescope
O. Scholten, S. Buitink, J. Bacelar, R. Braun, A. G. de Bruyn, H. Falcke, K. Singh, B. Stappers, R. G. Strom, and R. al Yahyaoui
Published 6 November 2009
191301  Full Text: PDF (188 kB)  | Buy Article
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Particle cascades initiated by ultrahigh energy neutrinos in the lunar regolith will emit an electromagnetic pulse with a time duration of the order of nanoseconds through a process known as the Askaryan effect. It has been shown that in an observing window around 150 MHz there is a maximum chance for detecting this radiation with radio telescopes commonly used in astronomy. In 50 h of observation time with the Westerbork Synthesis Radio Telescope array we have set a new limit on the flux of neutrinos, summed over all flavors, with energies in excess of 4×1022 eV.

Elementary Particles and Fields

Hydrodynamics with Triangle Anomalies
Dam T. Son and Piotr Surówka
Published 6 November 2009
191601  Full Text: PDF (98 kB)  | Buy Article
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We consider the hydrodynamic regime of theories with quantum anomalies for global currents. We show that a hitherto discarded term in the conserved current is not only allowed by symmetries, but is in fact required by triangle anomalies and the second law of thermodynamics. This term leads to a number of new effects, one of which is chiral separation in a rotating fluid at nonzero chemical potential. The new kinetic coefficients can be expressed, in a unique fashion, through the anomaly coefficients and the equation of state. We briefly discuss the relevance of this new hydrodynamic term for physical situations, including heavy-ion collisions.

Measurement of the WW Production Cross Section with Dilepton Final States in p[overline p] Collisions at sqrt(s)=1.96 TeV and Limits on Anomalous Trilinear Gauge Couplings
V. M. Abazov et al. (D0 Collaboration)
Published 2 November 2009
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We provide the most precise measurement of the WW production cross section in p[overline p] collisions to date at a center of mass energy of 1.96 TeV, and set limits on the associated trilinear gauge couplings. The WW-->[script-l]nu[script-l][prime]nu ([script-l], [script-l][prime]=e, µ) decay channels are analyzed in 1 fb-1 of data collected by the D0 detector at the Fermilab Tevatron Collider. The measured cross section is sigma(p[overline p]-->WW)=11.5±2.1(stat+syst)±0.7(lumi) pb. One- and two-dimensional 95% C.L. limits on trilinear gauge couplings are provided.

First Observation of [overline B]<sub>s</sub><sup>0</sup>-->D<sub>s</sub><sup>[plus-minus]</sup>K-/+ and Measurement of the Ratio of Branching Fractions [script B]([overline B]<sub>s</sub><sup>0</sup>-->D<sub>s</sub><sup>[plus-minus]</sup>K-/+)/[script B]([overline B]<sub>s</sub><sup>0</sup>-->D<sub>s</sub><sup>+</sup>pi-)
T. Aaltonen et al. (CDF Collaboration)
Published 3 November 2009
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A combined mass and particle identification fit is used to make the first observation of the decay [overline B]s<sup>0</sup>-->Ds<sup>[plus-minus]</sup>K-/+ and measure the branching fraction of [overline B]s<sup>0</sup>-->Ds<sup>[plus-minus]</sup>K-/+ relative to [overline B]s<sup>0</sup>-->Ds<sup>+</sup>pi-. This analysis uses 1.2 fb-1 integrated luminosity of p[overline p] collisions at sqrt(s)=1.96 TeV collected with the CDF II detector at the Fermilab Tevatron collider. We observe a [overline B]s<sup>0</sup>-->Ds<sup>[plus-minus]</sup>K-/+ signal with a statistical significance of 8.1sigma and measure [script B]([overline B]s<sup>0</sup>-->Ds<sup>[plus-minus]</sup>K-/+)/[script B]([overline B]s<sup>0</sup>-->Ds<sup>+</sup>pi-)=0.097±0.018(stat)±0.009(syst).

Measurement of Dijet Angular Distributions at sqrt(s)=1.96 TeV and Searches for Quark Compositeness and Extra Spatial Dimensions
V. M. Abazov et al. (D0 Collaboration)
Published 5 November 2009
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We present the first measurement of dijet angular distributions in p[overline p] collisions at sqrt(s)=1.96 TeV at the Fermilab Tevatron Collider. The measurement is based on a dataset corresponding to an integrated luminosity of 0.7 fb-1 collected with the D0 detector. Dijet angular distributions have been measured over a range of dijet masses, from 0.25 TeV to above 1.1 TeV. The data are in good agreement with the predictions of perturbative QCD and are used to constrain new physics models including quark compositeness, large extra dimensions, and TeV-1 scale extra dimensions. For all models considered, we set the most stringent direct limits to date.

Nuclear Physics

Precise Electromagnetic Tests of Ab Initio Calculations of Light Nuclei: States in 10Be
E. A. McCutchan, C. J. Lister, R. B. Wiringa, Steven C. Pieper, D. Seweryniak, J. P. Greene, M. P. Carpenter, C. J. Chiara, R. V. F. Janssens, T. L. Khoo, T. Lauritsen, I. Stefanescu, and S. Zhu
Published 4 November 2009
192501  Full Text: PDF (304 kB)  | Buy Article
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In order to test ab initio calculations of light nuclei, we have remeasured lifetimes in 10Be using the Doppler shift attenuation method (DSAM) following the 7Li(7Li,alpha)10Be reaction at 8 and 10 MeV. The new experiments significantly reduce systematic uncertainties in the DSAM technique. The Jpi=21<sup>+</sup> state at 3.37 MeV has tau=205±(5)stat±(7)sys fs corresponding to a B(E2[down-arrow]) of 9.2(3)e2 fm4 in broad agreement with many calculations. The Jpi=22<sup>+</sup> state at 5.96 MeV was found to have a B(E2[down-arrow]) of 0.11(2)e2 fm4 and provides a more discriminating test of nuclear models. New Green's function Monte Carlo calculations for these states and transitions with a number of Hamiltonians are also reported and compared to experiment.

Atomic, Molecular, and Optical Physics

Inelastic Collisions of Relativistic Electrons with Atomic Targets in a Laser Field
A. B. Voitkiv, B. Najjari, and J. Ullrich
Published 5 November 2009
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We consider collisions of relativistic electrons with atomic targets in a laser field assuming that the parameters of the latter are such that the field does not influence the target directly. Concentrating on target transitions we show that they can be substantially affected by the presence of the laser field. This may occur either via strong modifications in the motion of the relativistic electrons caused by the laser field or via the Compton effect when the incident electrons convert laser photon(s) into photons with frequencies equal to target transition frequencies.

Autler-Townes Effect in a Superconducting Three-Level System
Mika A. Sillanpää, Jian Li, Katarina Cicak, Fabio Altomare, Jae I. Park, Raymond W. Simmonds, G. S. Paraoanu, and Pertti J. Hakonen
Published 2 November 2009
193601  Full Text: PDF (364 kB)  | Buy Article
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When a three-level quantum system is irradiated by an intense coupling field resonant with one of the three possible transitions, the absorption peak of an additional probe field involving the remaining level is split. This process is known in quantum optics as the Autler-Townes effect. We observe these phenomena in a superconducting Josephson phase qubit, which can be considered an “artificial atom” with a multilevel quantum structure. The spectroscopy peaks can be explained reasonably well by a simple three-level Hamiltonian model. Simulation of a more complete model (including dissipation, higher levels, and cross coupling) provides excellent agreement with all of the experimental data.

Chirped Biphotons and their Compression in Optical Fibers
G. Brida, M. V. Chekhova, I. P. Degiovanni, M. Genovese, G. Kh. Kitaeva, A. Meda, and O. A. Shumilkina
Published 5 November 2009
193602  Full Text: PDF (486 kB)  | Buy Article
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We show that broadband biphoton wave packets produced via sponateous parametric down-conversion in crystals with linearly aperiodic poling can be easily compressed in time using the effect of group-velocity dispersion in optical fibers. This result could foster important developments in quantum metrology and lithography.

Nonlinear Dynamics, Fluid Dynamics, Classical Optics, etc.

Universal Critical Power for Nonlinear Schrödinger Equations with a Symmetric Double Well Potential
Andrea Sacchetti
Published 5 November 2009
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Here we consider stationary states for nonlinear Schrödinger equations in any spatial dimension n with symmetric double well potentials. These states may bifurcate as the strength of the nonlinear term increases and we observe two different pictures depending on the value of the nonlinearity power: a supercritical pitchfork bifurcation, and a subcritical pitchfork bifurcation with two asymmetric branches occurring as the result of saddle-node bifurcations. We show that in the semiclassical limit, or for a large barrier between the two wells, the first kind of bifurcation always occurs when the nonlinearity power is less than a critical value; in contrast, when the nonlinearity power is larger than such a critical value then we always observe the second scenario. The remarkable fact is that such a critical value is a universal constant in the sense that it does not depend on the shape of the double well potential and on the dimension n.

Dynamics of Transition from Static to Kinetic Friction
O. M. Braun, I. Barel, and M. Urbakh
Published 6 November 2009
194301  Full Text: PDF (490 kB)  | Buy Article
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We propose a model for a description of dynamics of cracklike processes that occur at the interface between two blocks prior to the onset of frictional motion. We find that the onset of sliding is preceded by well-defined detachment fronts initiated at the slider trailing edge and extended across the slider over limited lengths smaller than the overall length of the slider. Three different types of detachment fronts may play a role in the onset of sliding: (i) Rayleigh (surface sound) fronts, (ii) slow detachment fronts, and (iii) fast fronts. The important consequence of the precursor dynamics is that before the transition to overall sliding occurs, the initially uniform, unstressed slider is already transformed into a highly nonuniform, stressed state. Our model allows us to explain experimental observations and predicts the effect of material properties on the dynamics of the transition to sliding.

Acceleration Statistics of Neutrally Buoyant Spherical Particles in Intense Turbulence
Rachel D. Brown, Z. Warhaft, and Greg A. Voth
Published 5 November 2009
194501  Full Text: PDF (187 kB)  | Buy Article
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We measure acceleration statistics of neutrally buoyant spherical particles with a diameter 0.4<d/eta<27 in intense turbulence (400<Rlambda<815). High speed cameras image polystyrene tracer particles in a flow between counterrotating disks. The measurements of acceleration variance <a2> clearly resolve the transition from the tracer like behavior of small particles to the much smaller accelerations of large particles. Two models of this transition from small to large particle behavior are critically examined. For d>5eta, <a2> decreases with the diameter as d-2/3, in agreement with inertial range scaling arguments. Consistent with earlier work, we find that the scaled acceleration probability density function shows very little dependence on particle size.

Non-Newtonian Fluid Flow through Three-Dimensional Disordered Porous Media
Apiano F. Morais, Hansjoerg Seybold, Hans J. Herrmann, and José S. Andrade, Jr.
Published 5 November 2009
194502  Full Text: PDF (1382 kB)  | Buy Article
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We investigate the flow of various non-Newtonian fluids through three-dimensional disordered porous media by direct numerical simulation of momentum transport and continuity equations. Remarkably, our results for power-law (PL) fluids indicate that the flow, when quantified in terms of a properly modified permeability-like index and Reynolds number, can be successfully described by a single (universal) curve over a broad range of Reynolds conditions and power-law exponents. We also study the flow behavior of Bingham fluids described in terms of the Herschel-Bulkley model. In this case, our simulations reveal that the interplay of (i) the disordered geometry of the pore space, (ii) the fluid rheological properties, and (iii) the inertial effects on the flow is responsible for a substantial enhancement of the macroscopic hydraulic conductance of the system at intermediate Reynolds conditions.

Plasma and Beam Physics
Experimental Verification of Reversed Cherenkov Radiation in Left-Handed Metamaterial
Sheng Xi, Hongsheng Chen, Tao Jiang, Lixin Ran, Jiangtao Huangfu, Bae-Ian Wu, Jin Au Kong, and Min Chen
Published 2 November 2009
194801  Full Text: PDF (499 kB)  | Buy Article
See accompanying Viewpoint Physics 2, 91 (2009)
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By using a phased electromagnetic dipole array to model a moving charged particle, we experimentally verified a reversed Cherenkov radiation in the left-handed media in the frequency range from 8.1 to 9.5 GHz. Our results demonstrate the feasibility of new types of particle detectors and radiation generators.
Reversed Cherenkov-Transition Radiation by a Charge Crossing a Left-Handed Medium Boundary
Sergey N. Galyamin, Andrey V. Tyukhtin, Alexey Kanareykin, and Paul Schoessow
Published 2 November 2009
194802  Full Text: PDF (296 kB)  | Buy Article
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We analyze the radiation from a charged particle crossing the boundary between an ordinary medium and a “left-handed” metamaterial. We obtain exact and approximate expressions for the field components and develop algorithms for their computation. The spatial radiation in this system can be separated into three distinct components, corresponding to ordinary transition radiation having a relatively large magnitude, Cherenkov radiation, and reversed Cherenkov-transition radiation (RCTR). The last one is explained by reflection and refraction of reversed Cherenkov radiation at the interface. Conditions for generating of RCTR are obtained. We note properties of this radiation that have potential applications in the detection of charged particles and accelerator beams and for the characterization of metamaterial macroscopic parameters (epsilon, µ).

Electron Optical Injection with Head-On and Countercrossing Colliding Laser Pulses
H. Kotaki, I. Daito, M. Kando, Y. Hayashi, K. Kawase, T. Kameshima, Y. Fukuda, T. Homma, J. Ma, L.-M. Chen, T. Zh. Esirkepov, A. S. Pirozhkov, J. K. Koga, A. Faenov, T. Pikuz, H. Kiriyama, H. Okada, T. Shimomura, Y. Nakai, M. Tanoue, H. Sasao, D. Wakai, H. Matsuura, S. Kondo, S. Kanazawa, A. Sugiyama, H. Daido, and S. V. Bulanov
Published 2 November 2009
194803  Full Text: PDF (1378 kB)  | Buy Article
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A high stability electron bunch is generated by laser wakefield acceleration with the help of a colliding laser pulse. The wakefield is generated by a laser pulse; the second laser pulse collides with the first pulse at 180° and at 135° realizing optical injection of an electron bunch. The electron bunch has high stability and high reproducibility compared with single pulse electron generation. In the case of 180° collision, special measures have been taken to prevent damage. In the case of 135° collision, since the second pulse is countercrossing, it cannot damage the laser system.

Observation of Beam Loading in a Laser-Plasma Accelerator
C. Rechatin, X. Davoine, A. Lifschitz, A. Ben Ismail, J. Lim, E. Lefebvre, J. Faure, and V. Malka
Published 2 November 2009
194804  Full Text: PDF (507 kB)  | Buy Article
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Beam loading is the phenomenon which limits the charge and the beam quality in plasma based accelerators. An experimental study conducted with a laser-plasma accelerator is presented. Beam loading manifests itself through the decrease of the beam energy, the reduction of dark current, and the increase of the energy spread for large beam charge. 3D PIC simulations are compared to the experimental results and confirm the effects of beam loading. It is found that, in our experimental conditions, the trapped electron beams generate decelerating fields on the order of 1 (GV/m)/pC and that beam loading effects are optimized for trapped charges of about 20 pC.

Probing the Transverse Coherence of an Undulator X-Ray Beam Using Brownian Particles
M. D. Alaimo, M. A. C. Potenza, M. Manfredda, G. Geloni, M. Sztucki, T. Narayanan, and M. Giglio
Published 5 November 2009
194805  Full Text: PDF (572 kB)  | Buy Article
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We present a novel method to map the two-dimensional transverse coherence of an x-ray beam using the dynamical near-field speckles formed by scattering from colloidal particles. Owing to the statistical nature of the method, the coherence properties of synchrotron radiation from an undulator source is obtained with high accuracy. The two-dimensional complex coherence function is determined at the sample position and the imaging optical scheme further allowed us to evaluate the coherence factor at the undulator output despite the aberrations introduced by the focusing optics.

Is Diffusion Anomalous in Two-Dimensional Yukawa Liquids?
T. Ott and M. Bonitz
Published 3 November 2009
195001  Full Text: PDF (1142 kB)  | Buy Article
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There have recently been many predictions of “superdiffusion” in two-dimensional strongly coupled Yukawa systems, both by computer simulations and in dusty plasma experiments, with substantially varying diffusion exponents. Here we show that the results crucially depend on the strength of dissipation and the time instant of the measurement. For sufficiently large friction even subdiffusion is possible. However, there are strong indications that, in the long-time limit, anomalous diffusion vanishes and the system returns to normal diffusion, for dissipative as well as for frictionless systems.

Suppression of Ablation in Femtosecond Double-Pulse Experiments
M. E. Povarnitsyn, T. E. Itina, K. V. Khishchenko, and P. R. Levashov
Published 4 November 2009
195002  Full Text: PDF (577 kB)  | Buy Article
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We report the physical reasons of a curious decrease in the crater depth observed for long delays in experiments with two successive femtosecond pulses. Detailed hydrodynamic modeling demonstrates that the ablation mechanism is dumped when the delay between the pulses exceeds the electron-ion relaxation time. In this case, the interaction of the second laser pulse with the expanding target material leads to the formation of the second shock wave suppressing the rarefaction wave created by the first pulse. The evidence of this effect follows from the pressure and density profiles obtained at different delays after the first laser pulse.

Condensed Matter: Structure, etc.

Lifshitz Point in the Phase Diagram of Resonantly Interacting 6Li-40K Mixtures
K. B. Gubbels, J. E. Baarsma, and H. T. C. Stoof
Published 3 November 2009
195301  Full Text: PDF (138 kB)  | Buy Article
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We consider a strongly interacting 6Li-40K mixture, which is imbalanced both in the masses and the densities of the two fermionic species. At present, it is the experimentalist's favorite for reaching the superfluid regime. We construct an effective thermodynamic potential that leads to excellent agreement with Monte Carlo results for the normal state. We use it to determine the universal phase diagram of the mixture in the unitarity limit, where we find, in contrast to the mass-balanced case, the presence of a Lifshitz point. This point is characterized by the effective mass of the Cooper pairs becoming negative, which signals an instability towards a supersolid phase.

Multiresonant Spinor Dynamics in a Bose-Einstein Condensate
C. Klempt, O. Topic, G. Gebreyesus, M. Scherer, T. Henninger, P. Hyllus, W. Ertmer, L. Santos, and J. J. Arlt
Published 6 November 2009
195302  Full Text: PDF (334 kB)  | Buy Article
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We analyze the spinor dynamics of a 87Rb F=2 condensate initially prepared in the mF=0 Zeeman sublevel. We show that this dynamics, characterized by the creation of correlated atomic pairs in mF=±1, presents an intriguing multiresonant magnetic-field dependence induced by the trap inhomogeneity. This dependence is directly linked to the most unstable Bogoliubov spin excitations of the initial mF=0 condensate, showing that, in general, even a qualitative understanding of the pair-creation efficiency in a spinor condensate requires a careful consideration of the confinement.

Dewetting of Ultrathin Solid Films
O. Pierre-Louis, A. Chame, and Y. Saito
Published 4 November 2009
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Ultrathin crytalline solid films are found to dewet with a faceted rim. In the case of heterogeneous dewetting initiated from a linear trench or from periodically arranged holes, the dewetted area expands either with a faceted multilayer rim or in a layer-by-layer fashion. In the case of homogeneous dewetting, holes are accompanied with multilayer rims and the uncoverage increases as a power law of time. Results of kinetic Monte Carlo simulations are elucidated within the frame of nucleation theory and surface diffusion limited dynamics.

Nature of Atomic Bonding and Atomic Structure in the Phase-Change Ge2Sb2Te5 Glass
M. Xu, Y. Q. Cheng, H. W. Sheng, and E. Ma
Published 6 November 2009
195502  Full Text: PDF (967 kB)  | Buy Article
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Using electronic structure calculations, we demonstrate a global valence alternation in the amorphous Ge2Sb2Te5, a prototype phase-change alloy for data storage. The resulting p bonding profoundly influences the local atomic structure, leading to right-angle components similar to those in the crystalline counterpart of this chalcogenide glass. The dominance of p bonding is revealed by (i) distributions of the coordination number (CN) and the bond angle, for truly bonded atoms determined based on the electron localization function, and (ii) a direct evaluation of the p (and s) orbital occupation probability for the CN=3 Ge atoms that form 90° bonds with neighbors.

Condensed Matter: Electronic Properties, etc.

Nature and Strength of Interlayer Binding in Graphite
Leonardo Spanu, Sandro Sorella, and Giulia Galli
Published 6 November 2009
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We compute the interlayer bonding properties of graphite using an ab initio many-body theory. We carry out variational and diffusion quantum Monte Carlo calculations and find an equilibrium interlayer binding energy in good agreement with most recent experiments. We also analyze the behavior of the total energy as a function of interlayer separation at large distances comparing the results with the predictions of the random phase approximation.

Spin Hall Drag in Electronic Bilayers
S. M. Badalyan and G. Vignale
Published 2 November 2009
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We predict a new effect in electronic bilayers: spin Hall drag. The effect consists of the generation of spin accumulation across one layer by an electric current along the other layer. It arises from the combined action of spin-orbit and Coulomb interactions. Our theoretical analysis, based on the Boltzmann equation formalism, identifies two main contributions to the spin Hall drag resistivity: the side-jump contribution, which dominates at low temperature, going as T2, and the skew-scattering contribution, which is proportional to T3. The induced spin accumulation, while generally quite small, should be observable in optical rotation experiments.

Temperature Dependence of the Conductivity of Ballistic Graphene
Markus Müller, Matthias Bräuninger, and Björn Trauzettel
Published 2 November 2009
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We investigate the temperature dependence of conductivity in ballistic graphene using Landauer transport theory. We obtain results which are qualitatively in agreement with many features recently observed in transport measurements on high mobility suspended graphene. The conductivity sigma at high temperature T and low density n grows linearly with T, while at high n we find sigma~sqrt(|n|) with negative corrections at small T due to the T dependence of the chemical potential. At moderate densities the conductivity is a nonmonotonic function of T and n, exhibiting a minimum at T=0.693[h-bar]vsqrt(|n|) where v is the Fermi velocity. We discuss two kinds of Fabry-Perot oscillations in short nanoribbons and their stability at finite temperatures.

Bilayer Graphene Interferometry: Phase Jump and Wave Collimation
Sunghun Park and H.-S. Sim
Published 2 November 2009
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We theoretically study the phase of the reflection amplitude of an electron (massive Dirac fermion) at a lateral potential step in Bernal-stacked bilayer graphene. The phase shows an anomalous jump of pi, as the electron incidence angle (relative to the normal direction to the step) varies to pass ±pi/4. The jump is attributed to the Berry phase associated with the pseudospin-1/2 of the electron. This Berry-phase effect is robust against the band-gap opening due to the external electric gates generating the step. We propose an interferometry setup in which collimated waves can be generated and tuned. By using the setup, one can identify both the pi jump and the collimation angle.

Fractional Topological Insulators
Michael Levin and Ady Stern
Published 4 November 2009
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We analyze generalizations of two-dimensional topological insulators which can be realized in interacting, time reversal invariant electron systems. These states, which we call fractional topological insulators, contain excitations with fractional charge and statistics in addition to protected edge modes. In the case of sz conserving toy models, we show that a system is a fractional topological insulator if and only if sigmasH/e* is odd, where sigmasH is the spin-Hall conductance in units of e/2pi, and e* is the elementary charge in units of e.

Surface States of Topological Insulators: The Dirac Fermion in Curved Two-Dimensional Spaces
Dung-Hai Lee
Published 5 November 2009
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The surface of a topological insulator is a closed two-dimensional manifold. The surface states are described by the Dirac Hamiltonian in curved two-dimensional spaces. For a slablike sample with a magnetic field perpendicular to its top and bottom surfaces, there are chiral states delocalized on the four side faces. These “chiral sheets” carry both charge and spin currents. In strong magnetic fields, the quantized charge Hall effect [sigmaxy=(2n+1)e2/h] will coexist with spin Hall effect.

Theory of the Topological Anderson Insulator
C. W. Groth, M. Wimmer, A. R. Akhmerov, J. Tworzydlo, and C. W. J. Beenakker
Published 6 November 2009
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We present an effective medium theory that explains the disorder-induced transition into a phase of quantized conductance, discovered in computer simulations of HgTe quantum wells. It is the combination of a random potential and quadratic corrections [proportional]p2sigmaz to the Dirac Hamiltonian that can drive an ordinary band insulator into a topological insulator (having an inverted band gap). We calculate the location of the phase boundary at weak disorder and show that it corresponds to the crossing of a band edge rather than a mobility edge. Our mechanism for the formation of a topological Anderson insulator is generic, and would apply as well to three-dimensional semiconductors with strong spin-orbit coupling.

Localization of Metal-Induced Gap States at the Metal-Insulator Interface: Origin of Flux Noise in SQUIDs and Superconducting Qubits
SangKook Choi, Dung-Hai Lee, Steven G. Louie, and John Clarke
Published 3 November 2009
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The origin of magnetic flux noise in superconducting quantum interference devices with a power spectrum scaling as 1/f (f is frequency) has been a puzzle for over 20 years. This noise limits the decoherence time of superconducting qubits. A consensus has emerged that the noise arises from fluctuating spins of localized electrons with an areal density of 5×1017 m-2. We show that, in the presence of potential disorder at the metal-insulator interface, some of the metal-induced gap states become localized and produce local moments. A modest level of disorder yields the observed areal density.

Evidence for Complex Superconducting Order Parameter Symmetry in the Low-Temperature Phase of UPt3 from Josephson Interferometry
J. D. Strand, D. J. Van Harlingen, J. B. Kycia, and W. P. Halperin
Published 4 November 2009
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We present data on the modulation of the critical current with applied magnetic field in UPt3–Cu–Pb Josephson junctions and SQUIDs. The junctions were fabricated on polished surfaces of UPt3 single crystals. The shape of the resulting diffraction patterns provides phase-sensitive information on the superconducting order parameter. Our corner junction data show asymmetric patterns with respect to magnetic field, indicating a complex order parameter, and both our junction and SQUID measurements point to a phase shift of pi, supporting the E2u representation of the order parameter.

Proximity-Induced Pseudogap: Evidence for Preformed Pairs
Ofer Yuli, Itay Asulin, Yoav Kalcheim, Gad Koren, and Oded Millo
Published 5 November 2009
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The temperature evolution of the proximity effect in Au/La2-xSrxCuO4 and La1.55Sr0.45CuO4/  La2-xSrxCuO4 bilayers was investigated using scanning tunneling microscopy. Proximity-induced gaps, centered at the chemical potential, were found to persist above the superconducting transition temperature, Tc, and up to nearly the pseudogap crossover temperature in both systems. Such independence of the spectra on the details of the normal-metal cap layer is incompatible with a density-wave order origin. However, our results can be accounted for by a penetration of incoherent Cooper pairs into the normal metal above Tc.

Magnetic Excitations in the Kondo Liquid: Superconductivity and Hidden Magnetic Quantum Critical Fluctuations
Yi-feng Yang, Ricardo Urbano, Nicholas J. Curro, David Pines, and E. D. Bauer
Published 5 November 2009
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We report Knight-shift experiments on the superconducting heavy-electron material CeCoIn5 that allow one to track with some precision the behavior of the heavy-electron Kondo liquid in the superconducting state with results in agreement with BCS theory. An analysis of the 115In nuclear quadrupole resonance spin-lattice relaxation rate T1<sup>-1</sup> measurements under pressure reveals the presence of 2d magnetic quantum critical fluctuations in the heavy-electron component that are a promising candidate for the pairing mechanism in this material. Our results are consistent with an antiferromagnetic quantum critical point located at slightly negative pressure in CeCoIn5 and provide additional evidence for significant similarities between the heavy-electron materials and the high-Tc cuprates.

Rounding of First Order Transitions in Low-Dimensional Quantum Systems with Quenched Disorder
Rafael L. Greenblatt, Michael Aizenman, and Joel L. Lebowitz
Published 2 November 2009
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We prove that the addition of an arbitrarily small random perturbation to a quantum spin system rounds a first-order phase transition in the conjugate order parameter in d<=2 dimensions, or for cases involving the breaking of a continuous symmetry in d<=4. This establishes rigorously for quantum systems the existence of the Imry-Ma phenomenon which for classical systems was proven by Aizenman and Wehr.

Observation of the Underscreened Kondo Effect in a Molecular Transistor
Nicolas Roch, Serge Florens, Theo A. Costi, Wolfgang Wernsdorfer, and Franck Balestro
Published 3 November 2009
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We present the first quantitative experimental evidence for the underscreened Kondo effect, an incomplete compensation of a quantized magnetic moment by conduction electrons, as originally proposed by Nozières and Blandin. The device consists of an even charge spin S=1 molecular quantum dot, obtained by electromigration of C60 molecules into gold nanogaps and operated in a dilution fridge. The persistence of logarithmic singularities in the low temperature conductance is demonstrated by a comparison to the fully screened configuration obtained in odd charge spin S=1/2 Coulomb diamonds. We also discover an extreme sensitivity of the underscreened Kondo resonance to the magnetic field that we confirm on the basis of numerical renormalization group calculations.

Quasiparticles of Spatially Anisotropic Triangular Antiferromagnets in a Magnetic Field
Masanori Kohno
Published 4 November 2009
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The spectral properties of the spin-1/2 Heisenberg antiferromagnet on an anisotropic triangular lattice in a magnetic field are investigated using a weak-interchain-coupling approach combined with exact solutions of a chain. Dominant modes induced by interchain interactions in a magnetic field behave as quasiparticles which show distinctive features such as anomalous incommensurate ordering and high-energy modes. In terms of them, various unusual features observed in the anisotropic triangular antiferromagnet Cs2CuCl4 in a magnetic field are quantitatively explained in a unified manner.

Dependence of Magnetic Domain-Wall Motion on a Fast Changing Current
Lars Bocklage, Benjamin Krüger, Toru Matsuyama, Markus Bolte, Ulrich Merkt, Daniela Pfannkuche, and Guido Meier
Published 6 November 2009
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A dependence of current-induced domain-wall motion in nanowires on the temporal shape of current pulses is observed. The results show that the motion of the wall is amplified for alterations of the current on a time scale smaller than the intrinsic time scale of the domain wall which is a few nanoseconds in permalloy. This effect arises from an additional force on the wall by the spin-transfer torque due to a fast changing current and improves the efficiency of domain-wall motion. The observations provide an understanding for the efficient domain-wall motion with nanosecond current pulses.

Direct Observation of the Critical Relaxation of Polarization Clusters in BaTiO3 Using a Pulsed X-Ray Laser Technique
K. Namikawa, M. Kishimoto, K. Nasu, E. Matsushita, R. Z. Tai, K. Sukegawa, H. Yamatani, H. Hasegawa, M. Nishikino, M. Tanaka, and K. Nagashima
Published 5 November 2009
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We have developed a new method to investigate the relaxation time of the dipole moment in polarization clusters in BaTiO3. Time correlation of speckle intensities was measured by the use of a double pulsed soft x-ray laser. The evolution of the relaxation time of the dipole moment near the Curie temperature (TC) was investigated. The maximum relaxation time (~90 ps) is shown to appear at a temperature of 4.5 K above the TC, being coincident with the one where the maximum polarization takes place. This method is widely applicable to any other critical decay processes at phase transitions.

Molecular Dynamics Study of Dielectric Response in a Relaxor Ferroelectric
Ilya Grinberg, Young-Han Shin, and Andrew M. Rappe
Published 5 November 2009
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We use atomistic molecular dynamics simulations to study relaxor behavior in the 0.75PbMg1/3Nb2/3O3-0.25PbTiO3 material. Even for a fairly small simulation size of 1000 atoms, the system exhibits frequency dispersion and deviation from the Curie-Weiss law typical of relaxor materials. Analysis of the time autocorrelation functions for individual atoms allows us to identify the Nb atoms with a high concentration of neighboring Ti atoms as the nucleation sites for the relaxor behavior. This is due to the higher coupling between the cation displacements induced by the presence of overbonded oxygen atoms.

Soft Matter, Biological, and Interdisciplinary Physics

Soft X-Ray Diffraction Microscopy of a Frozen Hydrated Yeast Cell
Xiaojing Huang, Johanna Nelson, Janos Kirz, Enju Lima, Stefano Marchesini, Huijie Miao, Aaron M. Neiman, David Shapiro, Jan Steinbrener, Andrew Stewart, Joshua J. Turner, and Chris Jacobsen
Published 5 November 2009
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We report the first image of an intact, frozen hydrated eukaryotic cell using x-ray diffraction microscopy, or coherent x-ray diffraction imaging. By plunge freezing the specimen in liquid ethane and maintaining it below -170 °C, artifacts due to dehydration, ice crystallization, and radiation damage are greatly reduced. In this example, coherent diffraction data using 520 eV x rays were recorded and reconstructed to reveal a budding yeast cell at a resolution better than 25 nm. This demonstration represents an important step towards high resolution imaging of cells in their natural, hydrated state, without limitations imposed by x-ray optics.

Cryogenic X-Ray Diffraction Microscopy for Biological Samples
Enju Lima, Lutz Wiegart, Petra Pernot, Malcolm Howells, Joanna Timmins, Federico Zontone, and Anders Madsen
Published 5 November 2009
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X-ray diffraction microscopy (XDM) is well suited for nondestructive, high-resolution biological imaging, especially for thick samples, with the high penetration power of x rays and without limitations imposed by a lens. We developed nonvacuum, cryogenic (cryo-) XDM with hard x rays at 8 keV and report the first frozen-hydrated imaging by XDM. By preserving samples in amorphous ice, the risk of artifacts associated with dehydration or chemical fixation is avoided, ensuring the imaging condition closest to their natural state. The reconstruction shows internal structures of intact D. radiodurans bacteria in their natural contrast.

Dynamics of Enhanced Tracer Diffusion in Suspensions of Swimming Eukaryotic Microorganisms
Kyriacos C. Leptos, Jeffrey S. Guasto, J. P. Gollub, Adriana I. Pesci, and Raymond E. Goldstein
Published 5 November 2009
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In contexts such as suspension feeding in marine ecologies there is an interplay between Brownian motion of nonmotile particles and their advection by flows from swimming microorganisms. As a laboratory realization, we study passive tracers in suspensions of eukaryotic swimmers, the alga Chlamydomonas reinhardtii. While the cells behave ballistically over short intervals, the tracers behave diffusively, with a time-dependent but self-similar probability distribution function of displacements consisting of a Gaussian core and robust exponential tails. We emphasize the role of flagellar beating in creating oscillatory flows that exceed Brownian motion far from each swimmer.

Generalized Yvon-Born-Green Theory for Molecular Systems
J. W. Mullinax and W. G. Noid
Published 6 November 2009
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We employ a basis set representation for classical force fields to derive an original system of exact integral equations relating each mode in the force field to an associated set of structural correlation functions. This generalized Yvon-Born-Green theory provides a framework for interpreting complex many-body correlations and also for variationally determining optimal interaction potentials for proteins and other complex molecules directly from structural correlation functions.

How to Define Variation of Physical Properties Normal to an Undulating One-Dimensional Object
Hsiao-Ping Hsu, Kurt Binder, and Wolfgang Paul
Published 4 November 2009
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One-dimensional flexible objects are abundant in physics, from polymers to vortex lines to defect lines and many more. These objects structure their environment and it is natural to assume that the influence these objects exert on their environment depends on the distance from the line object. But how should this be defined? We argue here that there is an intrinsic length scale along the undulating line that is a measure of its stiffness (i.e., orientational persistence), which yields a natural way of defining the variation of physical properties normal to the undulating line. We exemplify how this normal variation can be determined from a computer simulation for the case of a so-called bottle-brush polymer, where side chains are grafted onto a flexible backbone.

Energy Landscape of Social Balance
Seth A. Marvel, Steven H. Strogatz, and Jon M. Kleinberg
Published 4 November 2009
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We model a close-knit community of friends and enemies as a fully connected network with positive and negative signs on its edges. Theories from social psychology suggest that certain sign patterns are more stable than others. This notion of social “balance” allows us to define an energy landscape for such networks. Its structure is complex: numerical experiments reveal a landscape dimpled with local minima of widely varying energy levels. We derive rigorous bounds on the energies of these local minima and prove that they have a modular structure that can be used to classify them.

Intrinsic Noise in Game Dynamical Learning
Tobias Galla
Published 6 November 2009
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Demographic noise has profound effects on evolutionary and population dynamics, as well as on chemical reaction systems and models of epidemiology. Such noise is intrinsic and due to the discreteness of the dynamics in finite populations. We here show that similar noise-sustained trajectories arise in game dynamical learning, where the stochasticity has a different origin: agents sample a finite number of moves of their opponents in between adaptation events. The limit of infinite batches results in deterministic modified replicator equations, whereas finite sampling leads to a stochastic dynamics. The characteristics of these fluctuations can be computed analytically using methods from statistical physics, and such noise can affect the attractors significantly, leading to noise-sustained cycling or removing periodic orbits of the standard replicator dynamics.

COMMENTS

Comment on “Ferromagnetic Microswimmers”
Andrej Vilfan and Holger Stark
Published 3 November 2009
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A Comment on the Letter by Feodor Y. Ogrin, Peter G. Petrov, and C. Peter Winlove [Phys Rev. Lett. 100, 218102 (2008)]. The authors of the Letter offer a Reply.

Ogrin, Petrov, and Winlove Reply:
Feodor Y. Ogrin, Peter G. Petrov, and C. Peter Winlove
Published 3 November 2009
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A Reply to the Comment by Andrej Vilfan and Holger Stark.