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Physical Review Letters

24 March 2006

Volume 96, Number 11 , Articles (11xxxx)

Articles published 18 March - 24 March 2006


PVLAS collaboration data providing evidence for light polarization rotation in vacuum with a transverse magnetic field. The signal (arrow) is at a frequency shift twice that of a rotating magnet. A light neutral spin-zero particle could cause such polarization rotation, though there are strong astrophysical constraints.

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LETTERS

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

Quantum Atom Optics with Fermions from Molecular Dissociation
K. V. Kheruntsyan
Published 20 March 2006
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We study a fermionic atom optics counterpart of parametric down-conversion with photons. This can be realized through dissociation of a Bose-Einstein condensate of molecular dimers consisting of fermionic atoms. We present a theoretical model describing the quantum dynamics of dissociation and find analytic solutions for mode occupancies and atomic pair correlations, valid in the short time limit. The solutions are used to identify upper bounds for the correlation functions, which are applicable to any fermionic system and correspond to ideal particle number-difference squeezing.

Multimode Uncertainty Relations and Separability of Continuous Variable States
Alessio Serafini
Published 21 March 2006
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A multimode uncertainty relation (generalizing the Robertson-Schrödinger relation) is derived as a necessary constraint on the second moments of n pairs of canonical operators. In turn, necessary conditions for the separability of multimode continuous variable states under (m+n)-mode bipartitions are derived from the uncertainty relation. These conditions are proven to be necessary and sufficient for (1+n)-mode Gaussian states and for (m+n)-mode bisymmetric Gaussian states.

Strongly Interacting Fermi Gases with Density Imbalance
J. Kinnunen, L. M. Jensen, and P. Törmä
Published 22 March 2006
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We consider density-imbalanced Fermi gases of atoms in the strongly interacting, i.e., unitarity, regime. The Bogoliubov–de Gennes equations for a trapped superfluid are solved. They take into account the finite size of the system, as well as give rise to both phase separation and Fulde-Ferrel-Larkin-Ovchinnikov-type oscillations in the order parameter. We show how radio-frequency spectroscopy reflects the phase separation, and can provide direct evidence of the FFLO-type oscillations via observing the nodes of the order parameter.

Topological Entanglement Entropy
Alexei Kitaev and John Preskill
Published 24 March 2006
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We formulate a universal characterization of the many-particle quantum entanglement in the ground state of a topologically ordered two-dimensional medium with a mass gap. We consider a disk in the plane, with a smooth boundary of length L, large compared to the correlation length. In the ground state, by tracing out all degrees of freedom in the exterior of the disk, we obtain a marginal density operator rho for the degrees of freedom in the interior. The von Neumann entropy of rho, a measure of the entanglement of the interior and exterior variables, has the form S(rho)=alphaL-gamma+[centered ellipsis], where the ellipsis represents terms that vanish in the limit L-->[infinity]. We show that -gamma is a universal constant characterizing a global feature of the entanglement in the ground state. Using topological quantum field theory methods, we derive a formula for gamma in terms of properties of the superselection sectors of the medium.

Detecting Topological Order in a Ground State Wave Function
Michael Levin and Xiao-Gang Wen
Published 24 March 2006
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A large class of topological orders can be understood and classified using the string-net condensation picture. These topological orders can be characterized by a set of data (N,di,Flmn<sup>ijk</sup>,deltaijk). We describe a way to detect this kind of topological order using only the ground state wave function. The method involves computing a quantity called the "topological entropy" which directly measures the total quantum dimension D=[summation]idi<sup>2</sup>.

Experimental Observation of Optical Rotation Generated in Vacuum by a Magnetic Field
E. Zavattini et al. (PVLAS Collaboration)
Published 24 March 2006
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See Also: Erratum
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We report the experimental observation of a light polarization rotation in vacuum in the presence of a transverse magnetic field. Assuming that data distribution is Gaussian, the average measured rotation is (3.9±0.5)×10-12 rad/pass, at 5 T with 44 000 passes through a 1 m long magnet, with lambda=1064 nm. The relevance of this result in terms of the existence of a light, neutral, spin-zero particle is discussed.

Photon Regeneration from Pseudoscalars at X-Ray Laser Facilities
Raúl Rabadán, Andreas Ringwald, and Kris Sigurdson
Published 24 March 2006
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See Also: Erratum
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Recently, the PVLAS Collaboration reported an anomalously large rotation of the polarization of light in the presence of a magnetic field in vacuum. As a possible explanation, they consider the existence of a light spin-zero particle coupled to two photons. We propose here a method of independently testing this result using a high-energy photon regeneration experiment (the x-ray analogue of "invisible light shining through walls") using the synchrotron x rays from a free-electron laser. With such an experiment the region of parameter space implied by PVLAS could be probed in a matter of minutes.

Experimental Synchronization of Independent Entangled Photon Sources
Tao Yang, Qiang Zhang, Teng-Yun Chen, Shan Lu, Juan Yin, Jian-Wei Pan, Zhi-Yi Wei, Jing-Rong Tian, and Jie Zhang
Published 20 March 2006
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We report the generation of independent entangled photon pairs from two synchronized but mutually incoherent laser sources. The quality of synchronization is confirmed by observing a violation of Bell's inequality with 3.2 standard deviations in an entanglement swapping experiment. The techniques developed in our experiment are not only important for realistic linear optical quantum-information processing, but also enable new tests of local realism.

Tunable Tsallis Distributions in Dissipative Optical Lattices
P. Douglas, S. Bergamini, and F. Renzoni
Published 24 March 2006
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We demonstrated experimentally that the momentum distribution of cold atoms in dissipative optical lattices is a Tsallis distribution. The parameters of the distribution can be continuously varied by changing the parameters of the optical potential. In particular, by changing the depth of the optical lattice, it is possible to change the momentum distribution from Gaussian, at deep potentials, to a power-law tail distribution at shallow optical potentials.

Gravitation and Astrophysics

Accurate Evolutions of Orbiting Black-Hole Binaries without Excision
M. Campanelli, C. O. Lousto, P. Marronetti, and Y. Zlochower
Published 22 March 2006
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See Also: Physics News Update
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We present a new algorithm for evolving orbiting black-hole binaries that does not require excision or a corotating shift. Our algorithm is based on a novel technique to handle the singular puncture conformal factor. This system, based on the Baumgarte-Shapiro-Shibata-Nakamura formulation of Einstein's equations, when used with a "precollapsed" initial lapse, is nonsingular at the start of the evolution and remains nonsingular and stable provided that a good choice is made for the gauge. As a test case, we use this technique to fully evolve orbiting black-hole binaries from near the innermost stable circular orbit regime. We show fourth-order convergence of waveforms and compute the radiated gravitational energy and angular momentum from the plunge. These results are in good agreement with those predicted by the Lazarus approach.

Gravitational-Wave Extraction from an Inspiraling Configuration of Merging Black Holes
John G. Baker, Joan Centrella, Dae-Il Choi, Michael Koppitz, and James van Meter
Published 22 March 2006
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We present new ideas for evolving black holes through a computational grid without excision, which enable accurate and stable evolutions of binary black hole systems with the accurate determination of gravitational waveforms directly from the wave zone region. Rather than excising the black hole interiors, our approach follows the "puncture" treatment of black holes, but utilizing a new gauge condition which allows the black holes to move successfully through the computational domain. We apply these techniques to an inspiraling binary, modeling the radiation generated during the final plunge and ringdown. We demonstrate convergence of the waveforms and good conservation of mass-energy, with just over 3% of the system's mass converted to gravitational radiation.

Inflationary Predictions for Scalar and Tensor Fluctuations Reconsidered
Latham A. Boyle, Paul J. Steinhardt, and Neil Turok
Published 24 March 2006
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We reconsider the predictions of inflation for the spectral index of scalar (energy density) fluctuations (ns) and the tensor/scalar ratio (r) using a discrete, model-independent measure of the degree of fine-tuning required to obtain a given combination of (ns, r). We find that, except for cases with numerous unnecessary degrees of fine-tuning, ns is less than 0.98, measurably different from exact Harrison-Zel'dovich. Furthermore, if ns>~0.95, in accord with current measurements, the tensor/scalar ratio satisfies r>~10-2, a range that should be detectable in proposed cosmic microwave background polarization experiments and direct gravitational wave searches.

Elementary Particles and Fields

Radiative Decays and the Nature of Heavy Quarkonia
Xavier Garcia i Tormo and Joan Soto
Published 24 March 2006
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We argue that the photon spectra in radiative decays of various heavy quarkonium states provide important information on their nature. If two of these states are in the strong coupling regime, we are able to produce a parameter-free model-independent formula, which holds at next-to-leading order and includes both direct and fragmentation contributions. When the formula is checked against recent CLEO data it favors Upsilon(2S) and Upsilon(3S) in the strong coupling regime and disfavors Upsilon(1S) in it.

Nuclear Physics

Photoabsorption on 4He with a Realistic Nuclear Force
Doron Gazit, Sonia Bacca, Nir Barnea, Winfried Leidemann, and Giuseppina Orlandini
Published 22 March 2006
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The 4He total photoabsorption cross section is calculated with the realistic nucleon-nucleon potential Argonne V18 and the three-nucleon force (3NF) Urbana IX. Final state interaction is included rigorously via the Lorentz integral transform method. A rather pronounced giant resonance with peak cross sections of 3.0 (3.2) mb is obtained with (without) the 3NF. Above 50 MeV strong 3NF effects, up to 35%, are present. Good agreement with experiment is found close to threshold. A comparison in the giant resonance region is inconclusive, since data do not show a unique picture.

Efimov Effect in Nuclear Three-Body Resonance Decays
E. Garrido, D. V. Fedorov, and A. S. Jensen
Published 20 March 2006
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We investigate the effects of the nearly fulfilled Efimov conditions on the properties of three-body resonances. Using the hyper-spheric adiabatic expansion method we compute energy distributions of fragments in a three-body decay of a nuclear resonance. As a realistic example we investigate the 1- state in the halo nucleus 11Li within a three-body 9Li+n+n model. Characteristic features appear as sharp peaks in the energy distributions. Their origin, as in the Efimov effect, is in the large two-body s-wave scattering lengths between the pairs of fragments.

Risk of Misinterpretation of Nearly Degenerate Pair Bands as Chiral Partners in Nuclei
C. M. Petrache, G. B. Hagemann, I. Hamamoto, and K. Starosta
Published 21 March 2006
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The experimental information on the observed nearly degenerate bands in the N=75 isotones, in particular 134Pr and 136Pm, which are often considered as the best candidates for chiral bands, is critically analyzed. Most properties of the bands, in particular, the recently measured branching ratios and lifetimes, are in clear disagreement with the interpretation of the two bands as chiral bands. For I=14–18 in 134Pr, where the observed energies are almost degenerate, we have obtained a value of 2.0(4) for the ratio of the transition quadrupole moments of the two bands, which implies a considerable difference in the nuclear shape associated with the two bands. The insufficiency of the near-degeneracy criterion to trace nuclear chirality is emphasized.

Probing Shell Structure and Shape Changes in Neutron-Rich Sulfur Isotopes through Transient-Field g-Factor Measurements on Fast Radioactive Beams of 38S and 40S.
A. D. Davies, A. E. Stuchbery, P. F. Mantica, P. M. Davidson, A. N. Wilson, A. Becerril, B. A. Brown, C. M. Campbell, J. M. Cook, D. C. Dinca, A. Gade, S. N. Liddick, T. J. Mertzimekis, W. F. Mueller, J. R. Terry, B. E. Tomlin, K. Yoneda, and H. Zwahlen
Published 24 March 2006
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The shell structure underlying shape changes in neutron-rich nuclei near N=28 has been investigated by a novel application of the transient-field technique to measure the first-excited-state g factors in 38S and 40S produced as fast radioactive beams. There is a fine balance between proton and neutron contributions to the magnetic moments in both nuclei. The g factor of deformed 40S does not resemble that of a conventional collective nucleus because spin contributions are more important than usual.

Hindrance of Heavy-Ion Fusion due to Nuclear Incompressibility
S. Misicu and H. Esbensen
Published 21 March 2006
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We propose a new mechanism to explain the unexpected steep falloff of fusion cross sections at energies far below the Coulomb barrier. The saturation properties of nuclear matter are causing a hindrance to large overlap of the reacting nuclei and consequently a sensitive change of the nuclear potential inside the barrier. We report in this Letter a good agreement with the data of coupled-channels calculation for the 64Ni+64Ni combination using the double-folding potential with Michigan-3-Yukawa-Reid effective N-N forces supplemented with a repulsive core that reproduces the nuclear incompressibility for total overlap.

Atomic, Molecular, and Optical Physics

Efficient Approach to Time-Dependent Density-Functional Perturbation Theory for Optical Spectroscopy
Brent Walker, A. Marco Saitta, Ralph Gebauer, and Stefano Baroni
Published 20 March 2006
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Using a superoperator formulation of linearized time-dependent density-functional theory, the dynamical polarizability of a system of interacting electrons is represented by a matrix continued fraction whose coefficients can be obtained from the nonsymmetric block-Lanczos method. The resulting algorithm, which is particularly convenient when large basis sets are used, allows for the calculation of the full spectrum of a system with a computational workload only a few times larger than needed for static polarizabilities within time-independent density-functional perturbation theory. The method is demonstrated with calculation of the spectrum of benzene, and prospects for its application to the large-scale calculation of optical spectra are discussed.

Enhancement and Quenching of Single-Molecule Fluorescence
Pascal Anger, Palash Bharadwaj, and Lukas Novotny
Published 21 March 2006
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We present an experimental and theoretical study of the fluorescence rate of a single molecule as a function of its distance to a laser-irradiated gold nanoparticle. The local field enhancement leads to an increased excitation rate whereas nonradiative energy transfer to the particle leads to a decrease of the quantum yield (quenching). Because of these competing effects, previous experiments showed either fluorescence enhancement or fluorescence quenching. By varying the distance between molecule and particle we show the first experimental measurement demonstrating the continuous transition from fluorescence enhancement to fluorescence quenching. This transition cannot be explained by treating the particle as a polarizable sphere in the dipole approximation.

Absorption Spectra of Quantum Aggregates Interacting via Long-Range Forces
A. Eisfeld and J. S. Briggs
Published 22 March 2006
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We present a simple formula by which the shape of the absorption spectrum of an aggregate of quantum "monomers" (cold atoms, molecules, quantum dots, nanoparticles, etc.) interacting via dipole-dipole forces can be calculated from the averaged spectrum of the quantum monomer itself. Spectral broadening, due to a wide variety of causes, is included explicitly so that the formula is applicable not only to the idealization of a discrete spectrum but also to the practical situation of a continuously broadened spectrum. In simple cases, analytic results are obtained showing the strong dependence of the aggregate spectrum on the precise nature of the broadening of the quantum monomer spectrum. The formula is compared with results of exact diagonalization of model aggregate Hamiltonians and with experiment.

Structural Transformations and Melting in Neon Clusters: Quantum versus Classical Mechanics
Pavel A. Frantsuzov, Dario Meluzzi, and Vladimir A. Mandelshtam
Published 20 March 2006
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The extraordinary complexity of Lennard-Jones (LJ) clusters, which exhibit numerous structures and "phases" when their size or temperature is varied, presents a great challenge for accurate numerical simulations, even without accounting for quantum effects. To study the latter, we utilize the variational Gaussian wave packet method in conjunction with the exchange Monte Carlo sampling technique. We show that the quantum nature of neon clusters has a substantial effect on their size-temperature "phase diagrams," particularly the critical parameters of certain structural transformations. We also give a numerical confirmation that none of the nonicosahedral structures observed for some classical LJ clusters are favorable in the quantum case.

Nonlinear Dynamics, Fluid Dynamics, Classical Optics, etc.

Quasi-Intrinsic Angular Momentum and the Measurement of Its Spectrum
Roberta Zambrini and Stephen M. Barnett
Published 22 March 2006
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We introduce the concept of quasi-intrinsic angular momentum to denote fields for which the mean value of the angular momentum is unaltered by a lateral shift of the rotation axis but the spectrum changes. This property is exemplified by the orbital angular momentum of a beam of light about its propagation direction. We propose an interferometric experiment to measure efficiently the exact angular momentum spectrum and variance for light beams with any arbitrary spatial distribution.

Enhanced Soliton Transport in Quasiperiodic Lattices with Introduced Aperiodicity
Andrey A. Sukhorukov
Published 23 March 2006
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We study linear transmission and nonlinear soliton transport through quasiperiodic structures, where the lattice profiles are described by multiple modulation frequencies. We show that resonant scattering at mixed-frequency resonances limits transmission efficiency of localized wave packets, leading to radiation and possible trapping of solitons. We obtain an explicit analytical expression for optimal quasiperiodic lattice profiles, where additional aperiodic modulations suppress mixed-frequency resonances, resulting in dramatic enhancement of soliton mobility. Our results can be applied to the design of photonic waveguide structures, and arrays of magnetic micro-traps for atomic Bose-Einstein condensates.

Stable Optical Trapping Based on Optical Binding Forces
Tomasz M. Grzegorczyk, Brandon A. Kemp, and Jin Au Kong
Published 24 March 2006
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Various trapping configurations have been realized so far, either based on the scattering force or the gradient force. In this Letter, we propose a new trapping regime based on the equilibrium between a scattering force and optical binding forces only. The trap is realized from the interaction between a single plane wave and a series of fixed small particles, and is efficient at trapping multiple free particles. The effects are demonstrated analytically upon computing the exact scattering from a collection of cylindrical particles and calculating the Lorentz force on each free particle via the Maxwell stress tensor.

Spatiotemporal Chaos: The Microscopic Perspective
Nicolas B. Garnier and Daniel K. Wójcik
Published 22 March 2006
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Extended nonequilibrium systems can be studied in the framework of field theory or from the dynamical systems perspective. Here we report numerical evidence that the sum of a well-defined number of instantaneous Lyapunov exponents for the complex Ginzburg-Landau equation is given by a simple function of the space average of the square of the macroscopic field. This relationship follows from an explicit formula for the time-dependent values of almost all the exponents.

Synchronization Reveals Topological Scales in Complex Networks
Alex Arenas, Albert Díaz-Guilera, and Conrad J. Pérez-Vicente
Published 22 March 2006
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We study the relationship between topological scales and dynamic time scales in complex networks. The analysis is based on the full dynamics towards synchronization of a system of coupled oscillators. In the synchronization process, modular structures corresponding to well-defined communities of nodes emerge in different time scales, ordered in a hierarchical way. The analysis also provides a useful connection between synchronization dynamics, complex networks topology, and spectral graph analysis.

Viscosity Destabilizes Sonoluminescing Bubbles
Ruediger Toegel, Stefan Luther, and Detlef Lohse
Published 20 March 2006
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In single-bubble sonoluminescence (SBSL) microbubbles are trapped in a standing sound wave, typically in water or water-glycerol mixtures. However, in viscous liquids such as glycol, methylformamide, or sulphuric acid it is not possible to trap the bubble in a stable position. This is very peculiar as larger viscosity normally stabilizes the dynamics. Suslick and co-workers call this new mysterious state of SBSL "moving-SBSL." We identify the history force (a force nonlocal in time) as the origin of this destabilization and show that the instability is parametric. A force balance model quantitatively accounts for the observed quasiperiodic bubble trajectories.

Folding of Viscous Threads in Diverging Microchannels
Thomas Cubaud and Thomas G. Mason
Published 20 March 2006
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We study the folding instability of a viscous thread surrounded by a less viscous miscible liquid flowing from a square to a diverging microchannel. Because of the change in the flow introduced by the diverging channel, the viscous thread minimizes viscous dissipation by oscillating to form bends rather than by simply dilating. The folding frequency and the thread diameter can be related to the volume flow rates and thus to the characteristic shear rate. Diffusive mixing at the boundary of the thread can significantly modify the folding flow morphologies. This microfluidic system enables us to control the bending of the thread and to enhance mixing between liquids having significantly different viscosities.

Nanoscale Fluid Flows in the Vicinity of Patterned Surfaces
Marek Cieplak, Joel Koplik, and Jayanth R. Banavar
Published 23 March 2006
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Molecular dynamics simulations of dense and rarefied fluids comprising small chain molecules in chemically patterned nanochannels predict a novel switching from Poiseuille to plug flow along the channel. We also demonstrate behavior akin to the lotus effect for a nanodrop on a chemically patterned substrate. Our results show that one can control and exploit the behavior of fluids at the nanoscale using chemical patterning.

Multifractal Dimension of Lagrangian Turbulence
Haitao Xu et al. (International Collaboration for Turbulence Research)
Published 24 March 2006
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We report experimental measurements of the Lagrangian multifractal dimension spectrum in an intensely turbulent laboratory water flow by the optical tracking of tracer particles. The Legendre transform of the measured spectrum is compared with measurements of the scaling exponents of the Lagrangian velocity structure functions, and excellent agreement between the two measurements is found, in support of the multifractal picture of turbulence. These measurements are compared with three model dimension spectra. When the nonexistence of structure functions of order less than -1 is accounted for, the models are shown to agree well with the measured spectrum.

Plasma and Beam Physics

Efficient Propagation of Polarization from Laser Photons to Positrons through Compton Scattering and Electron-Positron Pair Creation
T. Omori, M. Fukuda, T. Hirose, Y. Kurihara, R. Kuroda, M. Nomura, A. Ohashi, T. Okugi, K. Sakaue, T. Saito, J. Urakawa, M. Washio, and I. Yamazaki
Published 24 March 2006
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We have demonstrated for the first time the production of highly polarized short-pulse positrons with a finite energy spread in accordance with a new scheme that consists of two-quantum processes, such as inverse Compton scattering and electron-positron pair creation. Using a circularly polarized laser beam of 532 nm scattered off a high-quality, 1.28 GeV electron beam, we have obtained polarized positrons with an intensity of 2×104 e+/bunch. The magnitude of positron polarization has been determined to be 73±15(stat)±19(syst)% by means of a newly designed positron polarimeter.

Measurement of the Interaction Force among Particles in Three-Dimensional Plasma Clusters
T. Antonova, B. M. Annaratone, D. D. Goldbeck, V. Yaroshenko, H. M. Thomas, and G. E. Morfill
Published 20 March 2006
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The interaction forces between particles have been studied in a 3D plasma cluster under weak external confinement. A suitable combination of dc and rf applied to a small electrode provided gravity compensation, uniform over dimensions much larger than the cluster itself. The forces acting on the particles could be reconstructed due to unique three-dimensional diagnostics, which allow us to obtain coordinates and velocities of all the particles simultaneously. The measurements yield a maximum (external) confinement force of 1.4×10-15N and interparticle force that is repulsive at short distances and attractive at larger distances, with a maximum attractive force of 2.4×10-14N at particle separation 195 µm.

Spectrum of Magnetohydrodynamic Turbulence
Stanislav Boldyrev
Published 20 March 2006
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We propose a phenomenological theory of strong incompressible magnetohydrodynamic turbulence in the presence of a strong large-scale external magnetic field. We argue that in the inertial range of scales, magnetic-field and velocity-field fluctuations tend to align the directions of their polarizations. However, the perfect alignment cannot be reached; it is precluded by the presence of a constant energy flux over scales. As a consequence, the directions of shear-Alfvén fluid and magnetic-field fluctuations at each scale lambda become effectively aligned within the angle philambda[proportional]lambda1/4, which leads to scale-dependent depletion of the nonlinear interaction and to the field-perpendicular energy spectrum E(k[perpendicular])[proportional]k[perpendicular]<sup>-3/2</sup>. Our results may be universal, i.e., independent of the external magnetic field, since small-scale fluctuations locally experience a strong field produced by large-scale eddies.

Streamers in the JIPP T-llU Tokamak Plasmas
Y. Hamada et al. (JIPPT-IIU Group)
Published 22 March 2006
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It is shown that the low-density Ohmically heated tokamak plasmas have streamerlike eddies at the outer region at normalized minor radius of about 0.7 and high-frequency zonal flows of large amplitudes in the core. The amplitudes of the eddies e Phi-tilde/kTe and ñe/[overline n]e are of order of 0.5, similar to that of blobs in the tokamak plasma boundary. The waveforms are featured by pulses of complex shape with sharp fronts, similar to the results of streamer simulations by Garbet et al.. The time constant of the fronts is also in agreement with the simulation. The radial span of the eddies is estimated to be much larger than the poloidal span.

Early Out-Of-Equilibrium Beam-Plasma Evolution
M.-C. Firpo, A. F. Lifschitz, E. Lefebvre, and C. Deutsch
Published 22 March 2006
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We solve analytically the out-of-equilibrium initial stage that follows the injection of a radially finite electron beam into a plasma at rest and test it against particle-in-cell simulations. For initial large beam edge gradients and not too large beam radius, compared to the electron skin depth, the electron beam is shown to evolve into a ring structure. For low enough transverse temperatures, the filamentation instability eventually proceeds and saturates when transverse isotropy is reached. The analysis accounts for the variety of very recent experimental beam transverse observations.

Test of Thermal Transport Models through Dynamic Overpressure Stabilization of Ablation-Front Perturbation Growth in Laser-Driven CH Foils
O. V. Gotchev, V. N. Goncharov, J. P. Knauer, T. R. Boehly, T. J. B. Collins, R. Epstein, P. A. Jaanimagi, and D. D. Meyerhofer
Published 24 March 2006
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Heat-flow-induced dynamic overpressure at the perturbed ablation front of an inertial confinement fusion target can stabilize the ablative Richtmyer–Meshkov-like instability and mitigate the subsequent ablative Rayleigh-Taylor (RT) instability. A series of experiments was performed on the OMEGA laser to quantify the dynamic overpressure stabilization during the shock transit. Analysis of the experimental data using hydrocode simulations shows that the observed oscillatory evolution of the ablation-front perturbations depends on Dc, the size of the thermal conduction zone, and the fluid velocity in the blowoff region Vbl that are sensitive to the thermal transport model used. We show that the simulations match the experiment well when the time dependence of the heat-flux inhibition is taken into account using a recently developed nonlocal heat-transport model [V. N. Goncharov et al., Phys. Plasmas 13, 012702 (2006)].

Inductive and Electrostatic Acceleration in Relativistic Jet-Plasma Interactions
Johnny S. T. Ng and Robert J. Noble
Published 24 March 2006
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We report on the observation of rapid particle acceleration in numerical simulations of relativistic jet-plasma interactions and discuss the underlying mechanisms. The dynamics of a charge-neutral, narrow, electron-positron jet propagating through an unmagnetized electron-ion plasma was investigated using a three-dimensional, electromagnetic, particle-in-cell computer code. The interaction excited magnetic filamentation as well as electrostatic plasma instabilities. In some cases, the longitudinal electric fields generated inductively and electrostatically reached the cold plasma-wave-breaking limit, and the longitudinal momentum of about half the positrons increased by 50% with a maximum gain exceeding a factor of 2 during the simulation period. Particle acceleration via these mechanisms occurred when the criteria for Weibel instability were satisfied.

Condensed Matter: Structure, etc.

Novel Bidecahedral Morphology in Gold Nanoparticles Frozen from Liquid
Kenji Koga
Published 21 March 2006
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A bidecahedral morphology in which two truncated decahedral structures share two tetrahedral units, involving two types of symmetric-tilt grain boundaries, is observed as a novel and rare morphology of gold nanoparticles frozen from liquid in free space. This low-symmetry polyhedral morphology with eight multiply twinned domains is intermediate between the icosahedral and decahedral motifs.

Lattice Dynamics of Molybdenum at High Pressure
Daniel L. Farber, Michael Krisch, Daniele Antonangeli, Alexandre Beraud, James Badro, Florent Occelli, and Daniel Orlikowski
Published 21 March 2006
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We have determined the lattice dynamics of molybdenum at high pressure to 37 GPa using high-resolution inelastic x-ray scattering. Over the investigated pressure range, we find a significant decrease in the H-point phonon anomaly. We also present calculations based on density functional theory that accurately predict this pressure dependence. Based on these results, we infer that the likely explanation for the H-point anomaly in molybdenum is strong electron-phonon coupling, which decreases upon compression due to the shift of the Fermi level with respect to the relevant electronic bands.

New Growth Mode through Decorated Twin Boundaries
Sebastian Bleikamp, Arne Thoma, Celia Polop, Gerhard Pirug, Udo Linke, and Thomas Michely
Published 21 March 2006
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Scanning tunneling microscopy and low energy electron diffraction were used to investigate the growth of partly twinned Ir thin films on Ir(111). A transition from the expected layer-by-layer to a defect dominated growth mode with a fixed lateral length scale and increasing roughness is observed. During growth, the majority of the film is stably transformed to twinned stacking. This transition is initiated by the energetic avoidance of the formation of intrinsic stacking faults compared to two independent twin faults. The atomistic details of the defect kinetics are outlined.

First-Principles Calculations of Vibrational Lifetimes and Decay Channels: Hydrogen-Related Modes in Si
D. West and S. K. Estreicher
Published 22 March 2006
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The vibrational lifetimes and decay channels of local vibrational modes are calculated from first principles at various temperatures. Our method can be used to predict the temperature dependence of the lifetime of any normal mode in any crystal. We focus here on the stretch modes of H2<sup>*</sup>, HBC<sup>+</sup>, and VH·HV in Si. The frequencies are almost identical, but the lifetimes vary from 4 to 295 ps. The calculations correctly predict the lifetimes for T>50 K and illustrate the critical importance of pseudolocal modes in the decay processes of high-frequency local vibrational modes.

Acoustic Pulse Echoes Probed with Time-Resolved X-Ray Triple-Crystal Diffractometry
Yujiro Hayashi, Yoshihito Tanaka, Tomoyuki Kirimura, Noboru Tsukuda, Eiichi Kuramoto, and Tetsuya Ishikawa
Published 23 March 2006
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Acoustic pulse echoes generated by femtosecond laser irradiation were detected using time-resolved x-ray triple-crystal diffractometry. The determined time-dependent longitudinal strain component for pulse echoes in silicon and gallium arsenide plates showed that the polarity of the strain pulse was dependent on the optically induced initial stress, and that the bipolar pulse waveform was gradually deformed and broadened in the course of propagation. The three-dimensional wave front distortion of pulse echoes was shown simply as the pulse duration broadening, which was consistent with a boundary roughness for an unpolished plate.

Scaling Relations for Logarithmic Corrections
R. Kenna, D. A. Johnston, and W. Janke
Published 23 March 2006
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Multiplicative logarithmic corrections to scaling are frequently encountered in the critical behavior of certain statistical-mechanical systems. Here, a Lee-Yang zero approach is used to systematically analyze the exponents of such logarithms and to propose scaling relations between them. These proposed relations are then confronted with a variety of results from the literature.

Direct Identification of Critical Clusters in Chemical Vapor Deposition
H. Rauscher, J. Braun, and R. J. Behm
Published 23 March 2006
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The kinetics of heteroepitaxial island nucleation on the Si(111) surface during ultrahigh vacuum chemical vapor deposition with the precursor GeH4 was studied by scanning tunneling microscopy. The results can be described within the framework of rate-equation based nucleation theory, modified by an additional energetic barrier for the attachment of adatoms at steps. This barrier results from the passivation of steps by dissociation products from the GeH4 precursor. A critical nucleus size of 9 atoms is derived. Scanning tunneling microscopy images provide direct evidence for the existence of stable clusters consisting of 10 atoms and allow the unequivocal identification of their structure.

Condensed Matter: Electronic Properties, etc.

Two-Site Entropy and Quantum Phase Transitions in Low-Dimensional Models
Ö. Legeza and J. Sólyom
Published 21 March 2006
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We propose a new approach to study quantum phase transitions in low-dimensional lattice models. It is based on studying the von Neumann entropy of two neighboring central sites in a long chain. It is demonstrated that the procedure works equally well for fermionic and spin models, and the two-site entropy is a better indicator of quantum phase transition than calculating gaps, order parameters, or the single-site entropy. The method is especially convenient when the density-matrix renormalization-group algorithm is used.

Subgap Collective Tunneling and Its Staircase Structure in Charge Density Waves
Yu. I. Latyshev, P. Monceau, S. Brazovskii, A. P. Orlov, and T. Fournier
Published 23 March 2006
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Tunneling spectra of chain materials NbSe3 and TaS3 were studied in nanoscale mesa devices. Current-voltage I-V characteristics related to all charge density waves (CDWs) reveal universal spectra within the normally forbidden region of low V, below the electronic CDW gap 2Delta. The tunneling always demonstrates a threshold Vt[approximate]0.2Delta, followed, for both CDWs in NbSe3, by a staircase fine structure. T dependencies of Vt(T) and Delta(T) scale together for each CDW, while the low T values Vt(0) correlate with the CDWs' transition temperatures Tp. Fine structures of CDWs perfectly coincide when scaled along V/Delta. The results evidence the sequential entering of CDW vortices (dislocations) in the junction area with the tunneling current concentrated in their cores. The subgap tunneling proceeds via the phase channel: coherent phase slips at neighboring chains.

Mott-Anderson Transition Controlled by a Magnetic Field in Pyrochlore Molybdate
N. Hanasaki, M. Kinuhara, I. Kézsmárki, S. Iguchi, S. Miyasaka, N. Takeshita, C. Terakura, H. Takagi, and Y. Tokura
Published 24 March 2006
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The pyrochlore molybdate Gd2Mo2O7 locates near the phase boundary between the ferromagnetic-metallic and the spin-glass insulating state. This metal-insulator transition is governed on a large energy scale by the electron-correlation effect, while the geometrical frustration causes the random potential. The magnetic field can tune the randomness of the potential and control, under a suitable pressure, the continuous Mott-Anderson transition precisely. The critical exponent (µ=1.04±0.1) of the Mott-Anderson transition has been determined for this ferromagnetic orbital-degenerate electron system.

Critical Phenomena and the Quantum Critical Point of Ferromagnetic Zr1-xNbxZn2
D. A. Sokolov, M. C. Aronson, W. Gannon, and Z. Fisk
Published 24 March 2006
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We present a study of the magnetic properties of Zr1-xNbxZn2, using an Arrott plot analysis of the magnetization. The Curie temperature TC is suppressed to zero temperature for Nb concentration xC=0.083±0.002, while the spontaneous moment vanishes linearly with TC as predicted by the Stoner theory. The initial susceptibility chi displays critical behavior for x<=xC, with a critical exponent which smoothly crosses over from the mean field to the quantum critical value. For high temperatures and x<=xC, and for low temperatures and x>=xC we find that chi-1=chi0<sup>-1</sup>+aT4/3, where chi0<sup>-1</sup> vanishes as x-->xC. The resulting magnetic phase diagram shows that the quantum critical behavior extends over the widest range of temperatures for x=xC, and demonstrates how a finite transition temperature ferromagnet is transformed into a paramagnet, via a quantum critical point.

Orbital Ordering in LaMnO3: Electron-Electron versus Electron-Lattice Interactions
Wei-Guo Yin, Dmitri Volja, and Wei Ku
Published 24 March 2006
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The relative importance of electron-lattice (e-l) and electron-electron (e-e) interactions in ordering orbitals in LaMnO3 is systematically examined within the local-density approximation+HubbardU approximation of density functional theory. A realistic effective Hamiltonian is derived from novel Wannier state analysis of the electronic structure. Surprisingly, e-l interaction (~=0.9 eV) alone is found insufficient to stabilize the orbital ordered state. On the other hand, e-e interaction (~=1.7 eV) not only induces orbital ordering, but also greatly facilitates the Jahn-Teller distortion via enhanced localization. Further experimental means to quantify the competition between these two mechanisms are proposed.

Experimental Investigation of the Breakdown of the Onsager-Casimir Relations
C. A. Marlow, R. P. Taylor, M. Fairbanks, I. Shorubalko, and H. Linke
Published 20 March 2006
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We use magnetoconductance fluctuation measurements of phase-coherent semiconductor billiards to quantify the contributions to the nonlinear electric conductance that are asymmetric under reversal of magnetic field. We find that the average asymmetric contribution is linear in magnetic field (for magnetic flux much larger than 1 flux quantum) and that its magnitude depends on billiard geometry. In addition, we find an unexpected asymmetry in the power spectrum of the magnetoconductance with respect to reversal of magnetic field and bias voltage.

Electron States in a Lattice of Au Nanoparticles: The Role of Strain and Functionalization
Ronaldo J. C. Batista, Mário S. C. Mazzoni, Ignacio L. Garzón, Marcela R. Beltrán, and Helio Chacham
Published 23 March 2006
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We make use of first-principles calculations to study the effects of functionalization and compression on the electronic properties of 2D lattices of Au nanoparticles. We consider Au38 particles capped by methylthiol molecules and possibly functionalized by the dithiolated conjugated molecules benzenedimethanethiol and benzenedicarbothialdehyde. We find that the nonfunctionalized lattices are insulating, with negligible band dispersions even for a compression of 20% of the lattice constant. Distinct behaviors of the dispersion of the lowest conduction band as a function of compression are predicted for functionalized lattices: The band dispersion of the benzenedimethanethiol-functionalized lattice increases considerably with compression, while that of the benzenedicarbothialdehyde-functionalized lattice decreases.

Pressure Dependence of the Fulde-Ferrell-Larkin-Ovchinnikov State in CeCoIn5
C. F. Miclea, M. Nicklas, D. Parker, K. Maki, J. L. Sarrao, J. D. Thompson, G. Sparn, and F. Steglich
Published 20 March 2006
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Pressure studies of the thermodynamics of CeCoIn5 under magnetic fields H||c and H||ab have been made up to P=1.34 GPa. We recorded the signature of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state for all pressures when H||ab. Also remarkably, the FFLO regime suddenly expands for P=1.34 GPa. With the help of a microscopic theory for d-wave superconductivity, we have extracted the gyromagnetic ratio g and the Fermi velocities va and vc. Our study is the first evidence for the existence of the FFLO state away from the influence of the antiferromagnetic fluctuations. We find a close parallel between the T-P phase diagram of CeCoIn5 and the T-x phase diagram of the high-Tc cuprates, where x is the hole concentration.

Superconductivity in a Molecular Metal Cluster Compound
O. N. Bakharev, D. Bono, H. B. Brom, A. Schnepf, H. Schnöckel, and L. J. de Jongh
Published 21 March 2006
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Compelling evidence for band-type conductivity and even bulk superconductivity below Tc[approximate]8 K has been found in 69,71Ga NMR experiments in crystalline ordered, giant Ga84 cluster compounds. This material appears to represent the first realization of a theoretical model proposed by Friedel in 1992 for superconductivity in ordered arrays of weakly coupled, identical metal nanoparticles.

Phase Coexistence and Resistivity near the Ferromagnetic Transition of Manganites
A. S. Alexandrov, A. M. Bratkovsky, and V. V. Kabanov
Published 23 March 2006
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Pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair breaking in the ferromagnetic phase (the so-called current-carrier density collapse) has accounted for the first-order ferromagnetic-phase transition, colossal magnetoresistance, isotope effect, and pseudogap in doped manganites. Here we propose an explanation of the phase coexistence and describe the magnetization and resistivity of manganites near the ferromagnetic transition in the framework of the current-carrier density collapse. The present quantitative description of resistivity is obtained without any fitting parameters, by using the experimental resistivities far away from the transition and the experimental magnetization, and is essentially model-independent.

Kinks, Nodal Bilayer Splitting, and Interband Scattering in YBa2Cu3O6+x
S. V. Borisenko, A. A. Kordyuk, V. Zabolotnyy, J. Geck, D. Inosov, A. Koitzsch, J. Fink, M. Knupfer, B. Büchner, V. Hinkov, C. T. Lin, B. Keimer, T. Wolf, S. G. Chiuzbaian, L. Patthey, and R. Follath
Published 24 March 2006
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We apply the new-generation angle-resolved photoemission spectroscopy methodology to the most widely studied cuprate superconductor YBa2Cu3O6+x. Considering the nodal direction, we found noticeable renormalization effects known as kinks both in the quasiparticle dispersion and scattering rate, the bilayer splitting, and evidence for strong interband scattering—all the characteristic features of the nodal quasiparticles detected earlier in Bi2Sr2CaCu2O8+delta. The typical energy scale and the doping dependence of the kinks clearly point to their intimate relation with the spin-1 resonance seen in the neutron scattering experiments. Our findings strongly suggest a universality of the electron dynamics in the bilayer superconducting cuprates and a dominating role of the spin fluctuations in the formation of the quasiparticles along the nodal direction.

0-pi Transitions in Josephson Junctions with Antiferromagnetic Interlayers
Brian M. Andersen, I. V. Bobkova, P. J. Hirschfeld, and Yu. S. Barash
Published 24 March 2006
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We show that the dc Josephson current through superconductor-antiferromagnet-superconductor (S-AF-S) junctions manifests a remarkable atomic-scale dependence on the interlayer thickness. At low temperatures the junction is either a 0 or pi junction depending on whether the AF interlayer consists of an even or odd number of atomic layers. This is associated with different symmetries of the AF interlayers in the two cases. In the junction with odd AF interlayers an additional pi-0 transition can take place as a function of temperature. This originates from the interplay of spin-split Andreev bound states. Experimental implications of these theoretical findings are discussed.

Local Tunneling Spectroscopy as a Signature of the Fulde-Ferrell-Larkin-Ovchinnikov State in s- and d-Wave Superconductors
Qian Wang, H.-Y. Chen, C.-R. Hu, and C. S. Ting
Published 24 March 2006
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The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states for two-dimensional s- and d-wave superconductors (s- and d-SCs) are self-consistently studied under an in-plane magnetic field. While the stripe solution of the order parameter is found to have lower free energy in s-SCs, a square lattice solution appears to be energetically more favorable in the case of d-SCs. At certain symmetric sites, we find that the features in the local density of states (LDOS) can be ascribed to two types of bound states. We also show that the LDOS maps for d-SCs exhibit bias-energy-dependent checkerboard patterns. These characteristics can serve as signatures of the FFLO states.

Charge and Magnetization Inhomogeneities in Diluted Magnetic Semiconductors
Carsten Timm
Published 20 March 2006
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It is predicted that III-V diluted magnetic semiconductors can exhibit stripelike modulations of magnetization and carrier concentration. This inhomogeneity results from the strong dependence of the magnetization on the carrier concentration. Within Landau theory, a characteristic temperature T* below the Curie temperature is found so that below T* the equilibrium magnetization shows modulations, which are strongly anharmonic. The wavelength and amplitude of the modulation rise for decreasing temperature, starting from zero at T*. Above T*, the equilibrium state is homogeneous, but the coupling between charge and magnetization leads to the appearance of an electrically charged layer in domain walls.

Instanton Analysis of Hysteresis in the Three-Dimensional Random-Field Ising Model
Markus Müller and Alessandro Silva
Published 21 March 2006
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We study the magnetic hysteresis in the random-field Ising model in 3D. We discuss the disorder dependence of the coercive field Hc, and obtain an analytical description of the smooth part of the hysteresis below and above Hc, by identifying the disorder configurations (instantons) that are the most probable to trigger local avalanches. We estimate the critical disorder strength at which the hysteresis curve becomes continuous. From an instanton analysis at zero field we obtain a description of local two-level systems in the ferromagnetic phase.

Motional-Narrowing-Type Dephasing of Electron and Hole Spins of Itinerant Excitons in Magnetically Doped II-VI Bulk Semiconductors
Kai E. Rönnburg, Ernst Mohler, Hartmut G. Roskos, Kai Ortner, Charles R. Becker, and Laurens W. Molenkamp
Published 21 March 2006
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Time-resolved optical spin-quantum-beat measurements performed on magnetically doped II-VI bulk semiconductors reveal an increase of the electron spin dephasing time with rising temperature typical for motional narrowing. With the dephasing being notably faster than in undoped II-VI semiconductors, the magnetic dopants must play a key role, modifying the known dephasing mechanisms and introducing new ones. Focusing on the latter, we theoretically explore the spin dephasing channel arising from magnetization fluctuations sampled by the itinerant excitons. This mechanism suffices to explain quantitatively the results of our time-resolved Faraday-rotation experiments on optically excited Cd1-xMnxTe which we present here as a function of magnetic field, temperature and manganese dopant density. In addition to electron spin-quantum beats, some of our experiments reveal hole spin beats as well.

Enhancing Exchange Bias with Diluted Antiferromagnets
Jung-Il Hong, Titus Leo, David J. Smith, and Ami E. Berkowitz
Published 21 March 2006
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The exchange bias HE of coupled polycrystalline films of antiferromagnetic CoO and ferromagnetic Co was significantly enhanced by the systematic substitution of nonmagnetic Mg for Co in CoO. Samples in which either Co or Co1-xMgxO were deposited first were investigated at temperatures from 10 to 300 K. With Co1-xMgxO on the bottom, the increased interfacial uncompensated spin density of the single antiferromagnetic domain Co1-xMgxO crystallites produced the enhanced HE. With Co on the bottom, a thin interfacial oxide layer was primarily responsible for the strongly increased HE.

Magnetization Plateaus Induced by a Coupling to the Lattice
T. Vekua, D. C. Cabra, A. Dobry, C. Gazza, and D. Poilblanc
Published 21 March 2006
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We present a novel mechanism for the appearance of magnetization plateaus in quasi-one-dimensional quantum spin systems, which is induced by the coupling to the underlying lattice. We investigate in detail a simple model of a frustrated spin-1/2 Heisenberg chain coupled to adiabatic phonons under an external magnetic field, but the present mechanism is expected to be more general. Using field theoretic methods complemented by extensive density matrix renormalization group techniques, we show that magnetization plateaus at nontrivial rational values of the magnetization can be stabilized by the lattice coupling. We suggest that such a scenario could be relevant for some low dimensional frustrated spin-Peierls compounds.

Localized Excitation in the Hybridization Gap in YbAl3
A. D. Christianson, V. R. Fanelli, J. M. Lawrence, E. A. Goremychkin, R. Osborn, E. D. Bauer, J. L. Sarrao, J. D. Thompson, C. D. Frost, and J. L. Zarestky
Published 22 March 2006
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The intermediate valence compound YbAl3 exhibits a broad magnetic excitation in the inelastic neutron scattering spectrum with characteristic energy E1[approximate]50 meV, equal to the Kondo energy (TK~600–700 K). In the low temperature (T<Tcoh~40 K) Fermi liquid state, however, a new peak in the scattering occurs at E2[approximate]33 meV, which lies in the hybridization gap that exists in this compound. We report inelastic neutron scattering results for a single-crystal sample. The scattering at energies near E1 qualitatively has the momentum (Q) dependence expected for interband scattering across the indirect gap. The scattering near E2 has a very different Q dependence: it is a weak function of Q over a large fraction of the Brillouin zone and is smallest near (1/2,1/2,1/2). A possibility is that the peak at E2 arises from a spatially localized excitation in the hybridization gap.

Angle-Dependent X-Ray Magnetic Circular Dichroism from (Ga,Mn)As: Anisotropy and Identification of Hybridized States
K. W. Edmonds, G. van der Laan, A. A. Freeman, N. R. S. Farley, T. K. Johal, R. P. Campion, C. T. Foxon, B. L. Gallagher, and E. Arenholz
Published 22 March 2006
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Remarkably anisotropic Mn L2,3 x-ray magnetic circular dichroism spectra from the ferromagnetic semiconductor (Ga,Mn)As are reported. States with cubic and uniaxial symmetry are distinguished by careful analysis of the angle dependence of the spectra. The multiplet structures with cubic symmetry are qualitatively reproduced by calculations for an atomiclike d5 configuration in tetrahedral environment, and show zero anisotropy in the orbital and spin moments within the experimental uncertainty. However, hybridization with the host valence bands is reflected by the presence of a preedge feature with a uniaxial anisotropy and a marked dependence on the hole density.

Berry Phase, Topology, and Degeneracies in Quantum Nanomagnets
Patrick Bruno
Published 22 March 2006
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A topological theory of the diabolical points (degeneracies) of quantum magnets is presented. Diabolical points are characterized by their diabolicity index, for which topological sum rules are derived. The paradox of the missing diabolical points for Fe8 molecular magnets is clarified. A new method is also developed to provide a simple interpretation, in terms of destructive interferences due to the Berry phase, of the complete set of diabolical points found in biaxial systems such as Fe8.

Universal Distribution of Kondo Temperatures in Dirty Metals
P. S. Cornaglia, D. R. Grempel, and C. A. Balseiro
Published 23 March 2006
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Kondo screening of diluted magnetic impurities in a disordered host is studied analytically and numerically in one, two, and three dimensions. It is shown that in the TK-->0 limit the distribution of Kondo temperatures has a universal form P(TK)~TK<sup>-alpha</sup> that holds in the insulating phase and persists in the metallic phase close to the metal-insulator transition. Moreover, the exponent alpha depends only on the dimensionality. The most important consequence of this result is that the T dependence of thermodynamic properties is smooth across the metal-insulator transition in three dimensional systems.

Evidence of Large Magnetostructural Effects in Austenitic Stainless Steels
L. Vitos, P. A. Korzhavyi, and B. Johansson
Published 24 March 2006
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The surprisingly low magnetic transition temperatures in austenitic stainless steels indicate that in these Fe-based alloys magnetic disorder might be present at room temperature. Using a first-principles approach, we have obtained a theoretical description of the stacking fault energy in Fe100-c-nCrcNin alloys as a function of composition and temperature. Comparison of our results with experimental databases provides a strong evidence for large magnetic fluctuations in these materials. We demonstrate that the effects of alloying additions on the structural properties of steels contain a dominant magnetic contribution, which stabilizes the most common austenitic steels at normal service conditions.

Spin and Orbital Quantization of Electronic States as Origins of Second Harmonic Generation in Semiconductors
I. Sänger, D. R. Yakovlev, R. V. Pisarev, V. V. Pavlov, M. Bayer, G. Karczewski, T. Wojtowicz, and J. Kossut
Published 24 March 2006
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Basically different mechanisms of optical second harmonic generation (SHG) in semiconductors, induced by an external magnetic field H, have been identified experimentally by studying the diluted magnetic semiconductor (Cd,Mn)Te. For paramagnetic (Cd,Mn)Te the SHG response is governed by spin quantization of electronic states, in contrast with diamagnetic CdTe with its dominating orbital quantization. The mechanisms can be identified by the distinct magnetic field dependence of the SHG intensity which scales with the spin splitting in the paramagnetic case as compared to the H2 dependence observed for the diamagnetic case.

Efficient Single-Photon Sources Based on Low-Density Quantum Dots in Photonic-Crystal Nanocavities
Wen-Hao Chang, Wen-Yen Chen, Hsiang-Szu Chang, Tung-Po Hsieh, Jen-Inn Chyi, and Tzu-Min Hsu
Published 20 March 2006
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An efficient single-photon source based on low-density InGaAs quantum dots in a photonic-crystal nanocavity is demonstrated. The single-photon source features the effects of a photonic band gap, yielding a single-mode spontaneous emission coupling efficiency as high as beta=92% and a linear polarization degree up to p=95%. This appealing performance makes it well suited for practical implementation of polarization-encoded schemes in quantum cryptography.

Terahertz Radiation by an Ultrafast Spontaneous Polarization Modulation of Multiferroic BiFeO3 Thin Films
Kouhei Takahashi, Noriaki Kida, and Masayoshi Tonouchi
Published 22 March 2006
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Terahertz (THz) radiation has been observed from multiferroic BiFeO3 thin films via ultrafast modulation of spontaneous polarization upon carrier excitation with illumination of femtosecond laser pulses. The radiated THz pulses from BiFeO3 thin films were clarified to directly reflect the spontaneous polarization state, giving rise to a memory effect in a unique style and enabling THz radiation even at zero-bias electric field. On the basis of our findings, we demonstrate potential approaches to ferroelectric nonvolatile random access memory with nondestructive readability and ferroelectric domain imaging microscopy using THz radiation as a sensitive probe.

Influence of Copolymer Interface Orientation on the Optical Emission of Polymeric Semiconductor Heterojunctions
P. Sreearunothai, A. C. Morteani, I. Avilov, J. Cornil, D. Beljonne, R. H. Friend, R. T. Phillips, C. Silva, and L. M. Herz
Published 23 March 2006
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We have examined the Coulombic interactions at the interface in a blend of two copolymers with intramolecular charge-transfer character and optimized band offsets for photoinduced charge generation. The combination of both time-resolved measurements of photoluminescence, and quantum-chemical modeling of the heterojunction allows us to show that relative orientation across the heterojunction can lead to either a repulsive barrier (~65 meV) or an attractive interaction which can enhance the charge-transfer processes. We conclude that polymer orientation at the heterojunction can be as important as energy-band offsets in determining the dynamics of charge separation and optical emission.

Polarization Dependence of L- and M-Edge Resonant Inelastic X-Ray Scattering in Transition-Metal Compounds
Michel van Veenendaal
Published 23 March 2006
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The resonant inelastic x-ray scattering (RIXS) cross section at the L and M edges of transition-metal compounds is studied using an effective scattering operator. The intensities of the elastic peak and for spin-flip processes are derived. It is shown how the polarization dependence can be used to select transitions. Comparisons are made with experiment. A detailed analysis of the polarization and angular dependence of L- and M-edge RIXS for divalent copper compounds, such as the high-Tc superconductors, is given.

Observation of Electron Gas Cooling in Free Sodium Clusters
M. Maier, G. Wrigge, M. Astruc Hoffmann, P. Didier, and B. v. Issendorff
Published 24 March 2006
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Free size-selected Nan+ (n=16–250) clusters have been studied by femtosecond pump-probe photoelectron and photofragmentation spectroscopy. Thermal electron emission from the hot electron gas was used to monitor the energy transfer from the electronic system to lattice vibrations. The electron-phonon coupling constants determined for the different sizes can be described by the radius dependent function g(R)=(2.3+114 Å2/R2)×1016 W/m3 K. No strong quantum size effect was observed even for the smallest cluster size.

Soft Matter, Biological, and Interdisciplinary Physics

Core Precession and Global Modes in Granular Bulk Flow
Denis Fenistein, Jan-Willem van de Meent, and Martin van Hecke
Published 21 March 2006
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We report a novel transition to core precession for granular flows in a split-bottomed shear cell. This transition is related to a qualitative change in the 3D flow structure: For shallow layers of granular material, the shear zones emanating from the split reach the free surface, while for deep layers the shear zones meet below the surface, causing precession. The surface velocities reflect this transition by a change of symmetry. As a function of layer depth, we find that three qualitatively different smooth and robust granular flows can be created in this simple shearing geometry.

Brownian Forces in Sheared Granular Matter
A. Baldassarri, F. Dalton, A. Petri, S. Zapperi, G. Pontuale, and L. Pietronero
Published 23 March 2006
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We present results from a series of experiments on a granular medium sheared in a Couette geometry and show that their statistical properties can be computed in a quantitative way from the assumption that the resultant from the set of forces acting in the system performs a Brownian motion. The same assumption has been utilized, with success, to describe other phenomena, such as the Barkhausen effect in ferromagnets, and so the scheme suggests itself as a more general description of a wider class of driven instabilities.

Hydrodynamic Damping of Membrane Thermal Fluctuations near Surfaces Imaged by Fluorescence Interference Microscopy
Yoshihisa Kaizuka and Jay T. Groves
Published 20 March 2006
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Hydrodynamic coupling between closely apposed lipid bilayer membranes can substantially slow the time scale of thermal fluctuations. These effects could dominate the kinetics of macromolecular interactions at membrane interfaces, and may exert important influences over the interactions between living cells. Here, we study a reconstituted membrane system consisting of two planar bilayers with average separation distances of 30–60 nm. This spacing range is comparable to that of natural intercellular junctions. Using real-time fluorescence interference contrast imaging, spatial and temporal characteristics of the thermal fluctuation spectra are monitored simultaneously. Strong hydrodynamic damping of the fluctuation time scale is observed whereas fluctuation spatial dimensions are unaltered by proximity to another surface. These observations provide experimental confirmation of recent theoretical models.