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Physical Review Letters

20 November 2009

Volume 103, Number 21 , Articles (21xxxx)

Articles published 14 November - 20 November 2009


A contour plot of the nuclear potential energy surface for 72Kr along with the shapes corresponding to the marked points which include two minima: the ground state (blue) and a metastable state (green).

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LETTERS

General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.
Measure for the Degree of Non-Markovian Behavior of Quantum Processes in Open Systems
Heinz-Peter Breuer, Elsi-Mari Laine, and Jyrki Piilo
Published 16 November 2009
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We construct a general measure for the degree of non-Markovian behavior in open quantum systems. This measure is based on the trace distance which quantifies the distinguishability of quantum states. It represents a functional of the dynamical map describing the time evolution of physical states, and can be interpreted in terms of the information flow between the open system and its environment. The measure takes on nonzero values whenever there is a flow of information from the environment back to the open system, which is the key feature of non-Markovian dynamics.

Anderson Localization of Solitons
Krzysztof Sacha, Cord A. Müller, Dominique Delande, and Jakub Zakrzewski
Published 17 November 2009
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At low temperature, a quasi-one-dimensional ensemble of atoms with an attractive interaction forms a bright soliton. When exposed to a weak and smooth external potential, the shape of the soliton is hardly modified, but its center-of-mass motion is affected. We show that in a spatially correlated disordered potential, the quantum motion of a bright soliton displays Anderson localization. The localization length can be much larger than the soliton size and could be observed experimentally.

Finite-Temperature Pairing Gap of a Unitary Fermi Gas by Quantum Monte Carlo Calculations
Piotr Magierski, Gabriel Wlazlowski, Aurel Bulgac, and Joaquín E. Drut
Published 19 November 2009
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We calculate the one-body temperature Green's (Matsubara) function of the unitary Fermi gas via quantum Monte Carlo, and extract the spectral weight function A(p,omega) using the methods of maximum entropy and singular value decomposition. From A(p,omega) we determine the quasiparticle spectrum, which can be accurately parametrized by three functions of temperature: an effective mass m*, a mean-field potential U, and a gap Delta. Below the critical temperature Tc=0.15epsilonF the results for m*, U, and Delta can be accurately reproduced using an independent quasiparticle model. We find evidence of a pseudogap in the fermionic excitation spectrum for temperatures up to T*[approximate]0.20epsilonF>Tc.

Expectation Values in the Lieb-Liniger Bose Gas
M. Kormos, G. Mussardo, and A. Trombettoni
Published 20 November 2009
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We present a novel method to compute expectation values in the Lieb-Liniger model both at zero and finite temperature. These quantities, relevant in the physics of one-dimensional ultracold Bose gases, are expressed by a series that has a remarkable behavior of convergence. Among other results, we show the computation of the three-body expectation value at finite temperature, a quantity that rules the recombination rate of the Bose gas.

Perfect Distinguishability of Quantum Operations
Runyao Duan, Yuan Feng, and Mingsheng Ying
Published 20 November 2009
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We provide a feasible necessary and sufficient condition for when an unknown quantum operation (quantum device) secretly selected from a set of known quantum operations can be identified perfectly within a finite number of queries, and thus complete the characterization of the perfect distinguishability of quantum operations. We further design an optimal protocol which can achieve the perfect discrimination between two quantum operations by a minimal number of queries. Interestingly, we find that an optimal perfect discrimination between two isometries is always achievable without auxiliary systems or entanglement.

Gravitation and Astrophysics

Magnetic Fluctuation Power Near Proton Temperature Anisotropy Instability Thresholds in the Solar Wind
S. D. Bale, J. C. Kasper, G. G. Howes, E. Quataert, C. Salem, and D. Sundkvist
Published 16 November 2009
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The proton temperature anisotropy in the solar wind is known to be constrained by the theoretical thresholds for pressure-anisotropy-driven instabilities. Here, we use approximately 1×106 independent measurements of gyroscale magnetic fluctuations in the solar wind to show for the first time that these fluctuations are enhanced along the temperature anisotropy thresholds of the mirror, proton oblique firehose, and ion cyclotron instabilities. In addition, the measured magnetic compressibility is enhanced at high plasma beta (beta||>~1) along the mirror instability threshold but small elsewhere, consistent with expectations of the mirror mode. We also show that the short wavelength magnetic fluctuation power is a strong function of collisionality, which relaxes the temperature anisotropy away from the instability conditions and reduces correspondingly the fluctuation power.

Supersoft Symmetry Energy Encountering Non-Newtonian Gravity in Neutron Stars
De-Hua Wen, Bao-An Li, and Lie-Wen Chen
Published 16 November 2009
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Considering the non-Newtonian gravity proposed in grand unification theories, we show that the stability and observed global properties of neutron stars cannot rule out the supersoft nuclear symmetry energies at suprasaturation densities. The degree of possible violation of the inverse-square law of gravity in neutron stars is estimated using an equation of state of neutron-rich nuclear matter consistent with the available terrestrial laboratory data.

Gamma Rays from Ultracompact Primordial Dark Matter Minihalos
Pat Scott and Sofia Sivertsson
Published 20 November 2009
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Ultracompact minihalos have been proposed as a new class of dark matter structure. They would be produced by phase transitions in the early Universe or features in the inflaton potential, and constitute nonbaryonic massive compact halo objects today. We examine the prospects of detecting these minihalos in gamma rays if dark matter can self-annihilate. We compute present-day fluxes from minihalos produced in the e+e- annihilation epoch and the QCD and electroweak phase transitions. Even at a distance of 4 kpc, minihalos from the e+e- epoch would be eminently detectable today by the Fermi satellite or air Čerenkov telescopes, or even in archival EGRET data. Within 2 kpc, they would appear as extended sources to Fermi. At 4 kpc, minihalos from the QCD transition have similar predicted fluxes to dwarf spheroidal galaxies, so might also be detectable by present or upcoming experiments.

Elementary Particles and Fields

Measurement of D0-[overline D]0 Mixing from a Time-Dependent Amplitude Analysis of D0-->K+pi-pi0 Decays
B. Aubert et al. (BABAR Collaboration)
Published 18 November 2009
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We present evidence of D0-[overline D]0 mixing using a time-dependent amplitude analysis of the decay D0-->K+pi-pi0 in a data sample of 384 fb-1 collected with the BABAR detector at the PEP-II e+e- collider at the Stanford Linear Accelerator Center. Assuming CP conservation, we measure the mixing parameters xK pi pi[sup 0]<sup>[prime]</sup>=[2.61-0.68<sup>+0.57</sup>(stat)±0.39(syst)]%, yK pi pi[sup 0]<sup>[prime]</sup>=[-0.06-0.64<sup>+0.55</sup>(stat)±0.34(syst)]%. This result is inconsistent with the no-mixing hypothesis with a significance of 3.2 standard deviations. We find no evidence of CP violation in mixing.

Measurement of B-->K*(892)gamma Branching Fractions and CP and Isospin Asymmetries
B. Aubert et al. (BABAR Collaboration)
Published 19 November 2009
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We present an analysis of the decays B0-->K*0(892)gamma and B+-->K*+(892)gamma using a sample of about 383×106 B[overline B] events collected with the BABAR detector at the PEP-II asymmetric energy B factory. We measure the branching fractions [script B](B0-->K*0gamma)=(4.47±0.10±0.16)×10-5 and [script B](B+-->K*+gamma)=(4.22±0.14±0.16)×10-5. We constrain the direct CP asymmetry to be -0.033<[script A](B-->K*gamma)<0.028 and the isospin asymmetry to be 0.017<Delta0-<0.116, where the limits are determined by the 90% confidence interval and include both the statistical and systematic uncertainties.

Quark Structure of the Nucleon and Angular Asymmetry of Proton-Neutron Hard Elastic Scattering
Carlos G. Granados and Misak M. Sargsian
Published 17 November 2009
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We investigate an asymmetry in the angular distribution of hard elastic proton-neutron scattering with respect to the 90° center of mass scattering angle and demonstrate that it's magnitude is related to the helicity-isospin symmetry of the quark wave function of the nucleon. Our estimate of the asymmetry within the quark-interchange model of hard scattering demonstrates that the quark wave function of a nucleon based on the exact SU(6) symmetry predicts an angular asymmetry opposite to that of experimental observations. We found that the quark wave function based on the diquark picture of the nucleon produces a correct asymmetry. Comparison with the data allowed us to show that the vector diquarks contribute around 10% in the nucleon wave function and they are in negative phase relative to the scalar diquarks. These observations are essential in constraining QCD models of a nucleon.

Nuclear Physics

Global Calculation of Nuclear Shape Isomers
Peter Möller, Arnold J. Sierk, Ragnar Bengtsson, Hiroyuki Sagawa, and Takatoshi Ichikawa
Published 20 November 2009
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To determine which nuclei may exhibit shape isomerism, we use a well-benchmarked macroscopic-microscopic model to calculate potential-energy surfaces as functions of spheroidal (epsilon2), hexadecapole (epsilon4), and axial-asymmetry (gamma) shape coordinates for 7206 nuclei from A=31 to A=290. We analyze these and identify the deformations and energies of all minima deeper than 0.2 MeV. These minima may correspond to characteristic experimentally observable shape-isomeric states. Shape isomers mainly occur in the A=80 region, the A=100 region, and in an extended region centered around 208Pb. We compare our model to experimental results for Kr isotopes. Moreover, in a plot versus N and Z we show for each of the 7206 nuclei the calculated number of minima. The results reveal one fairly unexplored region of shape isomerism, which is experimentally accessible, namely the region northeast of 82<sup>208</sup>Pb.

Atomic, Molecular, and Optical Physics

Accurate Prediction of Nonadiabatic Transitions through Avoided Crossings
Volker Betz and Benjamin D. Goddard
Published 17 November 2009
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In the case of one nuclear degree of freedom, we derive an explicit formula for the nuclear wave function transmitted through an avoided crossing, and show that it agrees to high accuracy with precise numerical calculations.

Precision Lifetime Measurements of He- in a Cryogenic Electrostatic Ion-Beam Trap
P. Reinhed, A. Orbán, J. Werner, S. Rosén, R. D. Thomas, I. Kashperka, H. A. B. Johansson, D. Misra, L. Brännholm, M. Björkhage, H. Cederquist, and H. T. Schmidt
Published 18 November 2009
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We have developed a small purely electrostatic ion-beam trap which may be operated in thermal equilibrium at precisely controlled temperatures down to 10 K. Thus, we avoid magnetic field induced mixing of quantum states and may effectively eliminate any influence from absorption of photons from blackbody radiation. We report the first correction-free measurement of the lifetime of the 1s2s2p 4P5/2<sup>o</sup> level of 4He- yielding the high-precision result 359.0±0.7 µs. This result is an essential proof-of-principle for cryogenic electrostatic storage rings and traps for atomic and molecular physics.

Electron Localization in Molecular Fragmentation of H2 by Carrier-Envelope Phase Stabilized Laser Pulses
Manuel Kremer, Bettina Fischer, Bernold Feuerstein, Vitor L. B. de Jesus, Vandana Sharma, Christian Hofrichter, Artem Rudenko, Uwe Thumm, Claus Dieter Schröter, Robert Moshammer, and Joachim Ullrich
Published 20 November 2009
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Fully differential data for H2 dissociation in ultrashort (6 fs, 760 nm), linearly polarized, intense (0.44 PW/cm2) laser pulses with a stabilized carrier-envelope phase (CEP) were recorded with a reaction microscope. Depending on the CEP, the molecular orientation, and the kinetic energy release (KER), we find asymmetric proton emission at low KERs (0–3 eV), basically predicted by Roudnev and Esry, and much stronger than reported by Kling et al. Wave packet propagation calculations reproduce the salient features and discard, together with the observed KER-independent electron asymmetry, the first ionization step to be the reason for the asymmetric proton emission.

Complete Breakup of the Helium Atom by Proton and Antiproton Impact
Xiaoxu Guan and Klaus Bartschat
Published 17 November 2009
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We present a fully ab initio, nonperturbative, time-dependent approach to describe single and double ionization of helium by proton and antiproton impact. The problem is discretized by a flexible finite-element discrete-variable representation on the radial grid. Good agreement with the most recent experimental data for absolute angle-integrated cross sections is obtained for projectile energies between 3 keV and 6 MeV. Also, angle-differential cross sections for two-electron ejection are predicted for a proton impact energy of 6 MeV. The time evaluation of the ionization process is portrayed by displaying the electron density as a function of the projectile location.

Bandwidth-Tunable Single-Photon Source in an Ion-Trap Quantum Network
M. Almendros, J. Huwer, N. Piro, F. Rohde, C. Schuck, M. Hennrich, F. Dubin, and J. Eschner
Published 16 November 2009
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We report a tunable single-photon source based on a single trapped ion. Employing spontaneous Raman scattering and in-vacuum optics with large numerical aperture, single photons are efficiently created with controlled temporal shape and coherence time. These can be varied between 70 ns and 1.6 µs, as characterized by operating two sources simultaneously in two remote ion traps which reveals mutual and individual coherence through two-photon interference.

Spin Manipulation Using the Light-Shift Effect in Rubidium Atoms
T. Moriyasu, D. Nomoto, Y. Koyama, Y. Fukuda, and T. Kohmoto
Published 17 November 2009
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Optical manipulation of spin coherence in rubidium atoms is studied. The effect of off-resonant and circularly polarized light on optically induced magnetization is investigated. The change in precession frequency caused by the light-shift effect is verified. Absorption-free phase control of spin precession and pure spin rotation about an arbitrary axis are demonstrated. A theory of precession frequency shift that includes the effect of absorption is considered by using the density matrix and the experimental results are in agreement with the predictions of the theory. Thus, we show that it is possible to carry out off-resonant control of spin coherence and all-optical manipulation of spins.

Gently Modulating Optomechanical Systems
A. Mari and J. Eisert
Published 18 November 2009
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We introduce a framework of optomechanical systems that are driven with a mildly amplitude-modulated light field, but that are not subject to classical feedback or squeezed input light. We find that in such a system one can achieve large degrees of squeezing of a mechanical micromirror—signifying quantum properties of optomechanical systems—without the need of any feedback and control, and within parameters reasonable in experimental settings. Entanglement dynamics is shown of states following classical quasiperiodic orbits in their first moments. We discuss the complex time dependence of the modes of a cavity-light field and a mechanical mode in phase space. Such settings give rise to certifiable quantum properties within experimental conditions feasible with present technology.

Nonlinear Dynamics, Fluid Dynamics, Classical Optics, etc.
Phaseless Three-Dimensional Optical Nanoimaging
Alexander A. Govyadinov, George Y. Panasyuk, and John C. Schotland
Published 16 November 2009
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See Also: Phys. Rev. Focus
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We propose a method for optical nanoimaging in which the structure of a three-dimensional inhomogeneous medium may be recovered from far-field power measurements. Neither phase control of the illuminating field nor phase measurements of the scattered field are necessary. The method is based on the solution to the inverse scattering problem for a system consisting of a weakly-scattering dielectric sample and a strongly-scattering nanoparticle tip. Numerical simulations are used to illustrate the results.

Electric and Magnetic Dipole Coupling in Near-Infrared Split-Ring Metamaterial Arrays
Ivana Sersic, Martin Frimmer, Ewold Verhagen, and A. Femius Koenderink
Published 20 November 2009
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We present experimental observations of strong electric and magnetic interactions between split ring resonators (SRRs) in metamaterials. We fabricated near-infrared planar metamaterials with different inter-SRR spacings along different directions. Our transmission measurements show blueshifts and redshifts of the magnetic resonance, depending on SRR orientation relative to the lattice. The shifts agree well with simultaneous magnetic and electric near-field dipole coupling. We also find large broadening of the resonance, accompanied by a decrease in effective cross section per SRR with increasing density due to superradiant scattering. Our data shed new light on Lorentz-Lorenz approaches to metamaterials.

Finding Stationary Subspaces in Multivariate Time Series
Paul von Bünau, Frank C. Meinecke, Franz C. Király, and Klaus-Robert Müller
Published 20 November 2009
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Identifying temporally invariant components in complex multivariate time series is key to understanding the underlying dynamical system and predict its future behavior. In this Letter, we propose a novel technique, stationary subspace analysis (SSA), that decomposes a multivariate time series into its stationary and nonstationary part. The method is based on two assumptions: (a) the observed signals are linear superpositions of stationary and nonstationary sources; and (b) the nonstationarity is measurable in the first two moments. We characterize theoretical and practical properties of SSA and study it in simulations and cortical signals measured by electroencephalography. Here, SSA succeeds in finding stationary components that lead to a significantly improved prediction accuracy and meaningful topographic maps which contribute to a better understanding of the underlying nonstationary brain processes.

Flows around Confined Bubbles and Their Importance in Triggering Pinch-Off
Volkert van Steijn, Chris R. Kleijn, and Michiel T. Kreutzer
Published 19 November 2009
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We describe the breakup of a confined gas thread in a cross-flowing stream of liquid at capillary numbers Ca<10-2. The breakup is initiated, not by a Plateau-Rayleigh instability, but by liquid that flows from the tip of the thread to the neck where pinch-off occurs. This flow, faster than previously estimated, is driven by different curvatures at the tip and neck and runs through large gaps between thread and channel walls. Understanding how these curvatures evolve during bubble formation leads to accurate predictions of the moment of pinch-off.

Gravity-Capillary Lumps Generated by a Moving Pressure Source
James Diorio, Yeunwoo Cho, James H. Duncan, and T. R. Akylas
Published 20 November 2009
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The nonlinear wave pattern generated by a localized pressure source moving over a liquid free surface at speeds below the minimum phase speed (cmin) of linear gravity-capillary waves is investigated experimentally and theoretically. At these speeds, freely propagating fully localized solitary waves, or “lumps,” are known theoretically to be possible. For pressure-source speeds far below cmin, the surface response is a local depression similar to the case with no forward speed. As the speed is increased, a critical value is reached cc[approximate]0.9cmin where there is an abrupt transition to a wavelike state that features a steady disturbance similar to a steep lump behind the pressure forcing. As the speed approaches cmin, a second transition is found; the new state is unsteady and is characterized by continuous shedding of lumps from the tips of a V-shaped pattern.

Plasma and Beam Physics

First Direct Measurement of Optical Phonons in 2D Plasma Crystals
L. Couëdel, V. Nosenko, S. K. Zhdanov, A. V. Ivlev, H. M. Thomas, and G. E. Morfill
Published 17 November 2009
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Spectra of phonons with out-of-plane polarization were studied experimentally in a 2D plasma crystal. The dispersion relation was directly measured for the first time using a novel method of particle imaging. The out-of-plane mode was proven to have negative optical dispersion at small wave numbers, comparison with theory showed good agreement. The effect of the plasma wakes on the dispersion relation is briefly discussed.
Observation of a Helical Self-Organized State in a Compact Toroidal Plasma
C. D. Cothran, M. R. Brown, T. Gray, M. J. Schaffer, and G. Marklin
Published 16 November 2009
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A nonaxisymmetric stable magnetohydrodynamic (MHD) equilibrium within a prolate cylindrical conducting boundary has been produced experimentally. It has m=1 azimuthal symmetry, helical distortion, and flat lambda profile, all in agreement with the computed magnetically relaxed minimum magnetic energy Taylor state. Despite varied initial conditions determined by two helicity injectors on the device, this same equilibrium consistently emerges as the final state. These results therefore describe a new example of self-organization in an MHD plasma.

Characteristics of Light Reflected from a Dense Ionization Wave with a Tunable Velocity
A. Zhidkov, T. Esirkepov, T. Fujii, K. Nemoto, J. Koga, and S. V. Bulanov
Published 18 November 2009
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An optically dense ionization wave (IW) produced by two femtosecond (~10/30 fs) laser pulses focused cylindrically and crossing each other may become an efficient coherent x-ray converter in accordance with the Semenova-Lampe theory. The resulting velocity of a quasiplane IW in the vicinity of pulse intersection changes with the angle between the pulses from the group velocity of ionizing pulses to infinity allowing a tuning of the wavelength of x rays and their bunching. The x-ray spectra after scattering of a lower frequency and long coherent light pulse change from the monochromatic to high order harmoniclike with the duration of the ionizing pulses.

Laser-Driven Magnetic-Flux Compression in High-Energy-Density Plasmas
O. V. Gotchev, P. Y. Chang, J. P. Knauer, D. D. Meyerhofer, O. Polomarov, J. Frenje, C. K. Li, M. J.-E. Manuel, R. D. Petrasso, J. R. Rygg, F. H. Séguin, and R. Betti
Published 18 November 2009
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The demonstration of magnetic field compression to many tens of megagauss in cylindrical implosions of inertial confinement fusion targets is reported for the first time. The OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] was used to implode cylindrical CH targets filled with deuterium gas and seeded with a strong external field (>50 kG) from a specially developed magnetic pulse generator. This seed field was trapped (frozen) in the shock-heated gas fill and compressed by the imploding shell at a high implosion velocity, minimizing the effect of resistive flux diffusion. The magnetic fields in the compressed core were probed via proton deflectrometry using the fusion products from an imploding D3He target. Line-averaged magnetic fields between 30 and 40 MG were observed.

Generating Quasi-Single-Cycle Relativistic Laser Pulses by Laser-Foil Interaction
L. L. Ji (吉亮亮), B. F. Shen (沈百飞), X. M. Zhang (张晓梅), F. C. Wang (王凤超), Z. Y. Jin (金张英), C. Q. Xia (夏长权), M. Wen (温猛), W. P. Wang (王文鹏), J. C. Xu (徐建彩), and M. Y. Yu (郁明阳)
Published 19 November 2009
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A scheme for producing nearly single-cycle relativistic laser pulses is proposed. When a laser pulse interacts with an overdense thin foil, because of self-consistent nonlinear modulation, the latter will be more transparent to the more intense part of the laser, so that a transmitted pulse can be much shorter than the incident pulse. Using two-dimensional particle-in-cell simulation and analytical modeling, it is found that a transmitted pulse of duration 4 fs and peak intensity 3×1020 W/cm2 can be generated from a circularly polarized laser pulse. The intensity of the resulting pulse is only limited by that of the incident pulse, since this scheme involves only laser-plasma interaction.

Measurements of the Critical Power for Self-Injection of Electrons in a Laser Wakefield Accelerator
D. H. Froula, C. E. Clayton, T. Döppner, K. A. Marsh, C. P. J. Barty, L. Divol, R. A. Fonseca, S. H. Glenzer, C. Joshi, W. Lu, S. F. Martins, P. Michel, W. B. Mori, J. P. Palastro, B. B. Pollock, A. Pak, J. E. Ralph, J. S. Ross, C. W. Siders, L. O. Silva, and T. Wang
Published 19 November 2009
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A laser wakefield acceleration study has been performed in the matched, self-guided, blowout regime producing 720±50 MeV quasimonoenergetic electrons with a divergence DeltathetaFWHM of 2.85±0.15 mrad using a 10 J, 60 fs 0.8 µm laser. While maintaining a nearly constant plasma density (3×1018 cm-3), the energy gain increased from 75 to 720 MeV when the plasma length was increased from 3 to 8 mm. Absolute charge measurements indicate that self-injection of electrons occurs when the laser power P exceeds 3 times the critical power Pcr for relativistic self-focusing and saturates around 100 pC for P/Pcr>5. The results are compared with both analytical scalings and full 3D particle-in-cell simulations.

Condensed Matter: Structure, etc.

Multiparticle Composites in Density-Imbalanced Quantum Fluids
Evgeni Burovski, Giuliano Orso, and Thierry Jolicoeur
Published 16 November 2009
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We consider two-component one-dimensional quantum gases with a density imbalance. While generically such fluids are two-component Luttinger liquids, we show that if the ratio of the densities is a rational number, p/q, and mass asymmetry between components is sufficiently strong, one of the two eigenmodes acquires a gap. The gapped phase corresponds to (algebraic) ordering of (p+q)-particle composites. In particular, for attractive mixtures, this implies that the superconducting correlations are destroyed. We illustrate our predictions by numerical simulations of the fermionic Hubbard model with hopping asymmetry.

Bose-Einstein Condensation in Quantum Glasses
Giuseppe Carleo, Marco Tarzia, and Francesco Zamponi
Published 18 November 2009
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The role of geometrical frustration in strongly interacting bosonic systems is studied with a combined numerical and analytical approach. We demonstrate the existence of a novel quantum phase featuring both Bose-Einstein condensation and spin-glass behavior. The differences between such a phase and the otherwise insulating “Bose glasses” are elucidated.

Manifestation of Kohn Anomaly in 1/f Fluctuations in Metallic Carbon Nanotubes
Ju Hee Back, Cheng-Lin Tsai, Sunkook Kim, Saeed Mohammadi, and Moonsub Shim
Published 16 November 2009
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Low-frequency noise in metallic single walled carbon nanotubes is shown to be strongly dependent on the Fermi level position and the applied electric field across the nanotube. Resonance-like enhancement observed near optical phonon energy only when the Fermi level lies near the Dirac point is correlated to Raman G-band softening and broadening. The results suggest that the competition between zone-center and zone-boundary phonon scattering is the underlying origin of the large enhancement and resonance-like behavior of 1/f noise.

Self-Sustained Levitation of Dust Aggregate Ensembles by Temperature-Gradient-Induced Overpressures
Thorben Kelling and Gerhard Wurm
Published 20 November 2009
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In laboratory experiments we observe dust aggregates from 100 µm to 1 cm in size composed of micrometer-sized grains levitating over a hot surface. Depending on the dust sample aggregates start to levitate at a temperature of 400 K. Levitation of dust aggregates is restricted to a pressure range between 1–40 mbar. The levitating is caused by a Knudsen compressor effect. Based on thermal transpiration through the dust aggregates the pressure increases between surface and aggregates. Dust aggregates are typically balanced ~100 µm over the surface. On a slightly concave surface individual aggregates are trapped at the center. Ensembles of aggregates are confined in a 2D plane. Aggregates are subject to systematic and random translational and rotational motion. The levitated aggregates are well suited to study photophoretic or thermophoretic forces or the mutual interaction between dust aggregates.
Full Tunability of Strain along the fcc-bcc Bain Path in Epitaxial Films and Consequences for Magnetic Properties
J. Buschbeck, I. Opahle, M. Richter, U. K. Rößler, P. Klaer, M. Kallmayer, H. J. Elmers, G. Jakob, L. Schultz, and S. Fähler
Published 16 November 2009
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Strained coherent film growth is commonly either limited to ultrathin films or low strains. Here, we present an approach to achieve high strains in thicker films, by using materials with inherent structural instabilities. As an example, 50 nm thick epitaxial films of the Fe70Pd30 magnetic shape memory alloy are examined. Strained coherent growth on various substrates allows us to adjust the tetragonal distortion from c/abct=1.09 to 1.39, covering most of the Bain transformation path from fcc to bcc crystal structure. Magnetometry and x-ray circular dichroism measurements show that the Curie temperature, orbital magnetic moment, and magnetocrystalline anisotropy change over broad ranges.

Inaccuracy of Density Functional Theory Calculations for Dihydrogen Binding Energetics onto Ca Cation Centers
Janghwan Cha, Seokho Lim, Cheol Ho Choi, Moon-Hyun Cha, and Noejung Park
Published 19 November 2009
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We investigate the mechanism of dihydrogen adsorption onto Ca cation centers, which has been the significant focus of recent research for hydrogen storage. We particularly concentrate on reliability of commonly used density-functional theories, in comparison with correlated wave function theories. It is shown that, irrespective of the chosen exchange-correlation potentials, density-functional theories result in unphysical binding of H2 molecules onto Ca1+ system. This suggests that several previous publications could contain a serious overestimation of storage capacity at least in part of their results.

Condensed Matter: Electronic Properties, etc.

Spin Conservation and Fermi Liquid near a Ferromagnetic Quantum Critical Point
Andrey V. Chubukov and Dmitrii L. Maslov
Published 17 November 2009
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We propose a new low-energy theory for itinerant fermions near a ferromagnetic quantum critical point. We show that the full low-energy model includes, in addition to conventional interaction via spin fluctuations, another type of interaction, whose presence is crucial for the theory to satisfy SU(2) spin conservation. We demonstrate the consistency between a loopwise expansion and a Fermi liquid description for the full model. We further show that, prior to the ferromagnetic instability, the system develops a Pomeranchuk-type instability into a state with zero magnetization but with p-wave deformations of the Fermi surfaces of spin-up and spin-down electrons (a spin nematic).

Field-Induced Orbital Antiferromagnetism in Mott Insulators
K. A. Al-Hassanieh, C. D. Batista, G. Ortiz, and L. N. Bulaevskii
Published 19 November 2009
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We report on a new electromagnetic phenomenon that emerges in Mott insulators. The phenomenon manifests as antiferromagnetic ordering due to orbital electric currents which are spontaneously generated from the coupling between spin currents and an external homogenous magnetic field. This novel spin-charge-current effect provides the mechanism to measure the so-far elusive spin currents by means of unpolarized neutron scattering, nuclear magnetic resonance or muon spectroscopy. We illustrate this mechanism by solving a half-filled Hubbard model on a frustrated ladder.

Electric-Field-Enhanced Neutralization of Deep Centers in GaAs
D. G. Eshchenko, V. G. Storchak, S. P. Cottrell, and E. Morenzoni
Published 17 November 2009
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The charge dynamics of hydrogenlike centers in semi-insulating GaAs have been studied by muon spin resonance in the presence of electric field and RF excitation. Electric-field-enhanced neutralization of deep electron and hole traps by track-induced hot carriers results in an increase of the excess electron's or hole's lifetimes. Similar processes may take place in semiconductor devices working at high voltages and/or under irradiation. As a consequence of the deep traps neutralization, the muonium (µ++e-) center can capture a hole.

Diffusion and Ballistic Transport in One-Dimensional Quantum Systems
J. Sirker, R. G. Pereira, and I. Affleck
Published 19 November 2009
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It has been conjectured that transport in integrable one-dimensional systems is necessarily ballistic. The large diffusive response seen experimentally in nearly ideal realizations of the S=1/2 1D Heisenberg model is therefore puzzling and has not been explained so far. Here, we show that, contrary to common belief, diffusion is universally present in interacting 1D systems subject to a periodic lattice potential. We present a parameter-free formula for the spin-lattice relaxation rate which is in excellent agreement with experiment. Furthermore, we calculate the current decay directly in the thermodynamic limit using a time-dependent density matrix renormalization group algorithm and show that an anomalously large time scale exists even at high temperatures.

Field-Induced Kosterlitz-Thouless Transition in the N=0 Landau Level of Graphene
Kentaro Nomura, Shinsei Ryu, and Dung-Hai Lee
Published 20 November 2009
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At the charge neutral point, graphene exhibits a very unusual high-resistance metallic state and a transition to a complete insulating phase in a strong magnetic field. We propose that the current carriers in this state are the charged vortices of the XY valley-pseudospin order parameter, a situation which is dual to a conventional thin superconducting film. We study energetics and the stability of this phase in the presence of disorder.

Evidence for Two Competing Order Parameters in Underdoped Cuprate Superconductors from a Model Analysis of Fermi-Arc Effects
Andrés Greco
Published 17 November 2009
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Preformed pairs above Tc and the two-gap scenarios are two main proposals for describing the low-doping pseudogap phase of high-Tc cuprates. Very recent angle-resolved photoemission experiments have shown features which were interpreted as evidence for preformed pairs. Here it is shown that those results can be explained also in the context of the two-gap scenario if self-energy effects are considered. The discussion is based on the d charge-density wave theory or the flux phase of the t-J model.

Dissipative Enhancement of the Supercurrent in Tl2Ba2CaCu2O8 Intrinsic Josephson Junctions
P. A. Warburton, S. Saleem, J. C. Fenton, M. Korsah, and C. R. M. Grovenor
Published 17 November 2009
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We have measured dissipation-induced localization of the reaction coordinate for a metastable-state decay process in a model system with moderate damping. Specifically, the supercurrent in an array of Tl2Ba2CaCu2O8 intrinsic Josephson junctions is larger when all the junctions are in the zero-voltage state than when one or more junctions are in the voltage state since the dissipation is larger in the former case.

Local Current Injection into Mesoscopic Superconductors for the Manipulation of Quantum States
M. V. Milošević, A. Kanda, S. Hatsumi, F. M. Peeters, and Y. Ootuka
Published 19 November 2009
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We perform strategic current injection in a small mesoscopic superconductor and control the (non)equilibrium quantum states in an applied homogeneous magnetic field. In doing so, we realize a current-driven splitting of multiquanta vortices, current-induced transitions between states with different angular momenta, and current-controlled switching between otherwise degenerate quantum states. These fundamental phenomena form the basis for the electronic and logic applications discussed, and are confirmed in both theoretical simulations and multiple-small-tunnel-junction transport measurements.

Charging Effects in the Inductively Shunted Josephson Junction
Jens Koch, V. Manucharyan, M. H. Devoret, and L. I. Glazman
Published 19 November 2009
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The choice of impedance used to shunt a Josephson junction determines if the charge transferred through the circuit is quantized: a capacitive shunt renders the charge discrete, whereas an inductive shunt gives continuous charge. This discrepancy leads to a paradox in the limit of large inductances L. We show that while the energy spectra of the capacitively and inductively shunted junction are vastly different, their high-frequency responses become identical for large L. Inductive shunting thus opens the possibility to observe charging effects unimpeded by charge noise.

Model of Quasiparticles Coupled to a Frequency-Dependent Charge-Density-Wave Order Parameter in Cuprate Superconductors
G. Seibold, M. Grilli, and J. Lorenzana
Published 20 November 2009
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We investigate a model where superconducting electrons are coupled to a frequency dependent charge-density wave order parameter Deltar(omega). Our approach can reconcile the simultaneous existence of low-energy Bogoljubov quasiparticles and high energy electronic order as observed in scanning tunneling microscopy (STM) experiments. The theory accounts for the contrast reversal in the STM spectra between positive and negative bias observed above the pairing gap. An intrinsic relation between scattering rate and inhomogeneities follows naturally.
Magnetically Hindered Chain Formation in Transition-Metal Break Junctions
A. Thiess, Y. Mokrousov, S. Heinze, and S. Blügel
Published 16 November 2009
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Based on first-principles calculations, we demonstrate that magnetism impedes the formation of long chains in break junctions. We find a distinct softening of the binding energy of atomic chains due to the creation of magnetic moments that crucially reduces the probability of successful chain formation. Thereby, we are able to explain the long standing puzzle why most of the transition metals do not assemble as long chains in break junctions and thus provide indirect evidence that in general suspended atomic chains in transition-metal break junctions are magnetic.

Angular Momentum Conservation for Coherently Manipulated Spin Polarization in Photoexcited NiO: An Ab Initio Calculation
Georgios Lefkidis, G. P. Zhang, and W. Hübner
Published 16 November 2009
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In an ultrafast laser-induced magnetization-dynamics scenario we demonstrate for the first time an exact microscopic spin-switch mechanism. Combining ab initio electronic many-body theory and quantum optics analysis we show in detail how the coherently induced material polarization for every elementary process leads to angular-momentum exchange between the light and the irradiated antiferromagnetic NiO. Thus we answer the long-standing question where the angular momentum goes. The calculation also predicts a dynamic Kerr effect, which provides a signature for monitoring spin dynamics, by simply measuring the transient rotation and ellipticity of the reflected light.

Creating Polarization-Entangled Photon Pairs from a Semiconductor Quantum Dot Using the Optical Stark Effect
Andreas Muller, Wei Fang, John Lawall, and Glenn S. Solomon
Published 20 November 2009
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In typical epitaxial quantum dots (QDs) the ideally degenerate optical excitons are energy split, preventing the formation of two-photon entanglement in a biexciton decay. We use an external field, here a continuous-wave laser tuned to the QD in the ac Stark limit, to cancel the splitting and create two-photon entanglement. Quantum-state tomography is used to construct the two-photon density matrix. When the splitting is removed it satisfies well-known entanglement tests. Our approach shows that polarization-entangled photons can be routinely produced in semiconductor nanostructures.

Soft Matter, Biological, and Interdisciplinary Physics

Dynamic Length-Scale Characterization and Nonequilibrium Statistical Mechanics of Transport in Open-Cell Foams
Tyler R. Brosten, Sarah L. Codd, Robert S. Maier, and Joseph D. Seymour
Published 20 November 2009
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Nuclear magnetic resonance measurements of scale dependent dynamics in a random solid open-cell foam reveal a characteristic length scale for transport processes in this novel type of porous medium. These measurements and lattice Boltzmann simulations for a model foam structure indicate dynamical behavior analogous to lower porosity consolidated granular porous media, despite extremely high porosity in solid cellular foams. Scaling by the measured characteristic length collapses data for different foam structures as well as consolidated granular media. The nonequilibrium statistical mechanics theory of preasymptotic dispersion, developed for hierarchical porous media, is shown to model the hydrodynamic dispersive transport in a foam structure.

Dynamical Origin of the Effective Storage Capacity in the Brain's Working Memory
Christian Bick and Mikhail I. Rabinovich
Published 19 November 2009
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The capacity of working memory (WM), a short-term buffer for information in the brain, is limited. We suggest a model for sequential WM that is based upon winnerless competition amongst representations of available informational items. Analytical results for the underlying mathematical model relate WM capacity and relative lateral inhibition in the corresponding neural network. This implies an upper bound for WM capacity, which is, under reasonable neurobiological assumptions, close to the “magical number seven.”

Tetrahydrofuran Clathrate Hydrate Formation
Heiko Conrad, Felix Lehmkühler, Christian Sternemann, Arto Sakko, Dietmar Paschek, Laura Simonelli, Simo Huotari, Omid Feroughi, Metin Tolan, and Keijo Hämäläinen
Published 19 November 2009
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We report on the formation of tetrahydrofuran clathrate hydrate studied by x-ray Raman scattering measurements at the oxygen K edge. A comparison of x-ray Raman spectra measured from water-tetrahydrofuran mixtures and tetrahydrofuran hydrate at different temperatures supports stochastic hydrate formation models rather than models assuming hydrate precursors. This is confirmed by molecular dynamics simulations and density functional theory calculations of x-ray Raman spectra. In addition, changes in the spectra of tetrahydrofuran hydrate with temperatures close to the hydrate's dissociation temperature were observed and may be connected to changes in hydrate's local structure due to the formation of hydrogen bonds between guest and water molecules.
Zipf's Law in the Popularity Distribution of Chess Openings
Bernd Blasius and Ralf Tönjes
Published 16 November 2009
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See accompanying Viewpoint Physics 2, 97 (2009)
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We perform a quantitative analysis of extensive chess databases and show that the frequencies of opening moves are distributed according to a power law with an exponent that increases linearly with the game depth, whereas the pooled distribution of all opening weights follows Zipf's law with universal exponent. We propose a simple stochastic process that is able to capture the observed playing statistics and show that the Zipf law arises from the self-similar nature of the game tree of chess. Thus, in the case of hierarchical fragmentation the scaling is truly universal and independent of a particular generating mechanism. Our findings are of relevance in general processes with composite decisions.

COMMENTS

Comment on “Observation of a Push Force on the End Face of a Nanometer Silica Filament Exerted by Outgoing Light”
Iver Brevik
Published 18 November 2009
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A Comment on the Letter by Weilong She, Jianhui Yu, and Raohui Feng, [Phys. Rev. Lett. 101, 243601 (2008)]. The authors of the Letter offer a Reply.

She et al. Reply:
Weilong She, Jianhui Yu, and Raohui Feng
Published 18 November 2009
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A Reply to the Comment by Iver Brevik.

Comment on “Nonmonotonic Models are Not Necessary to Obtain Shear Banding Phenomena in Entangled Polymer Solutions”
Shi-Qing Wang
Published 18 November 2009
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A Comment on the Letter by J. M. Adams and P. D. Olmsted, Phys. Rev. Lett. 102, 067801 (2009). The authors of the Letter offer a Reply.

Adams and Olmsted Reply:
J. M. Adams and P. D. Olmsted
Published 18 November 2009
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A Reply to the Comment by Shi-Qing Wang.