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Physical Review Letters

3 August 2007

Volume 99, Number 5 , Articles (05xxxx)

Articles published 28 July - 3 August 2007


A diamond crystalline nucleus (red) embedded in liquid carbon (gray lines). Simulations show that the rate of diamond nucleation strongly depends on the local coordination of the liquid.

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LETTERS

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

Tests of Relativity by Complementary Rotating Michelson-Morley Experiments
Holger Müller, Paul Louis Stanwix, Michael Edmund Tobar, Eugene Ivanov, Peter Wolf, Sven Herrmann, Alexander Senger, Evgeny Kovalchuk, and Achim Peters
Published 30 July 2007
050401  Full Text: PDF (298 kB)  | Buy Article
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We report relativity tests based on data from two simultaneous Michelson-Morley experiments, spanning a period of more than 1 yr. Both were actively rotated on turntables. One (in Berlin, Germany) uses optical Fabry-Perot resonators made of fused silica; the other (in Perth, Australia) uses microwave whispering-gallery sapphire resonators. Within the standard model extension, we obtain simultaneous limits on Lorentz violation for electrons (5 coefficients) and photons (8) at levels down to 10-16, improved by factors between 3 and 50 compared to previous work.

Thermodynamical Limit of the Lipkin-Meshkov-Glick Model
Pedro Ribeiro, Julien Vidal, and Rémy Mosseri
Published 2 August 2007
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A method based on the analysis of the Majorana polynomial roots is introduced to compute the spectrum of the Lipkin-Meshkov-Glick model in the thermodynamical limit. A rich structure made of four qualitatively different regions is revealed in the parameter space, whereas the ground state study distinguishes between only two phases.

Quantum Glass Phases in the Disordered Bose-Hubbard Model
Pinaki Sengupta and Stephan Haas
Published 2 August 2007
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The phase diagram of the Bose-Hubbard model in the presence of off-diagonal disorder is determined using quantum Monte Carlo simulations. A sequence of quantum glass phases intervene at the interface between the Mott insulating and the superfluid phases of the clean system. In addition to the standard Bose glass phase, the coexistence of gapless and gapped regions close to the Mott insulating phase leads to a novel Mott glass regime which is incompressible yet gapless. Numerical evidence for the properties of these phases is given in terms of global (compressibility, superfluid stiffness) and local (compressibility, momentum distribution) observables.

Sideband Transitions and Two-Tone Spectroscopy of a Superconducting Qubit Strongly Coupled to an On-Chip Cavity
A. Wallraff, D. I. Schuster, A. Blais, J. M. Gambetta, J. Schreier, L. Frunzio, M. H. Devoret, S. M. Girvin, and R. J. Schoelkopf
Published 30 July 2007
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Sideband transitions are spectroscopically probed in a system consisting of a Cooper pair box strongly but nonresonantly coupled to a superconducting transmission line resonator. When the Cooper pair box is operated at the optimal charge bias point, the symmetry of the Hamiltonian requires a two-photon process to access sidebands. The observed large dispersive ac-Stark shifts in the sideband transitions induced by the strong nonresonant drives agree well with our theoretical predictions. Sideband transitions are important in realizing qubit-photon and qubit-qubit entanglement in the circuit quantum electrodynamics architecture for quantum information processing.

Phase Transitions from Saddles of the Potential Energy Landscape
Michael Kastner, Steffen Schreiber, and Oliver Schnetz
Published 30 July 2007
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The relation between saddle points of the potential of a classical many-particle system and the analyticity properties of its thermodynamic functions is studied. For finite systems, each saddle point is found to cause a nonanalyticity in the Boltzmann entropy, and the functional form of this nonanalytic term is derived. For large systems, the order of the nonanalytic term increases unboundedly, leading to an increasing differentiability of the entropy. Analyzing the contribution of the saddle points to the density of states in the thermodynamic limit, our results provide an explanation of how, and under which circumstances, saddle points of the potential energy landscape may (or may not) be at the origin of a phase transition in the thermodynamic limit. As an application, the puzzling observations by Risau-Gusman et al. [Phys. Rev. Lett. 95, 145702 (2005)] on topological signatures of the spherical model are elucidated.

Are Bosonic Replicas Faulty?
Vladimir Al. Osipov and Eugene Kanzieper
Published 31 July 2007
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Motivated by the ongoing discussion about a seeming asymmetry in the performance of fermionic and bosonic replicas, we present an exact, nonperturbative approach to both fermionic and bosonic zero-dimensional replica field theories belonging to the broadly interpreted beta=2 Dyson symmetry class. We then utilize the formalism developed to demonstrate that the bosonic replicas do correctly reproduce the microscopic spectral density in the QCD-inspired chiral Gaussian unitary ensemble. This disproves the myth that the bosonic replica field theories are intrinsically faulty.

Eigenvalue Statistics of the Real Ginibre Ensemble
Peter J. Forrester and Taro Nagao
Published 3 August 2007
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The real Ginibre ensemble consists of random N×N matrices formed from independent and identically distributed standard Gaussian entries. By using the method of skew orthogonal polynomials, the general n-point correlations for the real eigenvalues, and for the complex eigenvalues, are given as n×n Pfaffians with explicit entries. A computationally tractable formula for the cumulative probability density of the largest real eigenvalue is presented. This is relevant to May's stability analysis of biological webs.

Elementary Particles and Fields

Search for the Rare Decay B-->pil+l-
B. Aubert et al. (BABAR Collaboration)
Published 30 July 2007
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We have performed a search for the flavor-changing neutral-current decays B-->pi[script-l]+[script-l]-, where [script-l]+[script-l]- is either e+e- or µ+µ-, using a sample of 230×106 Upsilon(4S)-->B[overline B] decays collected with the BABAR detector. We observe no evidence of a signal and measure the upper limit on the isospin-averaged branching fraction to be [script B](B-->pi[script-l]+[script-l]-)<9.1×10-8 at 90% confidence level. We also search for the lepton-flavor-violating decays B-->pie±µ-/+ and measure an upper limit on the isospin-averaged branching fraction of [script B](B-->pie±µ-/+)<9.2×10-8 at 90% confidence level.

Gauge Coupling Unification and Light Exotica in String Theory
Stuart Raby and Akin Wingerter
Published 1 August 2007
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In this Letter we consider the consequences for the CERN Large Hadron Collider of light vectorlike exotica with fractional electric charge. It is shown that such states are found in orbifold constructions of the heterotic string. Moreover, these exotica are consistent with gauge coupling unification at one loop, even though they do not come in complete multiplets of SU(5).

Collider Signals of Unparticle Physics
Kingman Cheung, Wai-Yee Keung, and Tzu-Chiang Yuan
Published 1 August 2007
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Phenomenology of the notion of an unparticle [script U], recently perceived by Georgi, to describe a scale invariant sector with a nontrivial infrared fixed point at a higher energy scale is explored in details. Behaving like a collection of d[script U] (the scale dimension of the unparticle operator [script O][script U]) invisible massless particles, this unparticle can be unveiled by measurements of various energy distributions for the processes Z-->f[overline f][script U] and e-e+-->gamma[script U] at e-e+ colliders, as well as monojet production at hadron colliders. We also study the propagator effects of the unparticle through the Drell-Yan tree-level process and the one-loop muon anomaly.

Measurements of the Decay KL-->e+e-gamma
E. Abouzaid et al.
Published 3 August 2007
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The E799-II (KTeV) experiment at Fermilab has collected 83 262 KL-->e+e-gamma(gamma) events above a background of 79 events. We measure a decay width, normalized to the KL-->pi0pi0piD<sup>0</sup> (pi0-->gammagamma, pi0-->gammagamma, piD<sup>0</sup>-->e+e-gamma(gamma)) decay width, of Gamma(KL-->e+e-gamma(gamma))/Gamma(KL-->pi0pi0piD<sup>0</sup>)=(1.3302±0.0046stat±0.0102syst)×10-3. We also measure parameters of two KLgamma*gamma form factor models. In the Bergström-Massó-Singer parametrization, we find CalphaK*=-0.517±0.030stat±0.022syst. We separately fit for the first parameter of the D'Ambrosio-Isidori-Portolés model and find alphaDIP=-1.729±0.043stat±0.028syst.
Direct Observation of the Strange b Baryon Xi<sub>b</sub><sup>-</sup>
V. M. Abazov et al. (D0 Collaboration)
Published 3 August 2007
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We report the first direct observation of the strange b baryon Xib<sup>-</sup>([overline Xi ]b<sup>+</sup>). We reconstruct the decay Xib<sup>-</sup>-->J/psiXi-, with J/psi-->µ+µ-, and Xi--->Lambdapi--->ppi-pi- in p[overline p] collisions at sqrt(s)=1.96 TeV. Using 1.3 fb-1 of data collected by the D0 detector, we observe 15.2±4.4(stat)-0.4<sup>+1.9</sup>(syst) Xib<sup>-</sup> candidates at a mass of 5.774±0.011(stat)±0.015(syst) GeV. The significance of the observed signal is 5.5sigma, equivalent to a probability of 3.3×10-8 of it arising from a background fluctuation. Normalizing to the decay Lambdab-->J/psiLambda, we measure the relative rate ((sigma(Xi[sub b][sup -]) x [script B](Xi[sub b][sup -] --> J/psi Xi[sup -]))/(sigma(Lambda[sub b]) x [script B](Lambda[sub b] --> J/psi Lambda)))=0.28±0.09(stat)-0.08<sup>+0.09</sup>(syst).

Observation and Mass Measurement of the Baryon Xi<sub>b</sub><sup>-</sup>
T. Aaltonen et al. (CDF Collaboration)
Published 3 August 2007
052002  Full Text: PDF (177 kB)  | Buy Article
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We report the observation and measurement of the mass of the bottom, strange baryon Xib<sup>-</sup> through the decay chain Xib<sup>-</sup>-->J/psiXi-, where J/psi-->µ+µ-, Xi--->Lambdapi-, and Lambda-->ppi-. A signal is observed whose probability of arising from a background fluctuation is 6.6×10-15, or 7.7 Gaussian standard deviations. The Xib<sup>-</sup> mass is measured to be 5792.9±2.5(stat) ±1.7(syst) MeV/c2.

Nuclear Physics

Elliptic Flow for phi Mesons and (Anti)deuterons in Au+Au Collisions at sqrt(s[sub NN])=200 GeV
S. Afanasiev et al. (PHENIX Collaboration)
Published 31 July 2007
052301  Full Text: PDF (305 kB)  | Buy Article
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Differential elliptic flow (v2) for phi mesons and (anti)deuterons ([overline d])d is measured for Au+Au collisions at sqrt(s[sub NN])=200 GeV. The v2 for phi mesons follows the trend of lighter pi± and K± mesons, suggesting that ordinary hadrons interacting with standard hadronic cross sections are not the primary driver for elliptic flow development. The v2 values for ([overline d])d suggest that elliptic flow is additive for composite particles. This further validation of the universal scaling of v2 per constituent quark for baryons and mesons suggests that partonic collectivity dominates the transverse expansion dynamics.

High Resolution Spectroscopy of <sub>Lambda</sub><sup>12</sup>B by Electroproduction
M. Iodice et al. (Jefferson Lab Hall A Collaboration)
Published 1 August 2007
052501  Full Text: PDF (221 kB)  | Buy Article
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An experiment measuring electroproduction of hypernuclei has been performed in hall A at Jefferson Lab on a 12C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a ring imaging Cherenkov detector were added to the hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed Lambda<sup>12</sup>B spectrum shows for the first time identifiable strength in the core-excited region between the ground-state s-wave Lambda peak and the 11 MeV p-wave Lambda peak.

New Determination of the Astrophysical S Factor SE1 of the 12C(alpha,gamma)16O Reaction
X. D. Tang, K. E. Rehm, I. Ahmad, C. R. Brune, A. Champagne, J. P. Greene, A. A. Hecht, D. Henderson, R. V. F. Janssens, C. L. Jiang, L. Jisonna, D. Kahl, E. F. Moore, M. Notani, R. C. Pardo, N. Patel, M. Paul, G. Savard, J. P. Schiffer, R. E. Segel, S. Sinha, B. Shumard, and A. H. Wuosmaa
Published 3 August 2007
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A new measurement of the beta-delayed alpha decay of 16N has been performed using a set of high efficiency ionization chambers. Sources were made by implantation of a 16N beam, yielding very clean alpha spectra down to energies as low as 400 keV. Our data are in good agreement with earlier results. For the S factor SE1, we obtain a value of 74±21 keV b. In spite of improvements in the measurement, the error in SE1 remains relatively large because of the correlations among the fit parameters and the uncertainties inherent to the extrapolation.

Atomic, Molecular, and Optical Physics

Semiclassical Dynamics of Electron Transfer at Metal Surfaces
Hongzhi Cheng, Neil Shenvi, and John C. Tully
Published 3 August 2007
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See Also: Publisher's Note
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In this Letter, we demonstrate that nonadiabatic dynamics of molecular scattering from metal surfaces can be efficiently simulated by semiclassical Gaussian wave packet propagation on a local complex potential. The method relies on the wideband limit decoupling of the nuclear equations of motion on different electronic states. If the continuum diabatic potential surfaces are assumed to be parallel, the number of Gaussian wave packets spawned scales at most linearly with propagation time, allowing efficient propagation of nuclear dynamics.

Tailoring Optical Nonlinearities via the Purcell Effect
Peter Bermel, Alejandro Rodriguez, John D. Joannopoulos, and Marin Soljačić
Published 3 August 2007
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We predict that the effective nonlinear optical susceptibility can be tailored using the Purcell effect. While this is a general physical principle that applies to a wide variety of nonlinearities, we specifically investigate the Kerr nonlinearity. We show theoretically that using the Purcell effect for frequencies close to an atomic resonance can substantially influence the resultant Kerr nonlinearity for light of all (even highly detuned) frequencies. For example, in realistic physical systems, enhancement of the Kerr coefficient by one to two orders of magnitude could be achieved.

Bootstrapping Approach for Generating Maximally Path-Entangled Photon States
Kishore T. Kapale and Jonathan P. Dowling
Published 3 August 2007
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We propose a bootstrapping approach to the generation of maximally path-entangled states of photons, so-called “NOON states.” The strong atom-light interaction of cavity QED can be employed to generate NOON states with about 100 photons. These can then be used to boost the existing experimental Kerr nonlinearities based on quantum coherence effects, to facilitate NOON generation with an arbitrarily large number of photons. We also offer an alternative scheme that uses an atom-cavity dispersive interaction to obtain a sufficiently high Kerr nonlinearity necessary for arbitrary NOON generation.

Nonlinear Dynamics, Fluid Dynamics, Classical Optics, etc.

Fluctuational Electrodynamics in the Presence of Finite Thermal Sources
Vassilios Yannopapas and Nikolay V. Vitanov
Published 30 July 2007
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We present exact calculations of the spatial correlation of the blackbody radiation in the presence of spheres whose dimensions are smaller or comparable to the radiation wavelength. By going beyond the standard scalar coherence theory, we show that the spatial correlation function of a spherical thermal source is not universal but depends on the material properties of the source and exhibits near-field-induced features. Near-field effects are also manifested in the case of a linear chain of dielectric spheres where the correlation function probes the inhomogeneity of the chain. For this latter system we have established the conditions when the near-field effects cancel out and the correlation function takes the typical form of a conventional Lambertian source. For the case of a chain of metallic nanospheres, the increased spatial correlation of the far field leads to a directional thermal emission spectrum.

Grating-Assisted Phase Matching in Extreme Nonlinear Optics
Oren Cohen, Xiaoshi Zhang, Amy L. Lytle, Tenio Popmintchev, Margaret M. Murnane, and Henry C. Kapteyn
Published 31 July 2007
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We propose a new technique for phase matching high harmonic generation that can be used for generating bright, tabletop, tunable, and coherent x-ray sources at keV photon energies. A weak quasi-cw counterpropagating field induces a sinusoidal modulation in the phase of the emitted harmonics that can be used for correcting the large plasma-induced phase mismatch. We develop an analytical model that describes this grating-assisted x-ray phase matching and predicts that very modest intensities (<1010 W/cm2) of quasi-cw counterpropagating fields are required for implementation.

Guiding, Focusing, and Sensing on the Subwavelength Scale Using Metallic Wire Arrays
G. Shvets, S. Trendafilov, J. B. Pendry, and A. Sarychev
Published 2 August 2007
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We show that tapered arrays of thin metallic wires can manipulate electromagnetic fields on the subwavelength spatial scale. Two types of nanoscale imaging applications using terahertz and midinfrared waves are enabled: image magnification and radiation focusing. First, the tapered wire array acts as a multipixel TEM endoscope by capturing an electromagnetic field profile created by deeply subwavelength objects at the endoscope's tip and magnifying it for observation. Second, the image of a large mask at the endoscope's base is projected onto a much smaller image at the tip.

Dramatic Enhancement of High-Order Harmonic Generation
Eiji J. Takahashi, Tsuneto Kanai, Kenichi L. Ishikawa, Yasuo Nabekawa, and Katsumi Midorikawa
Published 2 August 2007
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We present a dramatic enhancement [Phys. Rev. Lett. 91, 043002 (2003)] of high-order harmonic generation by simultaneous irradiation of booster harmonics. A key feature of our experiment is the use of mixed gases (Xe and He) with different ionization energies. The harmonics from Xe atoms act as a booster to increase the harmonic yield from He by a factor of 4×103. The dominance of the dramatic enhancement effect is supported by simulation with the time-dependent Schrödinger equation as well as the observed spatial characteristic of the generated harmonics and dependence on medium conditions.

Changing Dynamical Complexity with Time Delay in Coupled Fiber Laser Oscillators
Anthony L. Franz, Rajarshi Roy, Leah B. Shaw, and Ira B. Schwartz
Published 2 August 2007
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We investigate the complexity of the dynamics of two mutually coupled systems with internal delays and vary the coupling delay over 4 orders of magnitude. Karhunen-Loève decomposition of spatiotemporal representations of fiber laser intensity data is performed to examine the eigenvalue spectrum and significant orthogonal modes. We compute the Shannon information from the eigenvalue spectra to quantify the dynamical complexity. A reduction in complexity occurs for short coupling delays while a logarithmic growth is observed as the coupling delay is increased.

Tailoring Self-Assembled Metallic Photonic Crystals for Modified Thermal Emission
S. E. Han, Andreas Stein, and D. J. Norris
Published 2 August 2007
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We predict that modified thermal emission can result from three-dimensional, self-assembled photonic crystals. In previous tungsten structures, known as inverse opals, strong absorption prevented any influence of the periodicity. We consider the origin of this effect and show how to tailor both absorption and surface coupling in experimentally realizable metallic inverse opals. Calculations for tungsten, molybdenum, and tantalum crystals show that their optical properties can be similar to or even better than the tungsten woodpile, where modified thermal emission has already been seen.

Maximum-Exponent Scaling Behavior of Optical Second-Harmonic Generation in Finite Multilayer Photonic Crystals
Marco Liscidini, Andrea Locatelli, Lucio Claudio Andreani, and Costantino De Angelis
Published 3 August 2007
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Scaling laws of second-harmonic generation (SHG) in nonlinear Bragg stacks (or finite one-dimensional photonic crystals) as a function of the number N of periods are explored. While it is known that SHG scales like the sixth power of N when phase matching is achieved, we find maximal scaling like the eighth power of N under appropriate non-phase-matching conditions with the pump and harmonic waves being resonant with band-edge states. In this framework we introduce the concept of self-adaptive coherence length that scales with the system length. An analytical treatment based on coupled-mode equations clarifies the conditions for obtaining different scaling laws as a function of filling factor in the photonic gap map.

Slow Dynamics in a Turbulent von Kármán Swirling Flow
A. de la Torre and J. Burguete
Published 2 August 2007
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An experimental study of a turbulent von Kármán flow in a cylinder is presented. The mean flow is stationary up to a Reynolds number Re=104 where a bifurcation takes place. The new regime breaks some symmetries of the problem and becomes time dependent because of equatorial vortices moving with a precession movement. In the exact counterrotating case, a bistable regime appears and spontaneous reversals of the azimuthal velocity are registered. A three-well potential model with additive noise reproduces this dynamic. A regime of periodic response is observed when a very weak forcing is applied.

Capillary Rise in Nanopores: Molecular Dynamics Evidence for the Lucas-Washburn Equation
D. I. Dimitrov, A. Milchev, and K. Binder
Published 31 July 2007
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When a capillary is inserted into a liquid, the liquid will rapidly flow into it. This phenomenon, well studied and understood on the macroscale, is investigated by molecular dynamics simulations for coarse-grained models of nanotubes. Both a simple Lennard-Jones fluid and a model for a polymer melt are considered. In both cases after a transient period (of a few nanoseconds) the meniscus rises according to a (time)1/2 law. For the polymer melt, however, we find that the capillary flow exhibits a slip length delta, comparable in size with the nanotube radius R. We show that a consistent description of the imbibition process in nanotubes is only possible upon modification of the Lucas-Washburn law which takes explicitly into account the slip length delta. We also demonstrate that the velocity field of the rising fluid close to the interface is not a simple diffusive spreading.

Intrinsic Dispersivity of Randomly Packed Monodisperse Spheres
U. M. Scheven, R. Harris, and M. L. Johns
Published 1 August 2007
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We determine the intrinsic longitudinal dispersivity ld of randomly packed monodisperse spheres by separating the intrinsic stochastic dispersivity ld from dispersion by unavoidable sample dependent flow heterogeneities. The measured ld, scaled by the hydrodynamic radius rh, coincide with theoretical predictions [Saffman, J. Fluid Mech. 7, 194 (1960)] for dispersion in an isotropic random network of identical capillaries of length l and radius a, for l/a=3.82, and with rescaled simulation results [Maier et al., Phys. Fluids 12, 2065 (2000)].

Plasma and Beam Physics

Observation of Megagauss-Field Topology Changes due to Magnetic Reconnection in Laser-Produced Plasmas
C. K. Li, F. H. Séguin, J. A. Frenje, J. R. Rygg, R. D. Petrasso, R. P. J. Town, O. L. Landen, J. P. Knauer, and V. A. Smalyuk
Published 2 August 2007
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The spatial structure and temporal evolution of megagauss magnetic fields generated by interactions of up to 4 laser beams with matter were studied with an innovative, time-gated proton radiography method that produces images of unprecedented clarity because it uses an isotropic, truly monoenergetic backlighter (14.7-MeV protons from D3He nuclear fusion reactions). Quantitative field maps reveal precisely and directly, for the first time, changes in the magnetic topology due to reconnection in a high-energy-density plasma (ne~1020–1022 cm-3, Te~1 keV).

Geodesic Plasma Compressibility Effects on the Magnetic Islands in a Tokamak
A. I. Smolyakov, X. Garbet, and M. Ottaviani
Published 2 August 2007
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It is shown that the stability of rotating magnetic islands in a tokamak plasma is affected by plasma compressibility related to the geodesic curvature in an inhomogeneous magnetic field. A robust contribution has been found to the Rutherford evolution equation. It is shown that the sign of the geodesic curvature contribution is opposite to the sign of the polarization term. It is suggested that this mechanism plays a crucial role in the stability of small scale magnetic islands.

Attraction of Positively Charged Particles in Highly Collisional Plasmas
S. A. Khrapak, G. E. Morfill, V. E. Fortov, L. G. D'yachkov, A. G. Khrapak, and O. F. Petrov
Published 2 August 2007
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It is shown that the electrostatic interaction potential between a pair of positively charged particles embedded in a highly collisional plasma has a long-range attractive asymptote. The effect is due to continuous plasma absorption on the particles. The relevance of this result to experimental investigations of complex (dusty) plasmas is discussed.

Nonambipolar Magnetic-Fluctuation-Induced Particle Transport and Plasma Flow in the MST Reversed-Field Pinch
W. X. Ding, D. L. Brower, D. Craig, B. H. Deng, S. C. Prager, J. S. Sarff, and V. Svidzinski
Published 3 August 2007
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First direct measurements of nonambipolar magnetic fluctuation-induced charge transport in the interior of a high-temperature plasma are reported. Global resistive tearing modes drive the charge transport which is measured in the vicinity of the resonant surface for the dominant core resonant mode. Finite charge transport has two important consequences. First, it generates a potential well along with locally strong electric field and electric field shear at the resonant surface. Second, this electric field induces a spontaneous E×B driven zonal flow.

Condensed Matter: Structure, etc.

Evidence for Out-of-Equilibrium Crystal Nucleation in Suspensions of Oppositely Charged Colloids
Eduardo Sanz, Chantal Valeriani, Daan Frenkel, and Marjolein Dijkstra
Published 1 August 2007
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We report a numerical study of the rate of crystal nucleation in a binary suspension of oppositely charged colloids. Two different crystal structures compete in the thermodynamic conditions under study. We find that the crystal phase that nucleates is metastable and, more surprisingly, its nucleation free-energy barrier is not the lowest one. This implies that, during nucleation, there is insufficient time for subcritical nuclei to relax to their lowest free-energy structure. Such behavior is in direct contradiction with the common assumption that the phase that crystallizes most readily is the one with the lowest free-energy barrier for nucleation. The phenomenon that we describe should be relevant for crystallization experiments where competing solid structures are not connected by an easy transformation.

Influence of Pressure on the Boson Peak: Stronger than Elastic Medium Transformation
K. Niss, B. Begen, B. Frick, J. Ollivier, A. Beraud, A. Sokolov, V. N. Novikov, and C. Alba-Simionesco
Published 3 August 2007
055502  Full Text: PDF (266 kB)  | Buy Article
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We study the changes in the low-frequency vibrational dynamics of poly(isobutylene) under pressure up to 1.4 GPa, corresponding to a density change of 20%. Combining inelastic neutron, x-ray, and Brillouin light scattering, we analyze the variations in the boson peak, transverse and longitudinal sound velocities, and the Debye level under pressure. We find that the boson peak variation under pressure cannot be explained by the elastic continuum transformation only. Surprisingly, the shape of the boson peak remains unchanged even at such high compression.

Dynamics of Step Bunching in Heteroepitaxial Growth on Vicinal Substrates
Mina Yoon, Ho Nyung Lee, Wei Hong, Hans M. Christen, Zhenyu Zhang, and Zhigang Suo
Published 3 August 2007
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When a heteroepitaxial film is grown on a vicinal substrate, the terrace steps at the growth front may bunch together to relieve strain, resulting in a rough surface. On the other hand, proper manipulation of the growth kinetics may suppress the inherent bunching instability, thus preserving step-flow growth. Here we show that the step dynamics in the early stages of growth can already determine whether the bunching instability is truly suppressed, prior to bunching actually taking place in the unstable regime. We determine the critical film thickness above which steps will bunch and exploit its scaling properties and usefulness for extracting intrinsic energy parameters. Experimental studies of SrRuO3 films grown on vicinal SrTiO3 substrates clearly establish the existence of the critical film thickness in step bunching.

Critical Behavior of the Chayes–Machta–Swendsen–Wang Dynamics
Youjin Deng, Timothy M. Garoni, Jonathan Machta, Giovanni Ossola, Marco Polin, and Alan D. Sokal
Published 31 July 2007
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We study the dynamic critical behavior of the Chayes-Machta dynamics for the Fortuin-Kasteleyn random-cluster model, which generalizes the Swendsen-Wang dynamics for the q-state Potts model to noninteger q, in two and three spatial dimensions, by Monte Carlo simulation. We show that the Li-Sokal bound z>=alpha/nu is close to but probably not sharp in d=2 and is far from sharp in d=3, for all q. The conjecture z>=beta/nu is false (for some values of q) in both d=2 and d=3.
Local Structure of Liquid Carbon Controls Diamond Nucleation
L. M. Ghiringhelli, C. Valeriani, E. J. Meijer, and D. Frenkel
Published 2 August 2007
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Diamonds melt at temperatures above 4000 K. There are no measurements of the steady-state rate of the reverse process, i.e., diamond nucleation from the melt, because experiments are difficult at these extreme temperatures and pressures. Using numerical simulations, we estimate the diamond nucleation rate and find that it increases by many orders of magnitude when the pressure is increased at constant supersaturation. The reason is that by increasing the pressure the local coordination of the liquid changes from threefold to fourfold, and we show that the free-energy cost to create a diamond-liquid interface is lower in the fourfold than in the threefold liquid. We speculate that this mechanism for nucleation control is relevant for crystallization in many network-forming liquids. We conclude that homogeneous diamond nucleation is likely in carbon-rich stars and unlikely in gaseous planets.

Evidence of Subsurface Oxygen Vacancy Ordering on Reduced CeO2(111)
Stefan Torbrügge, Michael Reichling, Atsushi Ishiyama, Seizo Morita, and Óscar Custance
Published 1 August 2007
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Surface and subsurface oxygen vacancies on the slightly reduced CeO2(111) surface have been studied by atomic resolution dynamic force microscopy at 80 K. Both types of defect are clearly identified by the comparison of the observed topographic features with the corresponding structures predicted from recent first-principles calculations. By combining two simultaneously acquired signals (the topography and the energy dissipated from the cantilever oscillation), we are able to unambiguously locate subsurface oxygen vacancies buried at the third surface atomic layer. We report evidence of local ordering of these subsurface defects that suggests the existence of a delicate balance between subtle interactions among adjacent subsurface oxygen vacancy structures.

Condensed Matter: Electronic Properties, etc.

Fermi Surface of CeIn3 above the Néel Critical Field
N. Harrison, S. E. Sebastian, C. H. Mielke, A. Paris, M. J. Gordon, C. A. Swenson, D. G. Rickel, M. D. Pacheco, P. F. Ruminer, J. B. Schillig, J. R. Sims, A. H. Lacerda, M.-T. Suzuki, H. Harima, and T. Ebihara
Published 31 July 2007
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We report measurements of the de Haas–van Alphen effect in CeIn3 in magnetic fields extending to [approximate]90 T, well above the Néel critical field of µ0Hc[approximate]61 T. The unreconstructed Fermi surface a sheet is observed in the high magnetic field polarized paramagnetic limit, but with its effective mass and Fermi surface volume strongly reduced in size compared to that observed in the low magnetic field paramagnetic regime under pressure. The spheroidal topology of this sheet provides an ideal realization of the transformation from a “large Fermi surface” accommodating f electrons to a “small Fermi surface” when the f-electron moments become polarized.

Superfluid to Mott-Insulator Transition in Bose-Hubbard Models
Manuela Capello, Federico Becca, Michele Fabrizio, and Sandro Sorella
Published 3 August 2007
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We study the superfluid-insulator transition in Bose-Hubbard models in one-, two-, and three-dimensional cubic lattices by means of a recently proposed variational wave function. In one dimension, the variational results agree with the expected Berezinskii-Kosterlitz-Thouless scenario of the interaction-driven Mott transition. In two and three dimensions, we find evidence that, across the transition, most of the spectral weight is concentrated at high energies, suggestive of preformed Mott-Hubbard sidebands. This result is compatible with the experimental data by Stoferle et al. [Phys. Rev. Lett. 92, 130403 (2004)].

Tunneling Anisotropic Magnetoresistance and Spin-Orbit Coupling in Fe/GaAs/Au Tunnel Junctions
J. Moser, A. Matos-Abiague, D. Schuh, W. Wegscheider, J. Fabian, and D. Weiss
Published 2 August 2007
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We report the observation of tunneling anisotropic magnetoresistance effect in the epitaxial metal-semiconductor system Fe/GaAs/Au. The observed twofold anisotropy of the resistance can be switched by reversing the bias voltage, suggesting that the effect originates from the interference of the spin-orbit coupling at the interfaces. Corresponding model calculations reproduce the experimental findings very well.

Efficient Atomic Self-Interaction Correction Scheme for Nonequilibrium Quantum Transport
C. Toher and S. Sanvito
Published 30 July 2007
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Density-functional theory calculations of electronic transport based on local exchange and correlation functionals contain self-interaction errors. As a consequence, insulating molecules in weak contact with metallic electrodes erroneously form highly conducting junctions. Here we present a fully self-consistent and still computationally undemanding self-interaction correction scheme that overcomes these limitations. The method is implemented in the transport code Smeagol and applied to the prototypical case of benzene molecules and gold electrodes. The Kohn-Sham highest occupied molecular orbital now reproduces closely the negative of the molecular ionization potential and is moved away from the gold Fermi energy. This leads to a drastic reduction of the low-bias current in much better agreement with experiments.

Transverse Field Effect in Graphene Ribbons
D. S. Novikov
Published 31 July 2007
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It is shown that a graphene ribbon, a ballistic strip of carbon monolayer, may serve as a quantum wire whose electronic properties can be continuously and reversibly controlled by an externally applied transverse voltage. The electron bands of armchair-edge ribbons undergo dramatic transformations: The Fermi surface fractures, Fermi velocity and effective mass change sign, and excitation gaps are reduced by the transverse field. These effects are manifest in the conductance plateaus, van Hove singularities, thermopower, and activated transport. The control over one-dimensional bands may help enhance effects of electron correlations, and be utilized in device applications.

Hall Resistivity of Granular Metals
Maxim Yu. Kharitonov and Konstantin B. Efetov
Published 1 August 2007
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We calculate the Hall conductivity sigmaxy and resistivity rhoxy of a granular system at large tunneling conductance gT>>1. We show that in the absence of Coulomb interaction the Hall resistivity depends neither on the tunneling conductance nor on the intragrain disorder and is given by the classical formula rhoxy=H/(n*ec), where n* differs from the carrier density n inside the grains by a numerical coefficient determined by the shape of the grains. The Coulomb interaction gives rise to logarithmic in temperature T correction to rhoxy in the range Gamma<~T<~min(gTEc,ETh), where Gamma is the tunneling escape rate, Ec is the charging energy, and ETh is the Thouless energy of the grain.

Dynamics of Quantum Dot Nuclear Spin Polarization Controlled by a Single Electron
P. Maletinsky, A. Badolato, and A. Imamoglu
Published 2 August 2007
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We present measurements of the buildup and decay of nuclear spin polarization in a single semiconductor quantum dot. Our experiment shows that we polarize the nuclei in a few milliseconds, while their decay dynamics depends drastically on external parameters. We show that a single electron can very efficiently depolarize nuclear spins in milliseconds whereas in the absence of the electron the nuclear spin lifetime is on the scale of seconds. This lifetime is further enhanced by 1–2 orders of magnitude by quenching the nonsecular nuclear dipole-dipole interactions with a magnetic field of 1 mT.

Specific-Heat Measurements of the Gap Structure of the Organic Superconductors kappa-(ET)2Cu[N(CN)2]Br and kappa-(ET)2Cu(NCS)2
O. J. Taylor, A. Carrington, and J. A. Schlueter
Published 30 July 2007
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We present high resolution heat capacity measurements of the organic superconductors kappa-(ET)2Cu[N(CN)2]Br and kappa-(ET)2Cu(NCS)2 in fields up to 14 T. We use the high field data to determine the normal state specific heat and hence extract the behavior of the electronic specific heat Cel in the superconducting state in zero and finite fields. We find that in both materials for T/Tc<~0.3, Cel(H=0)~T2 indicating d-wave superconductivity. The data are well described by a strong coupling d-wave model from our base temperature (T/Tc~0.1) right up to Tc. Our data help to resolve the controversy regarding the order parameter symmetry in these materials.

Radiation due to Josephson Oscillations in Layered Superconductors
L. N. Bulaevskii and A. E. Koshelev
Published 2 August 2007
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We derive the power of direct radiation into free space induced by Josephson oscillations in intrinsic Josephson junctions of layered superconductors. We consider the superradiation regime for a crystal cut in the form of a thin slice parallel to the c axis. We find that the radiation correction to the current-voltage characteristic in this regime depends only on crystal shape. We show that at a large number of junctions oscillations are synchronized providing high radiation power and efficiency in the terahertz frequency range. We discuss the crystal parameters and bias current optimal for radiation power and crystal cooling.

Supercurrent Pumping in Josephson Junctions with a Half-Metallic Ferromagnet
S. Takahashi, S. Hikino, M. Mori, J. Martinek, and S. Maekawa
Published 3 August 2007
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A Josephson current through a half-metallic ferromagnet between two conventional superconductors is theoretically studied. The spin dynamics such as magnon excitation plays a crucial role not only for the conversion between spin-singlet and spin-triplet pairs but also for the formation of the composite state of a triplet Cooper pair and magnon, by which the Josephson current flows between the superconductors. We propose the supercurrent pumping driven by the coherent precession of the magnetization by tuning the microwave frequency to the ferromagnetic resonance frequency in a ferromagnetic Josephson junction.

Hysteretic Behavior of Angular Dependence of Exchange Bias in FeNi/FeMn Bilayers
T. R. Gao, D. Z. Yang, S. M. Zhou, R. Chantrell, P. Asselin, J. Du, and X. S. Wu
Published 30 July 2007
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For FeNi/FeMn bilayers, the angular dependence of exchange bias shows hysteresis between clockwise and counterclockwise rotations, as a new signature. The hysteresis decreases for thick antiferromagnet layers. Calculations have clearly shown that the orientation of antiferromagnet spins also exhibits hysteresis between clockwise and counterclockwise rotations. This furnishes an interpretation of the macroscopic behavior of the ferromagnetic layer in terms of the thermally driven evolution of the magnetic state of the antiferromagnet layer.

Simulations of Ferrite-Dielectric-Wire Composite Negative Index Materials
Frederic J. Rachford, Douglas N. Armstead, Vincent G. Harris, and Carmine Vittoria
Published 30 July 2007
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We perform extensive finite difference time domain simulations of ferrite based negative index of refraction composites. A wire grid is employed to provide negative permittivity. The ferrite and wire grid interact to provide both negative and positive index of refraction transmission peaks in the vicinity of the ferrite resonance. Notwithstanding the extreme anisotropy in the index of refraction of the composite, negative refraction is seen at the composite air interface allowing the construction of a focusing concave lens with a magnetically tunable focal length.

Quantum Projection in an Ising Spin Liquid
D. M. Silevitch, C. M. S. Gannarelli, A. J. Fisher, G. Aeppli, and T. F. Rosenbaum
Published 30 July 2007
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A transverse magnetic field is used to scan the diagonal and off-diagonal susceptibility of the uniaxial quantum magnet, LiHo0.045Y0.955F4. Clusters of strongly coupled spins act as the primary source for the response functions, which result from a field-induced quantum projection of the system into a classically forbidden (meaning non-Ising) regime. Calculations based on spin pairs reproduce only some features of the data and fail to predict the measured off-diagonal response, providing evidence of a multispin collective state.

Effects of Two Energy Scales in Weakly Dimerized Antiferromagnetic Quantum Spin Chains
A. Brühl, B. Wolf, V. Pashchenko, M. Anton, C. Gross, W. Assmus, R. Valenti, S. Glocke, A. Klümper, T. Saha-Dasgupta, B. Rahaman, and M. Lang
Published 30 July 2007
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By means of thermal expansion and specific heat measurements on the high-pressure phase of (VO)2P2O7, the effects of two energy scales of the weakly dimerized antiferromagnetic S=1/2 Heisenberg chain are explored. The low-energy scale, given by the spin gap Delta, is found to manifest itself in a pronounced thermal expansion anomaly. A quantitative analysis, employing the density-matrix renormalization-group approach for transfer matrices calculations, shows that this feature originates from changes in the magnetic entropy with respect to Delta, [partial-derivative]Sm/[partial-derivative]Delta. This term, inaccessible by specific heat, is visible only in the weak-dimerization limit, where it reflects peculiarities of the excitation spectrum and its sensitivity to variations in Delta.

Universal Temperature Dependence of the Magnetization of Gapped Spin Chains
Yoshitaka Maeda, Chisa Hotta, and Masaki Oshikawa
Published 31 July 2007
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A Haldane chain under applied field is analyzed numerically, and a clear minimum of magnetization is observed as a function of temperature. We elucidate its origin using the effective theory near the critical field and propose a simple method to estimate the gap from the magnetization at finite temperatures. We also demonstrate that there exists a relation between the temperature dependence of the magnetization and the field dependence of the spin-wave velocity. Our arguments are universal for general axially symmetric one-dimensional spin systems.

Ultrametricity in the Edwards-Anderson Model
Pierluigi Contucci, Cristian Giardinà, Claudio Giberti, Giorgio Parisi, and Cecilia Vernia
Published 31 July 2007
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We test the property of ultrametricity for the spin-glass three-dimensional Edwards-Anderson model in zero magnetic field with numerical simulations up to 203 spins. We find an excellent agreement with the prediction of the mean field theory. Since ultrametricity is not compatible with a trivial structure of the overlap distribution, our result contradicts the droplet theory.

Large-Scale Monte Carlo Study of a Realistic Lattice Model for Ga1-xMnxAs
Yucel Yildirim, Gonzalo Alvarez, Adriana Moreo, and Elbio Dagotto
Published 1 August 2007
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Mn-doped GaAs is studied with a real-space Hamiltonian on an fcc lattice that reproduces the valence bands of undoped GaAs. Large-scale Monte Carlo (MC) simulations on a Cray XT3, using up to a thousand nodes, were needed. Spin-orbit interaction and the random distribution of the Mn ions are considered. The hopping amplitudes are functions of the GaAs Luttinger parameters. At the realistic coupling J~1.2 eV the MC Curie temperature and magnetization curves agree with experiments for x=8.5% annealed samples. Mn-doped GaSb and GaP are also briefly discussed.

Local Tunneling Spectroscopy across a Metamagnetic Critical Point in the Bilayer Ruthenate Sr3Ru2O7
K. Iwaya, S. Satow, T. Hanaguri, N. Shannon, Y. Yoshida, S. I. Ikeda, J. P. He, Y. Kaneko, Y. Tokura, T. Yamada, and H. Takagi
Published 2 August 2007
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The local spectroscopic signatures of metamagnetic criticality in Sr3Ru2O7 were explored using scanning tunneling microscopy (STM). Singular features in the tunneling spectrum were found close to the Fermi level, as would be expected in a Stoner picture of itinerant electron metamagnetism. These features showed a pronounced magnetic field dependence across the metamagnetic critical point, which cannot be understood in terms of a naive Stoner theory. In addition, a pseudogap structure was observed over several tens of meV, accompanied by a c(2×2) superstructure in STM images. This result represents a new electronic ordering at the surface in the absence of any measurable surface reconstruction.
Coexistence of Two Different Cr Ions by Self-Doping in Half-Metallic CrO2 Nanorods
Jeong Hyun Shim, Soonchil Lee, Joonghoe Dho, and Do-Hyung Kim
Published 2 August 2007
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We investigated the physical properties of Cr ions in half-metallic CrO2 using 53Cr nuclear magnetic resonance (NMR). The 53Cr NMR spectra showed two peaks with a similar intensity instead of the single peak anticipated from a uniform single valence Cr+4. Both Cr peaks exhibited a strong anisotropy in the hyperfine field and similar values in the enhancement factor and T2 relaxation time. These results suggest the coexistence of two different Cr ions (provisionally, Cr+4±1/3) by a self-doping effect, which is a likely origin of the metallic ferromagnetism in CrO2.

Transforming Nonlocality into a Frequency Dependence: A Shortcut to Spectroscopy
Matteo Gatti, Valerio Olevano, Lucia Reining, and Ilya V. Tokatly
Published 1 August 2007
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Measurable spectra are often derived from contractions of many-body Green's functions. One calculates hence more information than needed. Here we present and illustrate an in principle exact approach to construct effective potentials and kernels for the direct calculation of electronic spectra. In particular, a dynamical but local and real potential yields the spectral function needed to describe photoemission. We discuss for model solids the frequency dependence of this “photoemission potential” stemming from the nonlocality of the corresponding self-energy.

Chirping of an Optical Transition by an Ultrafast Acoustic Soliton Train in a Semiconductor Quantum Well
A. V. Scherbakov, P. J. S. van Capel, A. V. Akimov, J. I. Dijkhuis, D. R. Yakovlev, T. Berstermann, and M. Bayer
Published 1 August 2007
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Acoustic solitons formed during the propagation of a picosecond strain pulse in a GaAs crystal with a ZnSe/ZnMgSSe quantum well on top lead to exciton resonance energy shifts of up to 10 meV, and ultrafast frequency modulation, i.e., chirping, of the exciton transition. The effects are well described by a theoretical analysis based on the Korteweg–de Vries equation and accounting for the properties of the excitons in the quantum well.

Soft Matter, Biological, and Interdisciplinary Physics

Surface Triple Points and Multiple-Layer Transitions Observed by Tuning the Surface Field at Smectic Liquid-Crystal–Water Interfaces
Ch. Bahr
Published 31 July 2007
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We present an ellipsometric study of the interface between a smectic liquid crystal and water in the presence of a nonionic surfactant. The surfactant concentration serves as a handle to tune the surface field. For sufficiently large surfactant concentrations, a smectic phase is present at the interface in the temperature range above the smectic-A–isotropic bulk transition; when the bulk transition is approached, the thickness of this surface phase grows via a series of layer-by-layer transitions at which single smectic layers are formed. At lower surfactant concentrations, transitions appear at which the thickness of the surface phase jumps by multiple smectic layers, thereby implying the existence of triple points at which surface phases with different smectic layer numbers coexist. This is the first experimental demonstration of such surface triple points which are predicted by theoretical models.
Spontaneous Separation of Charged Grains
Amit Mehrotra, Fernando J. Muzzio, and Troy Shinbrot
Published 31 July 2007
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In 1867, Lord Kelvin described an experiment in which two streams of water droplets were connected so that each stream amplified the charge on the second stream [W. Thomson, Proc. R. Soc. London 16, 67 (1867).]. We present here a complementary effect in flowing grains that spontaneously separates similar and well-mixed grains into two charged streams of demixed grains. This effect has important consequences for industrial and natural processes.

Period-Doubling Bifurcation to Alternans in Paced Cardiac Tissue: Crossover from Smooth to Border-Collision Characteristics
Carolyn M. Berger, Xiaopeng Zhao, David G. Schaeffer, Hana M. Dobrovolny, Wanda Krassowska, and Daniel J. Gauthier
Published 30 July 2007
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We investigate, both experimentally and theoretically, the period-doubling bifurcation to alternans in heart tissue. Previously, this phenomenon has been modeled with either smooth or border-collision dynamics. Using a modification of existing experimental techniques, we find a hybrid behavior: Very close to the bifurcation point, the dynamics is smooth, whereas further away it is border-collision-like. The essence of this behavior is captured by a model that exhibits what we call an unfolded border-collision bifurcation. This new model elucidates that, in an experiment, where only a limited number of data points can be measured, the smooth behavior of the bifurcation can easily be missed.

Orientational Order and Instabilities in Suspensions of Self-Locomoting Rods
David Saintillan and Michael J. Shelley
Published 30 July 2007
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The orientational order and dynamics in suspensions of self-locomoting slender rods are investigated numerically. In agreement with previous theoretical predictions, nematic suspensions of swimming particles are found to be unstable at long wavelengths as a result of hydrodynamic fluctuations. Nevertheless, a local nematic ordering is shown to persist over short length scales and to have a significant impact on the mean swimming speed. The consequences of the large-scale orientational disorder for particle dispersion are also discussed.

Branching, Capping, and Severing in Dynamic Actin Structures
Ajay Gopinathan, Kun-Chun Lee, J. M. Schwarz, and Andrea J. Liu
Published 2 August 2007
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Branched actin networks at the leading edge of a crawling cell evolve via protein-regulated processes such as polymerization, depolymerization, capping, branching, and severing. A formulation of these processes is presented and analyzed to study steady-state network morphology. In bulk, we identify several scaling regimes in severing and branching protein concentrations and find that the coupling between severing and branching is optimally exploited for conditions in vivo. Near the leading edge, we find qualitative agreement with the in vivo morphology.

Solvent Effects on Charge Spatial Extent in DNA and Implications for Transfer
Yves A. Mantz, Francesco Luigi Gervasio, Teodoro Laino, and Michele Parrinello
Published 2 August 2007
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To clarify the role played by water in facilitating long-range DNA charge transport, carefully designed, state-of-the-art, self-interaction corrected density-functional quantum mechanical and molecular mechanical (SIC-QM/MM) simulations are performed for the first time on two ionized adenine:thymine bridge models in explicit water solvent at finite temperature. For random solvent configurations, the charge is partially delocalized. However, a charge localization on different, well-separated adenines can be induced and is correlated with a restructuring of their first solvation shells. Thus, the importance of water in the mechanism of long-range charge transport is explicitly demonstrated, and the microscopic conditions for a charge localization are revealed.

Peeling and Sliding in Nucleosome Repositioning
Tom Chou
Published 3 August 2007
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We investigate the mechanisms of histone sliding and detachment with a stochastic model that couples thermally induced, passive histone sliding with active motor-driven histone unwrapping. Analysis of a passive loop or twist defect-mediated histone sliding mechanism shows that diffusional sliding is enhanced as larger portions of the DNA is peeled off the histone. The mean times to histone detachment and the mean distance traveled by the motor complex prior to histone detachment are computed as functions of the intrinsic speed of the motor. Fast motors preferentially induce detachment over sliding. However, for a fixed motor speed, increasing the histone-DNA affinity (and thereby decreasing the passive sliding rate) increases the mean distance traveled by the motor.

Ranking Knots of Random, Globular Polymer Rings
M. Baiesi, E. Orlandini, and A. L. Stella
Published 1 August 2007
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An analysis of extensive simulations of interacting self-avoiding polygons on cubic lattice shows that the frequencies of different knots realized in a random, collapsed polymer ring decrease as a negative power of the ranking order, and suggests that the total number of different knots realized grows exponentially with the chain length. Relative frequencies of specific knots converge to definite values because the free energy per monomer, and its leading finite size corrections, do not depend on the ring topology, while a subleading correction only depends on the crossing number of the knots.

Nanoconfinement-Enhanced Conformational Response of Single DNA Molecules to Changes in Ionic Environment
Walter Reisner, Jason P. Beech, Niels B. Larsen, Henrik Flyvbjerg, Anders Kristensen, and Jonas O. Tegenfeldt
Published 1 August 2007
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We show that the ionic environment plays a critical role in determining the configurational properties of DNA confined in silica nanochannels. The extension of DNA in the nanochannels increases as the ionic strength is reduced, almost tripling over two decades in ionic strength for channels around 100×100 nm in dimension. Surprisingly, we find that the variation of the persistence length alone with ionic strength is not enough to explain our results. The effect is due mainly to increasing self-avoidance created by the reduced screening of electrostatic interactions at low ionic strength. To quantify the increase in self-avoidance, we introduce a new parameter into the de Gennes theory: an effective DNA width that gives the increase in the excluded volume due to electrostatic repulsion.

Stretch-Coil Transition and Transport of Fibers in Cellular Flows
Y.-N. Young and Michael J. Shelley
Published 2 August 2007
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It is shown that a slender elastic fiber moving in a Stokesian fluid can be susceptible to a buckling instability—termed the “stretch-coil” instability—when moving in the neighborhood of a hyperbolic stagnation point of the flow. When the stagnation point is part of an extended cellular flow, it is found that immersed fibers can move as random walkers across time-independent closed-streamline flow. It is also found that the flow is segregated into transport regions around hyperbolic stagnation points and their manifolds, and closed entrapment regions around elliptic points.

Experimental Growth Law for Bubbles in a Moderately “Wet” 3D Liquid Foam
Jérôme Lambert, Isabelle Cantat, Renaud Delannay, Rajmund Mokso, Peter Cloetens, James A. Glazier, and François Graner
Published 2 August 2007
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We used x-ray tomography to characterize the geometry of all bubbles in a liquid foam of average liquid fraction phil[approximate]17% and to follow their evolution, measuring the normalized growth rate [script G]=V-1/3(dV/dt) for 7000 bubbles. While [script G] does not depend only on the number of faces of a bubble, its average over f-faced bubbles scales as Gf~f-f0 for large f's at all times. We discuss the dispersion of [script G] and the influence of V and phil on [script G].

COMMENTS

Comment on “Electron Core-Hole Interaction and Its Induced Ionic Structural Relaxation in Molecular Systems under X-Ray Irradiation”
F. Schreiber, A. Gerlach, N. Koch, E. Zojer, M. Sokolowski, F. S. Tautz, M. Rohlfing, and E. Umbach
Published 1 August 2007
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A Comment on the Letter by Wei Ji, Zhong-Yi Lu, and Hongjun Gao, Phys. Rev. Lett. 97, 246101 (2006). The authors of the Letter offer a Reply.

Ji, Lu, and Gao Reply:
Wei Ji, Zhong-Yi Lu, and Hongjun Gao
Published 1 August 2007
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A Reply to the Comment by F. Schreiber et al.