Physical Review Letters -- February 17, 2006

BEC-BCS Crossover in "Magnetized" Feshbach-Resonantly Paired Superfluids

Published 13 February 2006

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We map out the detuning-magnetization phase diagram for a magnetized (unequal number of atoms in two pairing hyperfine states) gas of fermionic atoms interacting via an s-wave Feshbach resonance (FR). The phase diagram is dominated by the coexistence of a magnetized normal gas and a singlet-paired superfluid with the latter exhibiting a BCS-Bose Einstein condensate crossover with reduced FR detuning. On the BCS side of strongly overlapping Cooper pairs, a sliver of finite-momentum paired Fulde-Ferrell-Larkin-Ovchinnikov magnetized phase intervenes between the phase-separated and normal states. In contrast, for large negative detuning a uniform, polarized superfluid, that is, a coherent mixture of singlet Bose-Einstein-condensed molecules and fully magnetized single-species Fermi sea, is a stable ground state.

Qubit Assisted Probing of Coherence between Mesoscopic States of an Apparatus

Sougato Bose

Published 15 February 2006

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I present a general scheme through which the evidence of a superposition involving distinct states of a mesoscopic system can be probed. The scheme relies on a single qubit being coupled to a mesoscopic harmonic oscillator in such a way that it can be used to both prepare and probe a superposition of states of the oscillator. Two potentially realizable implementations, one with a single flux qubit coupled to an LC circuit, and the other with a single ion-trap qubit coupled to the collective motion of several ions, are proposed.

Driving Defect Modes of Bose-Einstein Condensates in Optical Lattices

Valeriy A. Brazhnyi, Vladimir V. Konotop, and Víctor M. Pérez-García

Published 16 February 2006

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We present an approximate analytical theory and direct numerical computation of defect modes of a Bose-Einstein condensate loaded in an optical lattice and subject to an additional localized (defect) potential. Some of the modes are found to be remarkably stable and can be driven along the lattice by means of a defect moving following a steplike function defined by the period of Josephson oscillations and the macroscopic stability of the atoms.

Critical Temperature of a Trapped Bose Gas: Comparison of Theory and Experiment

Matthew J. Davis and P. Blair Blakie

Published 16 February 2006

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We apply the projected Gross-Pitaevskii equation (PGPE) formalism to the experimental problem of the shift in critical temperature Tc of a harmonically confined Bose gas as reported in Gerbier et al., Phys. Rev. Lett. 92, 030405 (2004). The PGPE method includes critical fluctuations and we find the results differ from various mean-field theories, and are in best agreement with experimental data. To unequivocally observe beyond mean-field effects, however, the experimental precision must either improve by an order of magnitude, or consider more strongly interacting systems. This is the first application of a classical field method to make quantitative comparison with experiment.

Bose-Einstein Condensates in Rotating Lattices

Rajiv Bhat, M. J. Holland, and L. D. Carr

Published 17 February 2006

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Strongly interacting bosons in a two-dimensional rotating square lattice are investigated via a modified Bose-Hubbard Hamiltonian. Such a system corresponds to a rotating lattice potential imprinted on a trapped Bose-Einstein condensate. Second-order quantum phase transitions between states of different symmetries are observed at discrete rotation rates. For the square lattice we study, there are four possible ground-state symmetries.

Generic Bell Inequalities for Multipartite Arbitrary Dimensional Systems

W. Son, Jinhyoung Lee, and M. S. Kim

Published 17 February 2006

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We present generic Bell inequalities for multipartite arbitrary dimensional systems. The inequalities that any local realistic theory must obey are violated by quantum mechanics for even dimensional systems. A large set of variants are shown to naturally emerge from the generic Bell inequalities. We discuss particular variants of Bell inequalities that are violated for all the systems including odd dimensional systems.

Macroscopic Thermal Entanglement Due to Radiation Pressure

Aires Ferreira, Ariel Guerreiro, and Vlatko Vedral

Published 17 February 2006

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Can entanglement and the quantum behavior in physical systems survive at arbitrary high temperatures? In this Letter we show that this is the case for a electromagnetic field mode in an optical cavity with a movable mirror in a thermal state. We also identify two different dynamical regimes of generation of entanglement separated by a critical coupling strength.

Measuring Nonlinear Functionals of Quantum Harmonic Oscillator States

K. L. Pregnell

Published 13 February 2006

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Using only linear interactions and a local parity measurement we show how entanglement can be detected between two harmonic oscillators. The scheme generalizes to measure both linear and nonlinear functionals of an arbitrary oscillator state. This leads to many applications including purity tests, eigenvalue estimation, entropy, and distance measures—all without the need for nonlinear interactions or complete state reconstruction. Remarkably, experimental realization of the proposed scheme is already within the reach of current technology with linear optics.

Teleportation and Dense Coding with Genuine Multipartite Entanglement

Ye Yeo and Wee Kang Chua

Published 15 February 2006

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We present an explicit protocol [script E]

0 for faithfully teleporting an arbitrary two-qubit state via a genuine four-qubit entangled state. By construction, our four-partite state is not reducible to a pair of Bell states. Its properties are compared and contrasted with those of the four-party Greenberger-Horne-Zeilinger and W states. We also give a dense coding scheme [script D]

0 involving our state as a shared resource of entanglement. Both [script D]

0 and

0 indicate that our four-qubit state is a likely candidate for the genuine four-partite analogue to a Bell state.

Time-Optimal Quantum Evolution

Alberto Carlini, Akio Hosoya, Tatsuhiko Koike, and Yosuke Okudaira

Published 15 February 2006

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We present a general framework for finding the time-optimal evolution and the optimal Hamiltonian for a quantum system with a given set of initial and final states. Our formulation is based on the variational principle and is analogous to that for the brachistochrone in classical mechanics. We reduce the problem to a formal equation for the Hamiltonian which depends on certain constraint functions specifying the range of available Hamiltonians. For some simple examples of the constraints, we explicitly find the optimal solutions.

Demonstration of Quantum Telecloning of Optical Coherent States

Satoshi Koike, Hiroki Takahashi, Hidehiro Yonezawa, Nobuyuki Takei, Samuel L. Braunstein, Takao Aoki, and Akira Furusawa

Published 16 February 2006

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We demonstrate unconditional telecloning for the first time. In particular, we symmetrically and unconditionally teleclone coherent states of light from one sender to two receivers, achieving a fidelity for each clone of F=0.58±0.01, which surpasses the classical limit. This is a manipulation of a new type of multipartite entanglement whose nature is neither purely bipartite nor purely tripartite.

Fractionalization, Topological Order, and Quasiparticle Statistics

Masaki Oshikawa and T. Senthil

Published 13 February 2006

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We argue, based on general principles, that topological order is essential to realize fractionalization in gapped insulating phases in dimensions d

2. In d=2 with genus g, we derive the existence of the minimum topological degeneracy q^g if the charge is fractionalized in units of 1/q, irrespective of microscopic model or effective theory. Furthermore, if the quasiparticle is either boson or fermion, it must be at least q^2g.

Cold Atom Clock Test of Lorentz Invariance in the Matter Sector

Peter Wolf, Fréderic Chapelet, Sébastien Bize, and André Clairon

Published 15 February 2006

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We report on a new experiment that tests for a violation of Lorentz invariance (LI), by searching for a dependence of atomic transition frequencies on the orientation of the spin of the involved states (Hughes-Drever type experiment). The atomic frequencies are measured using a laser cooled ¹³³Cs atomic fountain clock, operating on a particular combination of Zeeman substates. We analyze the results within the framework of the Lorentz violating standard model extension (SME), where our experiment is sensitive to a largely unexplored region of the SME parameter space, corresponding to first measurements of four proton parameters and improvements by 11 and 13 orders of magnitude on the determination of four others. In spite of the attained uncertainties, and of having extended the search into a new region of the SME, we still find no indication of LI violation.

Cosmology with Massive Neutrinos Coupled to Dark Energy

A. W. Brookfield, C. van de Bruck, D. F. Mota, and D. Tocchini-Valentini

Published 13 February 2006

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Cosmological consequences of a coupling between massive neutrinos and dark energy are investigated. In such models, the neutrino mass is a function of a scalar field, which plays the role of dark energy. The evolution of the background and cosmological perturbations are discussed. We find that mass-varying neutrinos can leave a significant imprint on the anisotropies in the cosmic microwave background and even lead to a reduction of power on large angular scales.

Almost Certain Escape from Black Holes in Final State Projection Models

Seth Lloyd

Published 14 February 2006

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Recent models of the black-hole final state suggest that quantum information can escape from a black hole by a process akin to teleportation. These models rely on a controversial process called final-state projection. This Letter discusses the self-consistency of the final-state projection hypothesis and investigates escape from black holes for arbitrary final states and for generic interactions between matter and Hawking radiation. Quantum information escapes with fidelity [approximate]

(8/3

)²: only half a bit of quantum information is lost on average, independent of the number of bits that escape from the hole.

Testing for w<-1 in the Solar System

Jérôme Martin, Carlo Schimd, and Jean-Philippe Uzan

Published 16 February 2006

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In scalar-tensor theories of gravity, the equation of state of dark energy, w, can become smaller than -1 without violating any energy condition. The value of w today is tied to the level of deviations from general relativity which, in turn, is constrained by solar system and pulsars timing experiments. The conditions on these local constraints for w to be significantly less than -1 are established. It is demonstrated that this requires us to consider theories that differ from the Jordan-Fierz-Brans-Dicke theory and that involve either a steep coupling function or a steep potential. It is also shown how a robust measurement of w could probe scalar-tensor theories.

Observable N- Oscillation in High Scale Seesaw Models

Bhaskar Dutta, Yukihiro Mimura, and R. N. Mohapatra

Published 14 February 2006

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We discuss a realistic high scale (vB-L~10¹² GeV) supersymmetric seesaw model based on the gauge group SU(2)L×SU(2)R×SU(4)c where neutron-antineutron oscillation can be in the observable range. This is contrary to the naive dimensional arguments which say that tau

v

and should therefore be unobservable for seesaw scale vB-L

10⁵ GeV. Two reasons for this enhancement are (i) accidental symmetries which keep some of the diquark Higgs masses at the weak scale and (ii) a new supersymmetric contribution from a lower dimensional operator. The net result is that tau

v

v

rather than v B-L5

. The model also can explain the origin of matter via the leptogenesis mechanism and predicts light diquark states which can be produced at LHC.

Duality in Left-Right Symmetric Seesaw Mechanism

E. Kh. Akhmedov and M. Frigerio

Published 15 February 2006

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We consider type I+II seesaw mechanism, where the exchanges of both right-handed neutrinos and isotriplet Higgs bosons contribute to the neutrino mass. Working in the left-right symmetric framework and assuming the mass matrix of light neutrinos m

and the Dirac-type Yukawa couplings to be known, we find the triplet Yukawa coupling matrix f, which carries the information about the masses and mixing of the right-handed neutrinos. We show that in this case there exists a duality: for any solution f, there is a dual solution f^=m

/vL-f, where vL is the vacuum expectation value of the triplet Higgs boson. Thus, unlike in pure type I (II) seesaw, there is no unique allowed structure for the matrix f. For n lepton generations the number of solutions is 2ⁿ. We develop an exact analytic method of solving the seesaw nonlinear matrix equation for f.

Photoproduction on the Proton for Photon Energies from 1.527 to 2.227 GeV

M. Dugger et al. (CLAS Collaboration)

Published 15 February 2006

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Differential cross sections for the reaction gamma

p

p have been measured with the CLAS spectrometer and a tagged photon beam with energies from 1.527 to 2.227 GeV. The results reported here possess much greater accuracy than previous measurements. Analyses of these data suggest for the first time the coupling of the eta

N channel to both the S11(1535) and P11(1710) resonances, known to couple strongly to the eta

N channel in photoproduction on the proton, and the importance of J=3/2 resonances in the process.

Exclusive Measurement of the Nonmesonic Weak Decay of the He Hypernucleus

Published 14 February 2006

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We performed a coincidence measurement of two nucleons emitted from the nonmesonic weak decay of Lambda5

He formed via the ⁶Li(

⁺,K⁺) reaction. The energies of the two nucleons and the pair number distributions in the opening angle between them were measured. In both np and nn pairs, we observed a clean back-to-back correlation coming from the two-body weak reactions of Lambda

p

np and

n

nn, respectively. The ratio of the nucleon pair numbers was Nnn/Nnp=0.45±0.11(stat)±0.03(syst) in the kinematic region of cos theta

NN<-0.8. Since each decay mode was exclusively detected, the measured ratio should be close to the ratio of Gamma

(

p

np)/

(

n

nn). The ratio is consistent with recent theoretical calculations based on the heavy meson and/or direct-quark exchange picture.

Collective Non-Abelian Instabilities in a Melting Color Glass Condensate

Paul Romatschke and Raju Venugopalan

Published 14 February 2006

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We present first results for (3+1)D simulations of SU(2) Yang-Mills equations for matter expanding into the vacuum after a heavy ion collision. Violations of boost invariance cause a non-Abelian Weibel instability leading soft modes to grow with proper time tau

as exp(

), where g²µ is a scale arising from the saturation of gluons in the nuclear wave function. The scale for the growth rate Gamma

is set by a plasmon mass, defined as omega

pl=

0

, generated dynamically in the collision. We compare the numerical ratio Gamma

/

0 to the corresponding value predicted by the hard thermal loop formalism for anisotropic plasmas.

Strangeness Condensation by Expanding about the Fixed Point of the Harada-Yamawaki Vector Manifestation

G. E. Brown, Chang-Hwan Lee, Hong-Jo Park, and Mannque Rho

Published 17 February 2006

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Building on, and extending, the result of a higher-order in-medium chiral perturbation theory combined with renormalization group arguments and a variety of observations of the vector manifestation of Harada-Yamawaki hidden local symmetry theory, we obtain a surprisingly simple description of kaon condensation by fluctuating around the "vector manifestation" fixed point identified to be the chiral restoration point. Our development establishes that strangeness condensation takes place at ~3n0 where n0 is nuclear matter density. This result depends only on the renormalization-group (RG) behavior of the vector interactions, other effects involved in fluctuating about the bare vacuum in so many previous calculations being irrelevant in the RG about the fixed point. Our results have major effects on the collapse of neutron stars into black holes.

Fast Ground State Manipulation of Neutral Atoms in Microscopic Optical Traps

D. D. Yavuz, P. B. Kulatunga, E. Urban, T. A. Johnson, N. Proite, T. Henage, T. G. Walker, and M. Saffman

Published 14 February 2006

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We demonstrate Rabi flopping at MHz rates between ground hyperfine states of neutral ⁸⁷Rb atoms that are trapped in two micron sized optical traps. Using tightly focused laser beams we demonstrate high fidelity, site specific Rabi rotations with cross talk on neighboring sites separated by 8 µm at the level of 10^-3. Ramsey spectroscopy is used to measure a dephasing time of 870 µs, which is [approximate]

5000 times longer than the time for a

/2 pulse.

Chirped Attosecond Photoelectron Spectroscopy

G. L. Yudin, A. D. Bandrauk, and P. B. Corkum

Published 14 February 2006

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We study analytically the photoionization of a coherent superposition of electronic states and show that chirped pulses can measure attosecond time scale electron dynamics just as effectively as transform-limited attosecond pulses of the same bandwidth. The chirped pulse with a frequency-dependent phase creates the interfering photoelectron amplitudes that measure the electron dynamics. We show that at a given pump-probe time delay the differential asymmetry oscillates as a function of photoelectron energy. Our results suggest that the important parameter for attosecond science is not the pulse duration, but the bandwidth of phased radiation.

Fluorescence Emission of Excited Hydrogen Atoms after Core Excitation of Water Vapor

E. Melero García, A. Kivimäki, L. G. M. Pettersson, J. Álvarez Ruiz, M. Coreno, M. de Simone, R. Richter, and K. C. Prince

Published 17 February 2006

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The Balmer emission from atomic hydrogen has been recorded across the resonances at the oxygen K edge of the water molecule using synchrotron radiation excitation. The emission is observed to be strongest at excitations to Rydberg resonances. The observations are interpreted using a qualitative model for the dynamics of the core-to-Rydberg excited molecule. The model links the quantum state of the core-excited water molecule via resonant Auger decay and subsequent dissociation to the state of the fluorescing hydrogen atom.

Measurement of the Decay Rate of the Negative Ion of Positronium (Ps^-)

Frank Fleischer, Kai Degreif, Gerald Gwinner, Michael Lestinsky, Vitaly Liechtenstein, Florian Plenge, and Dirk Schwalm

Published 13 February 2006

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A new determination of the decay rate of the negative ion of positronium (Ps^-), using a beam-foil method and a stripping-based detection technique, is reported. The measured result of Gamma

=2.089(15) ns^-1 is a factor of 6 more precise than the previous experimental value of Gamma

=2.09(9) ns^-1, and is in excellent agreement with the theoretical value of Gamma

=2.086(6) ns^-1.

Harmonic Entanglement with Second-Order Nonlinearity

Nicolai B. Grosse, Warwick P. Bowen, Kirk McKenzie, and Ping Koy Lam

Published 13 February 2006

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We investigate the second-order nonlinear interaction as a means to generate entanglement between fields of differing wavelengths and show that perfect entanglement can, in principle, be produced between the fundamental and second-harmonic fields in these processes. Neither pure second-harmonic generation nor parametric oscillation optimally produce entanglement; such optimal entanglement is rather produced by an intermediate process.

Can Two-Photon Correlation of Chaotic Light Be Considered as Correlation of Intensity Fluctuations?

Giuliano Scarcelli, Vincenzo Berardi, and Yanhua Shih

Published 14 February 2006

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Two-photon correlation phenomena, including the historical experiment of Hanbury Brown and Twiss, may have to be described quantum mechanically, regardless of whether the source of radiation is classical or quantum. Supporting this point, we present a ghost imaging type of second-order spatial correlation experiment of chaotic light to show that the classical understanding based on the concept of statistical intensity fluctuations does not give a correct interpretation for the observation. From a practical point of view, this experiment demonstrates the possibility of having high contrast lensless two-photon imaging with chaotic light, suggesting imaging applications for radiations for which no effective lens is available.

Strong-Field Atomic Phase Matching

Carlos Trallero-Herrero, J. L. Cohen, and Thomas Weinacht

Published 16 February 2006

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We interpret a learning-control experiment with the goal of optimizing multiphoton population transfer in atomic sodium in the strong-field limit. Despite multiple experimental constraints, a learning algorithm discovers optimal pulses that can be understood in terms of a simple dynamic picture of the atom-field interaction. We show that the shaped pulses counteract the dynamic Stark-induced stimulated emission that would otherwise impede the efficient use of a

pulse to invert a multiphoton transition.

Observation of Discrete Surface Solitons

S. Suntsov, K. G. Makris, D. N. Christodoulides, G. I. Stegeman, A. Haché, R. Morandotti, H. Yang, G. Salamo, and M. Sorel

Published 13 February 2006

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We report the first observation of discrete optical surface solitons at the interface between a nonlinear self-focusing waveguide lattice and a continuous medium. The effect of power on the localization process of these optical self-trapped states at the edge of an AlGaAs waveguide array is investigated in detail. Our experimental results are in good agreement with theoretical predictions.

Nonlinear Magneto-Optical Bragg Gratings

Fredrik Jonsson and Christos Flytzanis

Published 13 February 2006

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In magneto-optical gratings the magnetic dipolar coupling superimposed on the electric dipolar one introduces nonreciprocity and spectral discrimination between circular polarization states, measured by a Zeeman-like splitting of the photonic Bragg resonances. In a nonlinear regime the degree of nonreciprocity is modified by the photoinduced interplay of these splittings and their Stark-like shifts. We predict novel magneto-optical modulation schemes for switching between orthogonal circular polarization states of transmission or reflection operated by means of an intense linearly polarized optical pulse train.

Conductivity and Permittivity of Two-Dimensional Metallic Photonic Crystals

A. Pimenov and A. Loidl

Published 15 February 2006

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Two-dimensional metallic photonic crystals with different filling factors were manufactured and investigated by broadband terahertz spectroscopy. This technique allowed an independent determination of conductivity and dielectric permittivity in an extremely large dynamic range. Nearly ideal plasmonic behavior is observed for all compositions. Transmittance maxima are observed close to the plasma frequency and attributed to the longitudinal resonance. The plasmon frequencies agree well with existing calculations, while damping effects are underestimated by almost 1 order of magnitude.

Observation of the Critical Regime Near Anderson Localization of Light

Martin Störzer, Peter Gross, Christof M. Aegerter, and Georg Maret

Published 15 February 2006

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The transition from diffusive transport to localization of waves should occur for any type of classical or quantum wave in any media as long as the wavelength becomes comparable to the transport mean free path [script-l]

^*. The signatures of localization and those of absorption, or bound states, can, however, be similar, such that an unequivocal proof of the existence of wave localization in disordered bulk materials is still lacking. Here we present time resolved measurements of light transport through strongly scattering samples with k [script-l]

^* values as low as 2.5. In transmission, we observe deviations from diffusion which cannot be explained by absorption, sample geometry, or reduction in transport velocity. Furthermore, the deviations from classical diffusion increase strongly with decreasing [script-l]

^* as expected for a phase transition. This constitutes an experimental realization of the critical regime in the approach to Anderson localization.

Delaying Transition to Turbulence by a Passive Mechanism

Jens H. M. Fransson, Alessandro Talamelli, Luca Brandt, and Carlo Cossu

Published 17 February 2006

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Reducing skin friction is important in nature and in many technological applications. This reduction may be achieved by reducing stresses in turbulent boundary layers, for instance tailoring biomimetic rough skins. Here we take a second approach consisting of keeping the boundary layer laminar as long as possible by forcing small optimal perturbations. Because of the highly non-normal nature of the underlying linearized operator, these perturbations are highly amplified and able to modify the mean velocity profiles at leading order. We report results of wind-tunnel experiments in which we implement this concept by using suitably designed roughness elements placed on the skin to enforce nearly optimal perturbations. We show that by using this passive control technique it is possible to sensibly delay transition to turbulence.

Coherent THz Synchrotron Radiation from a Storage Ring with High-Frequency RF System

F. Wang, D. Cheever, M. Farkhondeh, W. Franklin, E. Ihloff, J. van der Laan, B. McAllister, R. Milner, C. Tschalaer, D. Wang, D. F. Wang, A. Zolfaghari, T. Zwart, G. L. Carr, B. Podobedov, and F. Sannibale

Published 13 February 2006

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The generation of brilliant, stable, and broadband coherent synchrotron radiation (CSR) in electron storage rings depends strongly on ring rf system properties such as frequency and gap voltage. We have observed intense coherent radiation at frequencies approaching the THz regime produced by the MIT-Bates South Hall Ring, which employs a high-frequency S-band rf system. The measured CSR spectral intensity enhancement with 2 mA stored current was up to 10 000 times above background for wave numbers near 3 cm^-1. The measurements also uncovered strong beam instabilities that must be suppressed if such a very high rf frequency electron storage ring is to become a viable coherent THz source.

Intermediate Nonlinear Evolution of the Parker Instability: Formation of Convection-Induced Discontinuities and Absence of Finite-Time Singularities

P. Zhu, A. Bhattacharjee, and K. Germaschewski

Published 13 February 2006

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Direct numerical simulations of the line-tied Parker (or magnetic Rayleigh-Taylor) instability, based on the fully compressible ideal magnetohydrodynamic equations, are presented. In the intermediate nonlinear phase, the instability continues to grow exponentially in time and the plasma tends to develop convection-induced discontinuities in the form of shocklike coherent structures. No evidence of finite-time singularities is seen.

Electric Field in a Double Layer and the Imparted Momentum

A. Fruchtman

Published 14 February 2006

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It is shown that the net momentum delivered by the large electric field inside a one-dimensional double layer is zero. This is demonstrated through an analysis of the momentum balance in the double layer at the boundary between the ionosphere and the aurora cavity. For the recently observed double layer in a current-free plasma expanding along a divergent magnetic field, an analysis of the evolution of the radially averaged variables shows that the increase of plasma thrust results from the magnetic-field pressure balancing the plasma pressure in the direction of acceleration, rather than from electrostatic pressure.

Liquid-State Properties of a One-Component Plasma

Jérôme Daligault

Published 17 February 2006

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As a consequence of strong collective behavior, the microscopic dynamics of the one-component plasma (OCP) differs significantly from that of ordinary liquids. We show that, when particle caging dominates, the OCP transport coefficients nevertheless satisfy universal laws satisfied by dense ordinary fluids: the Stokes-Einstein relation, the Arrhenius law of viscosity, and several excess-entropy scaling relations. These results extend to long-range interaction potentials, the unifying description of atomic transport in condensed matter.

Anisotropic System of Quasiparticles in Superfluid Helium

I. N. Adamenko, K. E. Nemchenko, V. A. Slipko, and A. F. G. Wyatt

Published 13 February 2006

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The thermodynamic properties of anisotropic quasiparticle systems of He II are considered for all degrees of anisotropy. It is shown that the thermodynamic functions of a strongly anisotropic phonon-roton system are mainly determined by rotons at all temperatures. Analytical expressions for the roton thermodynamic functions are obtained for all degrees of anisotropy. The maximum anisotropy is limited by the criterion for thermodynamic stability, which is here derived for the whole temperature range.

Breaking of Chiral Symmetry and Spontaneous Rotation in a Spinor Bose-Einstein Condensate

Hiroki Saito, Yuki Kawaguchi, and Masahito Ueda

Published 15 February 2006

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We show that a spin-1 Bose-Einstein condensate with ferromagnetic interactions spontaneously generates a topological spin texture, in which the m=±1 components of the magnetic sublevels form vortices with opposite circulations. This phenomenon originates from an interplay between ferromagnetic interactions and spin conservation.

Stability of Quantum Breathers

L. S. Schulman, D. Tolkunov, and E. Mihokova

Published 13 February 2006

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Using two methods we show that a quantized discrete breather in a 1D lattice is stable. One method uses path integrals and compares correlations for a (linear) local mode with those of the quantum breather. The other takes a local mode as the zeroth order system relative to which numerical, cutoff-insensitive diagonalization of the Hamiltonian is performed.

Shock-Induced Structural Phase Transition, Plasticity, and Brittle Cracks in Aluminum Nitride Ceramic

Paulo S. Branicio, Rajiv K. Kalia, Aiichiro Nakano, and Priya Vashishta

Published 15 February 2006

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Atomistic mechanisms of fracture accompanying structural phase transformation (SPT) in AlN ceramic under hypervelocity impact are investigated using a 209×10⁶ atom molecular-dynamics simulation. The shock wave generated by the impact splits into an elastic wave and a slower SPT wave that transforms the wurtzite structure into the rocksalt phase. The interaction between the reflected elastic wave and the SPT wave front generates nanovoids and dislocations into the wurtzite phase. Nanovoids coalesce into mode I cracks while dislocations give rise to kink bands and mode II cracking.

Anomalous Latent Heat in Nonequilibrium Phase Transitions

A. E. Allahverdyan and K. G. Petrosyan

Published 13 February 2006

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We study first-order phase transitions in a two-temperature system, where due to the time-scale separation all the basic thermodynamical quantities (free energy, entropy, etc.) are well defined. The sign of the latent heat is found to be counterintuitive: it is positive when going from the phase where the temperatures and the entropy are higher to the one where these quantities are lower. The effect exists only out of equilibrium and requires conflicting interactions. It is displayed on a lattice gas model of ferromagnetically interacting spin-1/2 particles.

Glassy Behavior of Light

L. Angelani, C. Conti, G. Ruocco, and F. Zamponi

Published 16 February 2006

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We study the nonlinear dynamics of a multimode random laser using the methods of statistical physics of disordered systems. A replica-symmetry breaking phase transition is predicted as a function of the pump intensity. We thus show that light propagating in a random nonlinear medium displays glassy behavior; i.e., the photon gas has a multitude of metastable states and a nonvanishing complexity, corresponding to mode-locking processes in random lasers. The present work reveals the existence of new physical phenomena, and demonstrates how nonlinear optics and random lasers can be a benchmark for the modern theory of complex systems and glasses.

Anomalous Ion Accelerated Bulk Diffusion of Interstitial Nitrogen

Gintautas Abrasonis, Wolfhard Möller, and Xin Xin Ma

Published 13 February 2006

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Interstitial N diffusion under low energy (~700 eV) Ar⁺ bombardment at 673 K in ion beam nitrided austenitic stainless steel is investigated. Ar⁺ ion bombardment increases the N mobility in depths far beyond the ion penetration depth, resulting in an increased broadening of the N depth profile as a function of Ar⁺ flux. This effect cannot be explained by any established mechanism of radiation-enhanced diffusion. An explanation based on quasiparticle-enhanced mobility is proposed.

Large Slip of Aqueous Liquid Flow over a Nanoengineered Superhydrophobic Surface

Chang-Hwan Choi and Chang-Jin Kim

Published 16 February 2006

066001 Full Text: PDF (257 kB) | Buy Article
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While many recent studies have confirmed the existence of liquid slip over certain solid surfaces, there has not been a deliberate effort to design and fabricate a surface that would maximize the slip under practical conditions. Here, we have engineered a nanostructured superhydrophobic surface that minimizes the liquid-solid contact area so that the liquid flows predominantly over a layer of air. Measured through a cone-and-plate rheometer system, the surface has demonstrated dramatic slip effects: a slip length of ~20 µm for water flow and ~50 µm for 30 wt % glycerin. The essential geometrical characteristics lie with the nanoposts populated on the surface: tall and slender (i.e., needlelike) profile and submicron periodicity (i.e., pitch).

Less Strain Energy despite Fewer Misfit Dislocations: The Impact of Ordering

Th. Schmidt, R. Kröger, J. I. Flege, T. Clausen, J. Falta, A. Janzen, P. Zahl, P. Kury, M. Kammler, and M. Horn-von Hoegen

Published 13 February 2006

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The average strain state of Ge films grown on Si(111) by surfactant mediated epitaxy has been compared to the ordering of the interfacial misfit dislocation network. Surprisingly, a smaller degree of average lattice relaxation was found in films grown at higher temperature. On the other hand, these films exhibit a better ordered dislocation network. This effect energetically compensates the higher strain at higher growth temperature, leading to the conclusion that, apart from the formation of misfit dislocations, their ordering represents an important channel for lattice-strain energy relaxation.

Graphitic Nanofilms as Precursors to Wurtzite Films: Theory

Colin L. Freeman, Frederik Claeyssens, Neil L. Allan, and John H. Harding

Published 13 February 2006

066102 Full Text: PDF (260 kB) | Buy Article

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Periodic ab initio density functional calculations on ultrathin films of AlN, BeO, GaN, SiC, ZnO, and ZnS demonstrate the stabilization of thicker films terminating with the polar {0001} surface via charge transfer and metallization of the surface layers. In contrast thinner films remove the dipole by adopting a graphiticlike structure in which the atoms are threefold coordinate. This structure is thermodynamically the most favorable for these thinner films. Implications for the crystal growth of wurtzite materials are discussed.

Critical Adsorption of Polyelectrolytes onto Charged Spherical Colloids

Roland G. Winkler and Andrey G. Cherstvy

Published 13 February 2006

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The adsorption of a flexible polyelectrolyte in a salt solution onto an oppositely charged spherical surface is investigated. An analytical solution is derived, which is valid for any sphere radius and consistently recovers the result of a planar surface in the limit of large sphere radii, by substituting the Debye-Hückel potential via the Hulthén potential. Expressions for critical quantities such as the critical radius and the critical surface charge density are provide. A comparison of our theoretical results with experiments and computer simulations yields remarkable good agreement.

Self-Assembly of Gradient Concentric Rings via Solvent Evaporation from a Capillary Bridge

Jun Xu, Jianfeng Xia, Suck Won Hong, Zhiqun Lin, Feng Qiu, and Yuliang Yang

Published 13 February 2006

066104 Full Text: PDF (485 kB) | Buy Article

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A drop of solution containing nonvolatile solute is allowed to evaporate from a sphere-on-flat geometry. Left behind is a striking pattern of gradient concentric rings with unprecedented regularity. The center-to-center distance between adjacent rings, lambda

C-C, and the height of the ring, hd, are strongly affected by the concentration of the solution and the properties of the solvent. The nature of the formation of regular gradient ring patterns during the course of irreversible solvent evaporation is revealed through theoretical calculations based on the mass conservation of the solution.

Molecular Recoiling in Polymer Thin Film Dewetting

M. H. Yang, S. Y. Hou, Y. L. Chang, and A. C.-M. Yang

Published 14 February 2006

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The molecular recoiling force stemming from nonequilibrium chain conformation was found to play a very important role in the dewetting stability of polymer thin films. Correct measurements and inclusion of this molecular force into thermodynamic consideration are crucial for analyzing dewetting phenomena and nanoscale polymer chain physics. This force was measured using a simple method based on contour relaxation at the incipient dewetting holes. The recoiling stress was found to increase dramatically with molecular weight and decreasing film thickness. The corresponding forces were calculated to be in the range from 9.0 to 28.2 mN/m, too large to be neglected when compared to the dispersive forces (~10 mN/m) commonly operative in thin polymer films.

Transition Metal Oxides: Extra Thermodynamic Stability as Thin Films

Charles T. Campbell

Published 14 February 2006

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The oxides of many transition metals wet their own metal surface. The adhesion energy at this interface (Eadh,ox/m) provides extra stabilization, which lowers the O2 pressure required for oxide stability as a thin film below that required for bulk-oxide stability by the factor exp[(2 gamma

g/ox-Eadh,ox/m)/(tNoxRT)], where gamma

g/ox is the surface energy of the oxide, t is the oxide film thickness, and Nox is the oxygen concentration in the bulk oxide (moles O2 per volume). For oxide films only ~1 nm thick, this correction can be many orders of magnitude. This may extend to other compounds.

Formation and Splitting of Paired Hydroxyl Groups on Reduced TiO₂(110)

S. Wendt, J. Matthiesen, R. Schaub, E. K. Vestergaard, E. Lægsgaard, F. Besenbacher, and B. Hammer

Published 14 February 2006

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A combination of high-resolution scanning tunneling microscopy and density functional theory is utilized to study the interaction of water with the reduced TiO2(110)-(1×1) surface. As the direct product of water dissociation in oxygen vacancies, paired hydroxyl groups are formed. These pairs are immobile and stable unless they interact with adsorbed water molecules. As a result of these interactions, protons are transferred to adjacent oxygen rows, thereby forming single hydroxyl groups. Additionally, we show that hydroxyl groups facilitate the diffusion of water molecules over the oxygen rows.

Controlling Planar and Vertical Ordering in Three-Dimensional (In,Ga)As Quantum Dot Lattices by GaAs Surface Orientation

M. Schmidbauer, Sh. Seydmohamadi, D. Grigoriev, Zh. M. Wang, Yu. I. Mazur, P. Schäfer, M. Hanke, R. Köhler, and G. J. Salamo

Published 15 February 2006

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Anisotropic surface diffusion and strain are used to explain the formation of three-dimensional (In,Ga)As quantum dot lattices. The diffusion characteristics of the surface, coupled with the elastic anisotropy of the matrix, provides an excellent opportunity to influence the dot positions. In particular, quantum dots that are laterally organized into long chains or chessboard two-dimensional arrays vertically organized with strict vertical ordering or vertical ordering that is inclined to the sample surface normal are accurately predicted and observed.

Dopant-Induced Nanoscale Electronic Inhomogeneities in Ca_2-xSr_xRuO₄

Jiandi Zhang, Ismail, R. G. Moore, S.-C. Wang, H. Ding, R. Jin, D. Mandrus, and E. W. Plummer

Published 14 February 2006

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Ca2-xSrxRuO4 single crystals with 0.1

x

2.0 have been studied systematically using scanning tunneling microscopy (STM) and spectroscopy, low-energy electron diffraction, and angle resolved photoelectron spectroscopy (ARPES). In contrast with the well-ordered lattice structure, the local density of states at the surface clearly shows a strong doping dependent nanoscale electronic inhomogeneity, regardless of the fact of isovalent substitution. Remarkably, the surface electronic roughness measured by STM and the inverse spectral weight of quasiparticle states determined by ARPES are found to vary with x in the same manner as the bulk in-plane residual resistivity, following the Nordheim rule. For the first time, the surface measurements—especially those with STM—are shown to be in good agreement with the bulk transport results, all clearly indicating a doping-induced electronic disorder in the system.

The - Transition of Cerium Is Entropy Driven

B. Amadon, S. Biermann, A. Georges, and F. Aryasetiawan

Published 15 February 2006

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We emphasize, on the basis of experimental data and theoretical calculations, that the entropic stabilization of the gamma

phase is the main driving force of the alpha

-

transition of cerium in a wide temperature range below the critical point. Using a formulation of the total energy as a functional of the local density and of the f-orbital local Green's functions, we perform dynamical mean-field theory calculations within a new implementation based on the multiple linear muffin tin orbital (LMTO) method, which allows us to include semicore states. Our results are consistent with the experimental energy differences and with the qualitative picture of an entropy-driven transition, while also confirming the appearance of a stabilization energy of the alpha

phase as the quasiparticle Kondo resonance develops.

Probing Semiconductor Gap States with Resonant Tunneling

S. Loth, M. Wenderoth, L. Winking, R. G. Ulbrich, S. Malzer, and G. H. Döhler

Published 15 February 2006

066403 Full Text: PDF (1437 kB) | Buy Article

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Tunneling transport through the depletion layer under a GaAs {110} surface is studied with a low temperature scanning tunneling microscope (STM). The observed negative differential conductivity is due to a resonant enhancement of the tunneling probability through the depletion layer mediated by individual shallow acceptors. The STM experiment probes, for appropriate bias voltages, evanescent states in the GaAs band gap. Energetically and spatially resolved spectra show that the pronounced anisotropic contrast pattern of shallow acceptors occurs exclusively for this specific transport channel. Our findings suggest that the complex band structure causes the observed anisotropies connected with the zinc blende symmetry.

Exact Transformation for Spin-Charge Separation of Spin-1/2 Fermions without Constraints

Stellan Östlund and Mats Granath

Published 15 February 2006

066404 Full Text: PDF (155 kB) | Buy Article

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We demonstrate an exact local transformation which maps a purely Fermionic many-body system to a system of spinful bosons and spinless fermions, demonstrating a possible path to a non-Fermi-liquid state. We apply this to the half-filled Hubbard model and show how the transformation maps the ordinary spin half Fermionic degrees of freedom exactly and without introducing Hilbert space constraints to a chargelike quasicharge fermion and a spinlike quasispin Boson while preserving all the symmetries of the model. We present approximate solutions with localized charge which emerge naturally from the Hubbard model in this form. Our results strongly suggest that charge tends to remain localized for large values of the Hubbard U.

Thermodynamics and Excitations of Condensed Polaritons in Disordered Microcavities

F. M. Marchetti, J. Keeling, M. H. Szyma

ska, and P. B. Littlewood

Published 17 February 2006

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We study the thermodynamic condensation of microcavity polaritons using a realistic model of disorder in semiconductor quantum wells. This approach correctly describes the polariton inhomogeneous broadening in the low density limit, and treats scattering by disorder to all orders in the condensed regime. While the weak disorder changes the thermodynamic properties of the transition little, the effects of disorder in the condensed state are prominent in the excitations and can be seen in resonant Rayleigh scattering.

Consistency in Formulation of Spin Current and Torque Associated with a Variance of Angular Momentum

Yong Wang, Ke Xia, Zhao-Bin Su, and Zhongshui Ma

Published 13 February 2006

066601 Full Text: PDF (99 kB) | Buy Article

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Based on the Noether's theorem, we develop systematically and rigorously the spin-dependent formulation of the conservation laws. The effect of the electronic polarization due to the spin-orbit coupling is included in the Maxwell equations. The polarization is related to the antisymmetric components of spin current, and it provides a possibility to measure the spin current directly. The variances of spin and orbit angular momentum currents imply a torque on the "electric dipole" associated with the moving electron. The dependencies of the torque on the polarization and the force on the motions of spin-polarized electrons in a two-dimensional electron gas with the Rashba spin-orbit coupling are discussed.

Tunneling and Nonuniversality in Continuum Percolation Systems

C. Grimaldi and I. Balberg

Published 14 February 2006

066602 Full Text: PDF (194 kB) | Buy Article

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The values obtained experimentally for the conductivity critical exponent in numerous percolation systems, in which the interparticle conduction is by tunneling, were found to be in the range of t0 and about t0+10, where t0 is the universal conductivity exponent. These latter values are, however, considerably smaller than those predicted by the available "one-dimensional"-like theory of tunneling percolation. In this Letter, we show that this long-standing discrepancy can be resolved by considering the more realistic "three-dimensional" model and the limited proximity to the percolation threshold in all the many available experimental studies.

Dynamics of Localized Spins Coupled to the Conduction Electrons with Charge and Spin Currents

Masaru Onoda and Naoto Nagaosa

Published 16 February 2006

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The dynamics of the localized spins coupled to the conduction electrons is studied theoretically in the wide range of magnitudes of the charge and spin currents including the regime which has never been explored but is now possible in terms of the pure spin-current injection methods, e.g., the spin Hall effect and spin battery. The equations of motion for the two-spin system are investigated in detail, and its phase diagram of the dynamics is presented. It is found that the dynamics depends sensitively upon the relative magnitudes of the charge and spin currents; i.e., it shows steady state, periodic motion, and even chaotic behavior. The extension to the multispin system and its implications including a possible "spin-current detector" are also discussed.

Charging Mechanisms of Trapped Element-Selectively Excited Nanoparticles Exposed to Soft X Rays

M. Grimm, B. Langer, S. Schlemmer, T. Lischke, U. Becker, W. Widdra, D. Gerlich, R. Flesch, and E. Rühl

Published 13 February 2006

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Charging mechanisms of trapped, element-selectively excited free SiO2 nanoparticles by soft x rays are reported. The absolute charge state of the particles is measured and the electron emission probability is derived. Changes in electron emission processes as a function of photon energy and particle charge are obtained from the charging current. This allows us to distinguish contributions from primary photoelectrons, Auger electrons, and secondary electrons. Processes leading to no change in charge state after absorption of x-ray photons are identified. O 1s-excited SiO2 particles of low charge state indicate that the charging current follows the inner-shell absorption. In contrast, highly charged SiO2 nanoparticles are efficiently charged by resonant Auger processes, whereas direct photoemission and normal Auger processes do not contribute to changes in particle charge. These results are discussed in terms of an electrostatic model.

Transport Properties of Strongly Correlated Electrons in Quantum Dots Studied with a Simple Circuit Model

G. B. Martins, C. A. Büsser, K. A. Al-Hassanieh, E. V. Anda, A. Moreo, and E. Dagotto

Published 13 February 2006

066802 Full Text: PDF (1521 kB) | Buy Article

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Numerical calculations are shown to reproduce the main results of recent experiments involving nonlocal spin control in quantum dots [Craig et al., Science 304, 565 (2004).]. In particular, the experimentally reported zero-bias-peak splitting is clearly observed in our studies. To understand these results, a simple "circuit model" is introduced and shown to qualitatively describe the experiments. The main idea is that the splitting originates in a Fano antiresonance, which is caused by having one quantum dot side connected in relation to the current's path. This scenario provides an explanation of the results of Craig et al. that is an alternative to the RKKY proposal, also addressed here.

Two-Electron Linear Intersubband Light Absorption in a Biased Quantum Well

J. Dai, M. E. Raikh, and T. V. Shahbazyan

Published 13 February 2006

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We point out a novel manifestation of many-body correlations in the linear optical response of electrons confined in a quantum well. Namely, we demonstrate that along with the conventional absorption peak at a frequency omega

close to the intersubband energy Delta

, there exists an additional peak at frequency [h-bar]

2

. This new peak is solely due to electron-electron interactions, and can be understood as excitation of two electrons by a single photon. The actual peak line shape is comprised of a sharp feature, due to excitation of pairs of intersubband plasmons, on top of a broader band due to absorption by two single-particle excitations. The two-plasmon contribution allows us to infer intersubband plasmon dispersion from linear absorption experiments.

Semiclassical Theory of Chaotic Conductors

Stefan Heusler, Sebastian Müller, Petr Braun, and Fritz Haake

Published 15 February 2006

066804 Full Text: PDF (132 kB) | Buy Article

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We calculate the Landauer conductance through chaotic ballistic devices in the semiclassical limit, to all orders in the inverse number of scattering channels without and with a magnetic field. Families of pairs of entrance-to-exit trajectories contribute, similarly to the pairs of periodic orbits making up the small-time expansion of the spectral form factor of chaotic dynamics. As a clue to the exact result we find that close self-encounters slightly hinder the escape of trajectories into leads. Our result explains why the energy-averaged conductance of individual chaotic cavities, with disorder or "clean," agrees with predictions of random-matrix theory.

Interplay of Spin and Charge Channels in Zero-Dimensional Systems

M. N. Kiselev and Yuval Gefen

Published 17 February 2006

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We study the interplay of charge and spin (zero-mode) channels in quantum dots. The latter affects the former in the form of a distinct signature on the differential conductance. We also obtain both longitudinal and transverse spin susceptibilities. All these observables, underlain by spin fluctuations, become accentuated as one approaches the Stoner instability. The nonperturbative effects of zero-mode interaction are described in terms of the propagation of gauge bosons associated with charge [U(1)] and spin [SU(2)] fluctuations in the dot, while transverse spin fluctuations are analyzed perturbatively.

Parity of the Pairing Bosons in a High-Temperature Pb-Bi₂Sr₂CaCu₂O₈ Bilayer Superconductor by Angle-Resolved Photoemission Spectroscopy

S. V. Borisenko, A. A. Kordyuk, A. Koitzsch, J. Fink, J. Geck, V. Zabolotnyy, M. Knupfer, B. Büchner, H. Berger, M. Falub, M. Shi, J. Krempasky, and L. Patthey

Published 13 February 2006

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We report the observation of a novel effect in the bilayer Pb-Bi2Sr2CaCu2O8 (Pb-Bi2212) high-Tc superconductor by means of angle-resolved photoemission with circularly polarized excitation. Different scattering rates, determined as a function of energy separately for the bonding and antibonding copper-oxygen bands, strongly imply that the dominating scattering channel is odd with respect to layer exchange within a bilayer. This is inconsistent with a phonon-mediated scattering and favors the participation of the odd collective spin excitations in the scattering mechanism in near-nodal regions of the k space, suggesting a magnetic nature of the pairing mediator.

Nernst Effect and Disorder in the Normal State of High-T_c Cuprates

F. Rullier-Albenque, R. Tourbot, H. Alloul, P. Lejay, D. Colson, and A. Forget

Published 13 February 2006

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We have studied the influence of disorder induced by electron irradiation on the Nernst effect in optimally and underdoped YBa2Cu3O7- delta

single crystals. The fluctuation regime above Tc expands significantly with disorder, indicating that the Tc decrease is partly due to the induced loss of phase coherence. In pure crystals the temperature extension of the Nernst signal is found to be narrow whatever the hole doping, contrary to data reported in the low-Tc cuprate families. Our results show that the presence of intrinsic disorder can explain the enhanced range of the Nernst signal found in the pseudogap phase of the latter compounds.

Controllable Coupling between Flux Qubits

Yu-xi Liu, L. F. Wei, J. S. Tsai, and Franco Nori

Published 15 February 2006

067003 Full Text: PDF (158 kB)

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We propose an experimentally realizable method to control the coupling between two flux qubits. In our proposal, the bias fluxes are always fixed for these two inductively coupled qubits. The detuning of these two qubits can be initially chosen to be sufficiently large, so that their initial interbit coupling is almost negligible. When a variable frequency or time-dependent magnetic flux (TDMF) is applied to one of the qubits, a well-chosen frequency of the TDMF can be used to compensate the initial detuning and to couple two qubits. This proposed method avoids fast changes of either qubit frequencies or the amplitudes of the bias magnetic fluxes through the qubit loops, and also offers a remarkable way to implement any logic gate, as well as tomographically measure flux qubit states.

Field- and Pressure-Induced Phases in Sr₄Ru₃O₁₀: A Spectroscopic Investigation

Rajeev Gupta, M. Kim, H. Barath, S. L. Cooper, and G. Cao

Published 15 February 2006

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We have investigated the magnetic-field- and pressure-induced structural and magnetic phases of the triple-layer ruthenate Sr4Ru3O10. Magnetic-field-induced changes in the phonon spectra reveal dramatic spin-reorientation transitions and strong magnetoelastic coupling in this material. Further, we are able to deduce key magnetoelastic coupling parameters, and evidence that the magnetic moments are localized on the Ru sites. Additionally, pressure-dependent Raman measurements at different temperatures reveal an anomalous negative Gruneisen parameter associated with the B1g mode (~380 cm^-1) at low temperatures (T<75 K), which can be explained consistently with the field-dependent Raman data.

Elastic Scattering Susceptibility of the High Temperature Superconductor Bi₂Sr₂CaCu₂O₈₊: A Comparison between Real and Momentum Space Photoemission Spectroscopies

K. McElroy, G.-H. Gweon, S. Y. Zhou, J. Graf, S. Uchida, H. Eisaki, H. Takagi, T. Sasagawa, D.-H. Lee, and A. Lanzara

Published 15 February 2006

067005 Full Text: PDF (548 kB) | Buy Article

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The joint density of states of Bi2Sr2CaCu2O8+ delta

is calculated by evaluating the autocorrelation of the single particle spectral function A(k -->

,

) measured from angle resolved photoemission spectroscopy (ARPES). These results are compared with Fourier transformed (FT) conductance modulations measured by scanning tunneling microscopy (STM). Good agreement between the two experimental probes is found for two different doping values examined. In addition, by comparing the FT-STM results to the autocorrelated ARPES spectra with different photon polarization, new insight on the form of the STM matrix elements is obtained. This shines new light on unsolved mysteries in the tunneling data.

Exchange Bias Training Effect in Coupled All Ferromagnetic Bilayer Structures

Ch. Binek, S. Polisetty, Xi He, and A. Berger

Published 13 February 2006

067201 Full Text: PDF (217 kB) | Buy Article

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Exchange coupled bilayers of soft and hard ferromagnetic thin films show remarkable analogies to conventional antiferromagnetic/ferromagnetic exchange bias heterostructures. Not only do all these ferromagnetic bilayers exhibit a tunable exchange bias effect, they also show a distinct training behavior upon cycling the soft layer through consecutive hysteresis loops. In contrast with conventional exchange bias systems, such all ferromagnetic bilayer structures allow the observation of training induced changes in the bias-setting hardmagnetic layer by means of simple magnetometry. Our experiments show unambiguously that the exchange bias training effect is driven by deviations from equilibrium in the pinning layer. A comparison of our experimental data with predictions from a theory based upon triggered relaxation phenomena shows excellent agreement.

Transport in Almost Integrable Models: Perturbed Heisenberg Chains

P. Jung, R. W. Helmes, and A. Rosch

Published 14 February 2006

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The heat conductivity kappa

(T) of integrable models, like the one-dimensional spin-1/2 nearest-neighbor Heisenberg model, is infinite even at finite temperatures as a consequence of the conservation laws associated with integrability. Small perturbations lead to finite but large transport coefficients which we calculate perturbatively using exact diagonalization and moment expansions. We show that there are two different classes of perturbations. While an interchain coupling of strength J [perpendicular]

leads to

(T)

1/J

as expected from simple golden-rule arguments, we obtain a much larger kappa

(T)

1/J⁴ for a weak next-nearest-neighbor interaction J. This can be explained by a new approximate conservation law of the J-J Heisenberg chain.

Electronic Structure of Charge- and Spin-Controlled Sr_1-(x+y)La_x+yTi_1-xCr_xO₃

H. Iwasawa, K. Yamakawa, T. Saitoh, J. Inaba, T. Katsufuji, M. Higashiguchi, K. Shimada, H. Namatame, and M. Taniguchi

Published 14 February 2006

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We present the electronic structure of Sr1-(x+y)Lax+yTi1-xCrxO3 investigated by high-resolution photoemission spectroscopy. In the vicinity of the Fermi level, it was found that the electronic structure was composed of a Cr 3d local state with the t 2g3

configuration and a Ti 3d itinerant state. The energy levels of these Cr and Ti 3d states are well interpreted by the difference of the charge-transfer energy of both ions. The spectral weight of the Cr 3d state is completely proportional to the spin concentration x irrespective of the carrier concentration y, indicating that the spin density can be controlled by x as desired. In contrast, the spectral weight of the Ti 3d state is not proportional to y, depending on the amount of Cr doping.

Effect of Spin-Orbit Interaction on a Magnetic Impurity in the Vicinity of a Surface

L. Szunyogh, G. Zaránd, S. Gallego, M. C. Muñoz, and B. L. Györffy

Published 14 February 2006

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We propose a new mechanism for surface-induced magnetic anisotropy to explain the thickness dependence of the Kondo resistivity of thin films of dilute magnetic alloys. The surface anisotropy energy, generated by spin-orbit coupling on the magnetic impurity itself, is an oscillating function of the distance d from the surface and decays as 1/d². Numerical estimates based on simple models suggest that this mechanism, unlike its alternatives, gives rise to an effect of the desired order of magnitude.

Magnetic Vortex Core Dynamics in Cylindrical Ferromagnetic Dots

K. Yu. Guslienko, X. F. Han, D. J. Keavney, R. Divan, and S. D. Bader

Published 16 February 2006

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Physical Review Letters

17 February 2006

Volume 96, Number 6 , Articles (06xxxx)

Articles published 11 February - 17 February 2006

LETTERS

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

Gravitation and Astrophysics

Elementary Particles and Fields

Nuclear Physics

Atomic, Molecular, and Optical Physics

Nonlinear Dynamics, Fluid Dynamics, Classical Optics, etc.

Plasma and Beam Physics

Condensed Matter: Structure, etc.

Condensed Matter: Electronic Properties, etc.