Physical Review Letters

22 December 2006

Volume 97, Number 25 , Articles (25xxxx)

Articles published 16 December - 22 December 2006

Calculated distribution and intensity (from blue to red) of long-lived modes (dots) that form along periodic rays (dashed lines) in an optical resonator with a semistadium shape.

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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.

Soft Matter, Biological, and Interdisciplinary Physics

COMMENTS

ERRATA

LETTERS

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

Superfluid Transition in a Rotating Fermi Gas with Resonant Interactions

Martin Y. Veillette, Daniel E. Sheehy, Leo Radzihovsky, and Victor Gurarie

Published 19 December 2006
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We study a rotating atomic Fermi gas near a narrow s-wave Feshbach resonance in a uniaxial trap with frequencies , z. We predict the upper-critical angular velocity, c2(,T), as a function of temperature T and detuning across the BEC-BCS crossover. The suppression of superfluidity at c2 is distinct in the BCS and BEC regimes, with the former controlled by depairing and the latter by the dilution of bosonic molecules. At low T and z, in the BCS and crossover regimes of 0c, c2 is implicitly given by 2, vanishing as c2~(1-/c)^1/2 near c2F+Fln(F/) (with the BCS gap and the resonance width), and extending the bulk result c22²/F to a trap. In the BEC regime of <0 we find c2, where molecular superfluidity is destroyed only by large quantum fluctuations associated with comparable boson and vortex densities.

Comparing Contact and Dipolar Interactions in a Bose-Einstein Condensate

Axel Griesmaier, Jürgen Stuhler, Tobias Koch, Marco Fattori, Tilman Pfau, and Stefano Giovanazzi

Published 21 December 2006
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We have measured the relative strength dd of the magnetic dipole-dipole interaction compared with the contact interaction in a dipolar chromium Bose-Einstein condensate. We analyze the asymptotic velocities of expansion of the condensate with different orientations of the atomic magnetic moments. By comparing the experimental results with numerical solutions of the hydrodynamic equations for dipolar condensates, we obtain dd=0.159±0.034. We use this result to determine the s-wave scattering length a=(5.08±1.06×10^-9) m=(96±20) a0 of ⁵²Cr. This is fully consistent with our previous measurements on the basis of Feshbach resonances and therefore confirms the validity of the theoretical approach used to describe the dipolar Bose-Einstein condensate.

Experimental Evidence for the Breakdown of a Hartree-Fock Approach in a Weakly Interacting Bose Gas

J.-B. Trebbia, J. Esteve, C. I. Westbrook, and I. Bouchoule

Published 22 December 2006
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We investigate the physics underlying the presence of a quasicondensate in a nearly one dimensional, weakly interacting trapped atomic Bose gas. We show that a Hartree-Fock (mean-field) approach fails to predict the existence of the quasicondensate in the center of the cloud: the quasicondensate is generated by interaction-induced correlations between atoms and not by a saturation of the excited states. Numerical calculations based on Bogoliubov theory give an estimate of the crossover density in agreement with experimental results.

Quest for Quantum Superpositions of a Mirror: High and Moderately Low Temperatures

József Zsolt Bernád, Lajos Diósi, and Tamás Geszti

Published 22 December 2006
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The Born-Markov master equation analysis of the vibrating mirror and photon experiment proposed by Marshall, Simon, Penrose, and Bouwmeester is completed by including the important issues of temperature and friction. We find that at the level of cooling available to date, visibility revivals are purely classical, and no quantum effect can be detected by the setup, no matter how strong the photon-mirror coupling is. Checking proposals of universal nonenvironmental decoherence is ruled out by dominating thermal decoherence; a conjectured coordinate-diffusion contribution to decoherence may become observable on reaching moderately low temperatures.

Security of Quantum Bit String Commitment Depends on the Information Measure

Harry Buhrman, Matthias Christandl, Patrick Hayden, Hoi-Kwong Lo, and Stephanie Wehner

Published 19 December 2006
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Unconditionally secure nonrelativistic bit commitment is known to be impossible in both the classical and the quantum world. However, when committing to a string of n bits at once, how far can we stretch the quantum limits? In this Letter, we introduce a framework of quantum schemes where Alice commits a string of n bits to Bob, in such a way that she can only cheat on a bits and Bob can learn at most b bits of information before the reveal phase. Our results are twofold: we show by an explicit construction that in the traditional approach, where the reveal and guess probabilities form the security criteria, no good schemes can exist: a+b is at least n. If, however, we use a more liberal criterion of security, the accessible information, we construct schemes where a=4log2n+O(1) and b=4, which is impossible classically. Our findings significantly extend known no-go results for quantum bit commitment.

Relativity and Lorentz Invariance of Entanglement Distillability

L. Lamata, M. A. Martin-Delgado, and E. Solano

Published 19 December 2006
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We study entanglement distillability of bipartite mixed spin states under Wigner rotations induced by Lorentz transformations. We define weak and strong criteria for relativistic isoentangled and isodistillable states to characterize relative and invariant behavior of entanglement and distillability. We exemplify these criteria in the context of Werner states, where fully analytical methods can be achieved and all relevant cases presented.

Quantum Information Becomes Classical When Distributed to Many Users

Giulio Chiribella and Giacomo Mauro D'Ariano

Published 21 December 2006
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Any physical transformation that equally distributes quantum information over a large number M of users can be approximated by a classical broadcasting of measurement outcomes. The accuracy of the approximation is at least of the order O(M^-1). In particular, quantum cloning of pure and mixed states can be approximated via quantum state estimation. As an example, for optimal qubit cloning with 10 output copies, a single user has an error probability perr0.45 in distinguishing classical from quantum output, a value close to the error probability of the random guess.

All-Optical Measurement-Based Quantum-Information Processing in Quantum Dots

Avinash Kolli, Brendon W. Lovett, Simon C. Benjamin, and Thomas M. Stace

Published 22 December 2006
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Parity measurements on qubits can generate the entanglement resource necessary for scalable quantum computation. Here we describe a method for fast optical parity measurements on electron spin qubits within coupled quantum dots. The measurement scheme, which can be realized with existing technology, consists of the optical excitation of excitonic states followed by monitored relaxation. Conditional on the observation of a photon, the system is projected into the odd/even-parity subspaces. Our model incorporates all the primary sources of error, including detector inefficiency, effects of spatial separation and nonresonance of the dots, and also unwanted excitations. Through an analytical treatment we establish that the scheme is robust to such effects. Two applications are presented: a realization of a controlled-NOT gate, and a technique for growing large scale graph states.

Effective Coupling between Two Brownian Particles

O. S. Duarte and A. O. Caldeira

Published 18 December 2006
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We use the system-plus-reservoir approach to study the dynamics of a system composed of two independent Brownian particles. We present an extension of the well-known model of a bath of oscillators which is capable of inducing an effective coupling between the two particles depending on the choice made for the spectral function of the bath oscillators. The coupling is nonlinear in the variables of interest, and an exponential dependence on these variables is imposed in order to guarantee the translational invariance of the model if the two particles are not subject to any external potential. The effective equations of motion for the particles are obtained by the Laplace transform method, and, besides recovering all the local dynamical properties for each particle, we end up with an effective interaction potential between them. We explicitly analyze one of its possible forms.

Gravitation and Astrophysics

Enhanced Polarization of the Cosmic Microwave Background Radiation from Thermal Gravitational Waves

Kaushik Bhattacharya, Subhendra Mohanty, and Akhilesh Nautiyal

Published 18 December 2006
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If inflation was preceded by a radiation era, then at the time of inflation there will exist a decoupled thermal distribution of gravitons. Gravitational waves generated during inflation will be amplified by the process of stimulated emission into the existing thermal distribution of gravitons. Consequently, the usual zero temperature scale invariant tensor spectrum is modified by a temperature dependent factor. This thermal correction factor amplifies the B-mode polarization of the cosmic microwave background radiation by an order of magnitude at large angles, which may now be in the range of observability of the Wilkinson Microwave Anisotropy Probe.

Elementary Particles and Fields

Absolute Branching Fraction Measurements for D⁺ and D⁰ Inclusive Semileptonic Decays

N. E. Adam et al. (CLEO Collaboration)

Published 21 December 2006
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We present measurements of the inclusive branching fractions for the decays D⁺Xe⁺e and D⁰Xe⁺e, using 281 pb^-1 of data collected on the (3770) resonance with the CLEO-c detector. We find (D⁰Xe⁺e)=(6.46±0.17±0.13)% and (D⁺Xe⁺e)=(16.13±0.20±0.33)%. Using the known D meson lifetimes, we obtain the ratio /=0.985±0.028±0.015, confirming isospin invariance at the level of 3%. The positron momentum spectra from D⁺ and D⁰ have consistent shapes.

Evidence of the Purely Leptonic Decay B^-^-

K. Ikado et al. (Belle Collaboration)

Published 22 December 2006
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We present the first evidence of the decay B^-^-, using 414 fb^-1 of data collected at the (4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. Events are tagged by fully reconstructing one of the B mesons in hadronic modes. We detect the signal with a significance of 3.5 standard deviations including systematics and measure the branching fraction to be (B^-^-)=(1.79(stat)(syst))×10^-4. This implies that fB=0.229(stat)(syst) GeV and is the first direct measurement of this quantity.

Longitudinal Double-Spin Asymmetry and Cross Section for Inclusive Jet Production in Polarized Proton Collisions at =200 GeV

B. I. Abelev et al. (STAR Collaboration)

Published 18 December 2006
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We report a measurement of the longitudinal double-spin asymmetry ALL and the differential cross section for inclusive midrapidity jet production in polarized proton collisions at =200 GeV. The cross section data cover transverse momenta 5<pT<50 GeV/c and agree with next-to-leading order perturbative QCD evaluations. The ALL data cover 5<pT<17 GeV/c and disfavor at 98% C.L. maximal positive gluon polarization in the polarized nucleon.

Measurement of High-p_T Single Electrons from Heavy-Flavor Decays in p+p Collisions at =200 GeV

A. Adare et al. (PHENIX Collaboration)

Published 21 December 2006
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The momentum distribution of electrons from decays of heavy flavor (charm and bottom) for midrapidity |y|<0.35 in p+p collisions at =200 GeV has been measured by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider over the transverse momentum range 0.3<pT<9 GeV/c. Two independent methods have been used to determine the heavy-flavor yields, and the results are in good agreement with each other. A fixed-order-plus-next-to-leading-log perturbative QCD calculation agrees with the data within the theoretical and experimental uncertainties, with the data/theory ratio of 1.71±0.02^stat±0.18^sys for 0.3<pT<9 GeV/c. The total charm production cross section at this energy has also been deduced to be =567±57^stat±193^sys µb.

Small-x Behavior of Parton Distributions from the Observed Froissart Energy Dependence of the Deep-Inelastic-Scattering Cross Sections

M. M. Block, Edmond L. Berger, and Chung-I Tan

Published 22 December 2006
252003 Full Text: PDF (324 kB) | Buy Article

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We fit the reduced cross section for deep-inelastic electron scattering data to a three parameter ln²s fit, A+ln²(s/s0), where s=(1-x)+m², and Q² is the virtuality of the exchanged photon. Over a wide range in Q² (0.11Q²1200 GeV²) all of the fits satisfy the logarithmic energy dependence of the Froissart bound. We can use these results to extrapolate to very large energies and hence to very small values of Bjorken x—well beyond the range accessible experimentally. As Q², the structure function F(x,Q²) exhibits Bjorken scaling, within experimental errors. We obtain new constraints on the behavior of quark and antiquark distribution functions at small x.

Nuclear Physics

Plasma Instabilities in an Anisotropically Expanding Geometry

Paul Romatschke and Anton Rebhan

Published 18 December 2006
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We study (3+1)D kinetic (Boltzmann-Vlasov) equations for relativistic plasma particles in a one dimensionally expanding geometry motivated by ultrarelativistic heavy-ion collisions. We set up local equations in terms of Yang-Mills potentials and auxiliary fields that allow simulations of hard- (expanding-) loop dynamics on a lattice. We determine numerically the evolution of plasma instabilities in the linear (Abelian) regime and also derive their late-time behavior analytically, which is consistent with recent numerical results on the evolution of the so-called melting color-glass condensate. We also find a significant delay in the onset of growth of plasma instabilities which are triggered by small rapidity fluctuations, even when the initial state is highly anisotropic.

Atomic, Molecular, and Optical Physics

Memory in Nonlinear Ionization of Transparent Solids

P. P. Rajeev, M. Gertsvolf, E. Simova, C. Hnatovsky, R. S. Taylor, V. R. Bhardwaj, D. M. Rayner, and P. B. Corkum

Published 18 December 2006
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We demonstrate a shot-to-shot reduction in the threshold laser intensity for ionization of bulk glasses illuminated by intense femtosecond pulses. For SiO2 the threshold change serves as positive feedback reenforcing the process that produced it. This constitutes a memory in nonlinear ionization of the material. The threshold change saturates with the number of pulses incident at a given spot. Irrespective of the pulse energy, the magnitude of the saturated threshold change is constant (~20%). However, the number of shots required to reach saturation does depend on the pulse energy. Recognition of a memory in ionization is vital to understand multishot optical or electrical breakdown phenomena in dielectrics.

Magnetic-Field Induced Enhancement in the Fluorescence Yield Spectrum of Doubly Excited States in Helium

Magnus Ström, Conny Såthe, Marcus Agåker, Johan Söderström, Jan-Erik Rubensson, Stefano Stranges, Robert Richter, Michele Alagia, T. W. Gorczyca, and F. Robicheaux

Published 19 December 2006
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An influence of static magnetic fields on the fluorescence yield spectrum of He in the vicinity of the N=2 thresholds has been observed. The experimental results are in excellent agreement with predictions based on multichannel quantum defect theory, and it is demonstrated that the Rydberg electron mixing due to the diamagnetic interaction is essential for the description of the observed fluorescence yield intensity enhancement.

Navigating Localized Wave Packets in Phase Space

W. Zhao, J. J. Mestayer, J. C. Lancaster, F. B. Dunning, C. O. Reinhold, S. Yoshida, and J. Burgdörfer

Published 20 December 2006
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The ability to localize and to steer Rydberg wave packets in phase space using tailored sequences of half-cycle pulses is demonstrated. Classical phase-space portraits are used to explain the method and to illustrate the level of control that can be achieved. This is confirmed experimentally by positioning a phase-space-localized wave packet at the center of a stable island or navigating it around its periphery. This work provides a valuable starting point for further engineering of electronic wave functions.

Nonperturbative Calculation of the Two-Loop Lamb Shift in Li-Like Ions

V. A. Yerokhin, P. Indelicato, and V. M. Shabaev

Published 21 December 2006
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A calculation valid to all orders in the nuclear-strength parameter is presented for the two-loop Lamb shift, notably for the two-loop self-energy correction, to the 2p-2s transition energies in heavy Li-like ions. The calculation removes the largest theoretical uncertainty for these transitions and yields the first experimental identification of two-loop QED effects in the region of the strong binding field.

Structure and Magnetization of Small Monodisperse Platinum Clusters

Xiong Liu, Matthias Bauer, Helmut Bertagnolli, Emil Roduner, Joris van Slageren, and Fritz Phillipp

Published 18 December 2006
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Magnetization measurements of well-characterized monodisperse Pt clusters consisting of 13±2 atoms in a zeolite confirm the predicted extraordinary magnetic polarization with up to 8 unpaired electrons on a cluster, corresponding to a magnetic moment of 0.65(5)µB per atom. The effect is partly quenched by hydrogen chemisorption. The study provides insight into the electronic structure of the cluster and is fundamental for an understanding of how magnetism develops in small clusters.

Experimental Evidence for Spin-Orbit Interactions in Positronium-Xe Collisions

Haruo Saito and Toshio Hyodo

Published 19 December 2006
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Spin-orbit interaction of positronium (Ps) with the surrounding atoms, predicted by Mitroy and Novikov [Phys. Rev. Lett. 90, 183202 (2003)], has been detected experimentally. We have found that the lifetime of the magnetically unperturbed ortho-Ps in Xe gas of 1 atm decreases significantly when a magnetic field of 1.0 T is applied. This decrease is attributed to the Ps spin conversion caused by spin-orbit interaction during Ps-Xe collision. The annihilation cross section of ortho-Ps due to this interaction has been determined to be 3 times as large as that expected by Mitroy and Novikov.

Quantum Correlations of Two Optical Fields Close to Electromagnetically Induced Transparency

A. Sinatra

Published 19 December 2006
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We show that three-level atoms excited by two cavity modes in a configuration close to electromagnetically induced transparency can produce strongly squeezed bright beams or correlated beams which can be used for quantum nondemolition measurements. The input intensity is the experimental “knob” for tuning the system into a squeezer or a quantum nondemolition device. The quantum correlations become ideal at a critical point characterized by the appearance of a switching behavior in the mean fields intensities. Our predictions, based on a realistic fully quantum three-level model including cavity losses and spontaneous emission, allow direct comparison with future experiments.

Nonlinear Dynamics, Fluid Dynamics, Classical Optics, etc.

Formation of Long-Lived, Scarlike Modes near Avoided Resonance Crossings in Optical Microcavities

Jan Wiersig

Published 19 December 2006
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We study the formation of long-lived states near avoided resonance crossings in open systems. For three different optical microcavities (rectangle, ellipse, and semistadium) we provide numerical evidence that these states are localized along periodic rays, resembling scarred states in closed systems. Our results shed light on the morphology of long-lived states in open mesoscopic systems.

Subcycle Pulsed Focused Vector Beams

Qiang Lin, Jian Zheng, and Wilhelm Becker

Published 19 December 2006
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An accurate description of a subcycle pulsed beam (SCPB) is presented based on the complex-source model. The fields are exact solutions of Maxwell's equations and applicable to a focused pulsed beam with a pulse duration down to and below one cycle of the carrier wave and with arbitrary polarization state. Depending on the pulse duration, the pulse is blueshifted, and its wings are chirped. This effect, which we refer to as “self-induced blueshift” goes beyond the carrier-envelope description. The corresponding phase is a temporal analog of the Gouy phase. The energy gain of a relativistic electron swept over by an SCPB is very sensitive to the proper form chosen to describe the pulse.

Attosecond Temporal Gating with Elliptically Polarized Light

N. Dudovich, J. Levesque, O. Smirnova, D. Zeidler, D. Comtois, M. Yu. Ivanov, D. M. Villeneuve, and P. B. Corkum

Published 22 December 2006
253903 Full Text: PDF (226 kB) | Buy Article

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Temporal gating allows high accuracy time-resolved measurements of a broad range of ultrafast processes. By manipulating the interaction between an atom and an intense laser field, we extend gating into the nonlinear medium in which attosecond optical and electron pulses are generated. Our gate is an amplitude gate induced by ellipticity of the fundamental pulse. The gate modulates the spectrum of the high harmonic emission and we use the measured modulation to characterize the sub-laser-cycle dynamics of the recollision electron wave packet.

Long-Range Propagation of Plasmon Polaritons in a Thin Metal Film on a One-Dimensional Photonic Crystal Surface

Valery N. Konopsky and Elena V. Alieva

Published 22 December 2006
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We present experimental results on ultralong-range surface plasmon polaritons, propagating in a thin metal film on a one-dimensional (1D) photonic crystal surface over a distance of several millimeters. This propagation length is about 2 orders of magnitude higher than the one in the ordinary Kretschmann configuration at the same optical frequency. We show that a long-range surface plasmon polaritons propagation may take place not only in a (quasi)symmetrical scheme, where a thin metal film is located between two media with (approximately) the same refraction index, but also in a scheme where the thin metal film is located between an appropriate 1D photonic crystal and an arbitrary (air, water, etc.) medium. The ultralong-range surface plasmon polaritons are potentially important for biosensors, plasmonics, and other applications.

Dissipative Solitons that Cannot be Trapped

Rosa Pardo and Víctor M. Pérez-García

Published 18 December 2006
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We show that dissipative solitons in systems with high-order nonlinear dissipation cannot survive in the presence of trapping potentials of the rigid wall or asymptotically increasing type. Solitons in such systems can survive in the presence of a weak potential but only with energies out of the interval of existence of linear quantum mechanical stationary states.

First Experimental Observation of Superscars in a Pseudointegrable Barrier Billiard

E. Bogomolny, B. Dietz, T. Friedrich, M. Miski-Oglu, A. Richter, F. Schäfer, and C. Schmit

Published 20 December 2006
254102 Full Text: PDF (536 kB) | Buy Article

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With a perturbation body technique intensity distributions of the electric field strength in a flat microwave billiard with a barrier inside up to mode numbers as large as about 700 were measured. A method for the reconstruction of the amplitudes and phases of the electric field strength from those intensity distributions has been developed. Recently predicted superscars have been identified experimentally and—using the well-known analogy between the electric field strength and the quantum mechanical wave function in a two-dimensional microwave billiard—their properties determined.

Reciprocal Oscillons and Nonmonotonic Fronts in Forced Nonequilibrium Systems

Arik Yochelis, John Burke, and Edgar Knobloch

Published 22 December 2006
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The formation of oscillons in a synchronously oscillating background is studied in the context of both damped and self-exciting oscillatory media. Using the forced complex Ginzburg-Landau equation we show that such states bifurcate from finite amplitude homogenous states near the 2:1 resonance boundary. In each case we identify a region in parameter space containing a finite multiplicity of coexisting stable oscillons with different structure. Stable time-periodic monotonic and nonmonotonic frontlike states are present in an overlapping region. Both types of structure are related to the presence of a Maxwell point between the zero and finite amplitude homogeneous states.

Plasma and Beam Physics

Waveguide-Based Off-Axis Holography with Hard X Rays

C. Fuhse, C. Ollinger, and T. Salditt

Published 21 December 2006
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We present an off-axis holography experiment based on the coherent cone beams emitted from a pair of x-ray waveguides. A magnified off-axis hologram is recorded, from which the phase of the optical transmission function of a sample is obtained by digital holographic reconstruction. A spatial resolution of about 100 nm has been achieved at 10.4 keV photon energy. Spatial resolution is determined by the cross-sectional dimensions of the waveguide and could approach a fundamental limit of about 10 nm in future experiments. In addition, we propose a new experimental setup that might overcome this limitation.

Magnetic Reconnection and Plasma Dynamics in Two-Beam Laser-Solid Interactions

P. M. Nilson, L. Willingale, M. C. Kaluza, C. Kamperidis, S. Minardi, M. S. Wei, P. Fernandes, M. Notley, S. Bandyopadhyay, M. Sherlock, R. J. Kingham, M. Tatarakis, Z. Najmudin, W. Rozmus, R. G. Evans, M. G. Haines, A. E. Dangor, and K. Krushelnick

Published 19 December 2006
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We present measurements of a magnetic reconnection in a plasma created by two laser beams (1 ns pulse duration, 1×10¹⁵ W cm^-2) focused in close proximity on a planar solid target. Simultaneous optical probing and proton grid deflectometry reveal two high velocity, collimated outflowing jets and 0.7—1.3 MG magnetic fields at the focal spot edges. Thomson scattering measurements from the reconnection layer are consistent with high electron temperatures in this region.

Dynamic Alignment in Driven Magnetohydrodynamic Turbulence

Joanne Mason, Fausto Cattaneo, and Stanislav Boldyrev

Published 20 December 2006
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Motivated by recent analytic predictions, we report numerical evidence showing that in driven incompressible magnetohydrodynamic turbulence the magnetic- and velocity-field fluctuations locally tend to align the directions of their polarizations. This dynamic alignment is stronger at smaller scales with the angular mismatch between the polarizations decreasing with the scale approximately as ^1/4. This can naturally lead to a weakening of the nonlinear interactions and provide an explanation for the energy spectrum E(k)k^-3/2 that is observed in numerical experiments of strongly magnetized turbulence.

Strong ExB Shear Flows in the Transport-Barrier Region in H-Mode Plasma

H. Xia, M. G. Shats, and H. Punzmann

Published 20 December 2006
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We report the first experimental observation of stationary zonal flow in the transport-barrier region of the H-mode plasma. Strong peaks in Er shear mark the width of this region. A strong m=n=0 low-frequency (f<0.6 kHz) zonal flow is observed in regions of increased Er, suggesting a substantial contribution of zonal flow to the spatial modulation of Er radial profiles. Radial localization of the zonal flow is correlated with a region of zero magnetic shear and low-order (7/5) rational surfaces.

Condensed Matter: Structure, etc.

Direct Observation of Local Distortion of a Crystal Lattice with Picometer Accuracy Using Atomic Resolution Neutron Holography

L. Cser, G. Krexner, M. Markó, I. Sharkov, and Gy. Török

Published 19 December 2006
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We demonstrate that neutron holography permits us to extend the determination of atomic positions beyond nearest neighbors at least up to the fourth neighboring shell around cadmium probe atoms alloyed into a lead crystal. The accuracy achieved is sufficient to allow quantitative determination of displacements of atoms due to elastic distortions induced by impurity atoms. The atomic positions derived from the holographic data are in good agreement with those expected theoretically due to Friedel oscillations in this system. In addition, the atomic positions are in qualitative agreement with results obtained in an independent experiment studying the diffuse distortion scattering around Bragg peaks.

Probing the Structure and Energetics of Dislocation Cores in SiGe Alloys through Monte Carlo Simulations

Ioannis N. Remediakis, David E. Jesson, and Pantelis C. Kelires

Published 20 December 2006
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We present a methodology for the investigation of dislocation energetics in segregated alloys based on Monte Carlo simulations which equilibrate the topology and composition of the dislocation core and its surroundings. An environment-dependent partitioning of the system total energy into atomic contributions allows us to link the atomistic picture to continuum elasticity theory. The method is applied to extract core energies and radii of 60° glide dislocations in segregated SiGe alloys which are inaccessible by other methods.

Megabar-Pressure Infrared Study of Hydrogen Deuteride

Akobuije Chijioke and Isaac F. Silvera

Published 22 December 2006
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Solid hydrogen deuteride (HD) has been studied to a pressure of 159 GPa and to low temperatures using near infrared spectroscopy. Of the two high pressure phases observed in hydrogen and deuterium, known as the BSP (broken-symmetry phase) and the A phase, only the BSP had been observed in the lower pressure region of the phase line of HD and it was unusually different from the homonuclear diatomic species with a reentrant behavior. In this Letter the BSP phase line is identified to its maximum pressure of 159 GPa. Infrared absorption reveals a transition to the A phase, observed for the first time in HD with onset at 157±3 GPa. A new phase of electric dipolar order that should occur at low temperature is discussed.

Electron-Ion Interference and Onsager Reciprocity in Mixed Ionic-Electronic Transport in TiO₂

Doh-Kwon Lee and Han-Ill Yoo

Published 19 December 2006
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Onsager's reciprocity theorem has been extensively discussed for fluid systems and its validity generally accepted. The application of the theorem to solid state, however, is not always transparent due to additional complications that have no counterparts in fluids, and its validity has not been so exhaustively examined experimentally either. Here we show that in the phenomenon of mixed ionic-electronic conduction in TiO2 (rutile), the theorem is verified experimentally, and the Onsager cross coefficient can be even larger than a direct one, contrary to the conventional belief.

Atomistic Mechanism of Boron Diffusion in Silicon

Davide De Salvador, Enrico Napolitani, Salvatore Mirabella, Gabriele Bisognin, Giuliana Impellizzeri, Alberto Carnera, and Francesco Priolo

Published 20 December 2006
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B diffuses in crystalline Si by reacting with a Si self-interstitial (I) with a frequency g and so forming a fast migrating BI complex that can migrate for an average length . We experimentally demonstrate that both g and strongly depend on the free hole concentration p. At low p, g has a constant trend and increases with p, while at high p, g has a superlinear trend and decreases with p. This demonstrates that BI forms in the two regimes by interaction with neutral and double positive I, respectively, and its charge state has to change by interaction with free holes before diffusing.

How Do Oxygen Molecules Move into Silver Contacts and Change Their Electronic Transport Properties?

Yuanhua Qi, Daren Guan, Yuansheng Jiang, Yujun Zheng, and Chengbu Liu

Published 20 December 2006
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We present first principles simulations of the elongation process of the silver contact at the O2 atmosphere. The electronic transport properties are calculated. It is found that the O2 molecule can move into the silver contact during elongation and the corresponding mechanism is given. We demonstrate that there are two transmission channels around the Fermi level in an Ag-O2 contact system. The breaking process of an Ag-Ag bond is found to play an important role in determining the evolution of the system conductance during the elongation.

Condensed Matter: Electronic Properties, etc.

Evidence of Long-Wavelength Collective Excitations in Magnetic Superlattices

Nikolay I. Polushkin, Steven A. Michalski, Lanping Yue, and Roger D. Kirby

Published 18 December 2006
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We report on a mechanism of dynamic dipolar coupling in magnetic superlattices via long-wavelength nonevanescent fields. In the spin excitation spectra of our heterophase stripe structures, such interactions mediate a singletdoublet crossover in the frequency regime driven by the orientation of an external static field. This crossover is a new feature observed in collective behavior of superlattices, though there is some analogy of this phenomenon with birefringence taking place in optical superlattices. We envision applying the collective effects described here in microwave photonic devices.

Energy Dependence on Fractional Charge for Strongly Interacting Subsystems

Steven M. Valone and Susan R. Atlas

Published 19 December 2006
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The energies of a pair of strongly interacting subsystems with arbitrary noninteger charges are examined from closed- and open-system perspectives. An ensemble representation of the charge dependence is derived, valid at all interaction strengths. Transforming from resonance-state ionicity to ensemble charge dependence imposes physical constraints on the occupation numbers in the strong-interaction limit. For open systems, the chemical potential is evaluated using microscopic and thermodynamic models, leading to a novel correlation between ground-state charge and an electronic temperature.

Fermi-Edge Singularity in a Spin-Incoherent Luttinger Liquid

Gregory A. Fiete

Published 20 December 2006
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We theoretically investigate the Fermi-edge singularity in a spin-incoherent Luttinger liquid. Both cases of finite and infinite core hole mass are explored, as well as the effect of a static external magnetic field of arbitrary strength. For a finite mass core hole the absorption edge behaves as (-th)/ for frequencies just above the threshold frequency th. The exponent depends on the interaction parameter g of the interacting one dimensional system, the electron-hole coupling, and is independent of the magnetic field strength, the momentum, and the mass of the excited core hole (in contrast to the spin-coherent case). In the infinite mass limit, the spin-incoherent problem can be mapped onto an equivalent problem in a spinless Luttinger liquid for which the logarithmic factor is absent, and backscattering from the core hole leads to a universal contribution to the exponent .

Critical Fidelity at the Metal-Insulator Transition

Gim Seng Ng, Joshua Bodyfelt, and Tsampikos Kottos

Published 20 December 2006
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Using a Wigner Lorentzian random matrix ensemble, we study the fidelity, F(t), of systems at the Anderson metal-insulator transition, subject to small perturbations that preserve the criticality. We find that there are three decay regimes as perturbation strength increases: the first two are associated with a Gaussian and an exponential decay, respectively, and can be described using linear response theory. For stronger perturbations F(t) decays algebraically as F(t)~t^-D, where D is the correlation dimension of the local density of states.

Detection of Current-Induced Spins by Ferromagnetic Contacts

. Adagideli, G. E. W. Bauer, and B. I. Halperin

Published 20 December 2006
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Detection of current-induced spin accumulation via ferromagnetic contacts is discussed. Onsager's relations forbid that in a two-probe configuration, spins excited by currents in time-reversal symmetric systems can be detected by switching the magnetization of a ferromangetic detector contact. Nevertheless, current-induced spins can be transferred as a torque to a contact magnetization and can affect the charge currents in many-terminal configurations. We demonstrate the general concepts by solving the microscopic transport equations for the diffuse Rashba system with magnetic contacts.

Bistability-Mediated Carrier Recombination at Light-Induced Boron-Oxygen Complexes in Silicon

Mao-Hua Du, Howard M. Branz, Richard S. Crandall, and S. B. Zhang

Published 22 December 2006
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A first-principles study of the BO2 complex in B-doped Czochralski Si reveals a defect-bistability-mediated carrier recombination mechanism, which contrasts with the standard fixed-level Shockley-Read-Hall model of recombination. An O2 dimer distant from B causes only weak carrier recombination, which nevertheless drives O2 diffusion under light to form the BO2 complex. Although BO2 and O2 produce nearly identical defect levels in the band gap, the recombination at BO2 is substantially faster than at O2 because the charge state of the latter inhibits the hole capture step of recombination.

Spatial Extent of Wave Functions of Gate-Induced Hole Carriers in Pentacene Field-Effect Devices as Investigated by Electron Spin Resonance

Kazuhiro Marumoto, Shin-ichi Kuroda, Taishi Takenobu, and Yoshihiro Iwasa

Published 22 December 2006
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An electron spin resonance (ESR) method is applied to a pentacene field-effect device to investigate gate-induced hole carriers in such devices. Clear field-induced ESR signals are observed, demonstrating that all of the field-injected carriers have S=1/2 spins. Anisotropic ESR signals due to unpaired electrons show the molecular orientation at the interface in the devices. The spatial extent of the spin density distribution (wave function) of the carriers is evaluated from the ESR linewidth, accounting for the hyperfine structure, to be of the order of 10 molecules.

Dirac and Normal Fermions in Graphite and Graphene: Implications of the Quantum Hall Effect

Igor A. Luk'yanchuk and Yakov Kopelevich

Published 20 December 2006
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Spectral analysis of the Shubnikov–de Haas magnetoresistance oscillations and the quantum Hall effect (QHE) measured in quasi-2D highly oriented pyrolytic graphite (HOPG) [Phys. Rev. Lett. 90, 156402 (2003)] reveals two types of carriers: normal (massive) electrons with Berry phase 0 and Dirac-like (massless) holes with Berry phase . We demonstrate that recently reported integer- and semi-integer QHEs for bilayer and single-layer graphenes take place simultaneously in HOPG samples.

Interaction-Induced Resonance in Conductance and Thermopower of Quantum Wires

Anders Mathias Lunde, Karsten Flensberg, and Leonid I. Glazman

Published 20 December 2006
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We study the effect of electron-electron interaction on the transport properties of short clean quantum wires adiabatically connected to reservoirs. Interactions lead to resonances in a multichannel wire at particular values of the Fermi energy. We investigate in detail the resonance in a two-channel wire. The (negative) conductance correction peaks at the resonance, and decays exponentially as the Fermi energy is tuned away, the resonance width being given by the temperature. Likewise, the thermopower shows a characteristic structure, which is surprisingly well approximated by the so-called Mott formula. Finally, fourfold splitting of the resonance in a magnetic field provides a unique signature of the effect.

Nanometer Scale Electronic Reconstruction at the Interface between LaVO₃ and LaVO₄

L. Fitting Kourkoutis, Y. Hotta, T. Susaki, H. Y. Hwang, and D. A. Muller

Published 21 December 2006
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Electrons at interfaces, driven to minimize their free energy, are distributed differently than in bulk. This can be dramatic at interfaces involving heterovalent compounds. Here we profile an abrupt interface between V 3d² LaVO3 and V 3d⁰ LaVO4 using electron energy loss spectroscopy. Although no bulk phase of LaVOx with a V 3d¹ configuration exists, we find a nanometer-wide region of V 3d¹ at the LaVO3/LaVO4 interface, rather than a mixture of V 3d⁰ and V 3d². The two-dimensional sheet of 3d¹ electrons is a prototypical electronic reconstruction at an interface between competing ground states.

Edge Excitations and Non-Abelian Statistics in the Moore-Read State: A Numerical Study in the Presence of Coulomb Interaction and Edge Confinement

Xin Wan, Kun Yang, and E. H. Rezayi

Published 22 December 2006
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We study the ground state and low-energy excitations of fractional quantum Hall systems on a disk at a filling fraction =5/2, with Coulomb interaction and a background confining potential. We find the Moore-Read ground state is stable within a finite but narrow window in parameter space. The corresponding low-energy excitations contain a fermionic branch and a bosonic branch, with widely different velocities. A short-range repulsive potential can stabilize a charge +e/4 quasihole at the center, leading to a different edge excitation spectrum due to the change of boundary conditions for Majorana fermions, clearly indicating the non-Abelian nature of the quasihole.

Novel Route to a Finite Center-of-Mass Momentum Pairing State for Superconductors: A Current-Driven Fulde-Ferrell-Larkin-Ovchinnikov State

Hyeonjin Doh, Matthew Song, and Hae-Young Kee

Published 18 December 2006
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The previously studied Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is stabilized by a magnetic field via the Zeeman coupling in spin-singlet superconductors. Here we suggest a novel route to achieve nonzero center-of-mass momentum pairing states in superconductors with Fermi surface nesting. We investigate two-dimensional superconductors under a uniform external current, proportional to a finite pair momentum of qe. We find that an FFLO state with a spontaneous pair momentum of qs is stabilized above a certain critical current that depends on the direction of the external current. A finite qs arises in order to make the total pair-momentum of qt(=qs+qe) perpendicular to the nesting vector, which is independent of spin states of Cooper pairs. We also discuss experimental signatures of the FFLO state.

Crossover between Channeling and Pinning at Twin Boundaries in YBa₂Cu₃O₇ Thin Films

A. Palau, J. H. Durrell, J. L. MacManus-Driscoll, S. Harrington, T. Puig, F. Sandiumenge, X. Obradors, and M. G. Blamire

Published 18 December 2006
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The critical current (Jc) of highly twinned YBa2Cu3O7 films has been measured as a function of temperature, magnetic field, and angle. For much of the parameter space we observe a strong suppression of Jc for fields in the twin boundary (TB) directions; this is quantitatively modeled as flux-cutting-mediated vortex channeling. For certain temperatures and fields a crossover occurs to a regime in which channeling is blocked and the TBs act as planar pinning centers so that TB pinning enhances the overall Jc. In this regime, intrinsic pinning along the TBs is comparable to that between the twins.

Quantum Cascade Phenomenon in Bi₂Sr₂CaCu₂O₈₊ Single Crystals

V. M. Krasnov

Published 19 December 2006
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We study interlayer transport in Bi2Sr2CaCu2O8+ cuprates, which represent stacks of atomic scale intrinsic Josephson junctions. A series of resonant dips in conductance is observed at condition when bremsstrahlung and recombination bands in nonequilibrium spectrum of Josephson junctions overlap. The phenomenon is explained in terms of self-detection of a new type of collective strongly nonequilibrium state in natural atomic superlattices, bearing certain resemblance with operation of a quantum cascade laser. Conclusions are supported by in situ generation-detection experiments and by numerical simulations.

Antiferromagnetism and Superconductivity in Layered Organic Conductors: Variational Cluster Approach

P. Sahebsara and D. Sénéchal

Published 20 December 2006
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The -(ET)2X layered conductors (where ET stands for BEDT-TTF) are studied within the dimer model as a function of the diagonal hopping t and Hubbard repulsion U. Antiferromagnetism and d-wave superconductivity are investigated at zero temperature using variational cluster perturbation theory (VCPT). For large U, Néel antiferromagnetism exists for t<t, with t~0.9. For fixed t, as U is decreased (or pressure increased), a dx²-y² superconducting phase appears. When U is decreased further, then a dxy order takes over. There is a critical value of t~0.8 of t beyond which the AF and dSC phases are separated by the Mott disordered phase.

Enhancement of Pairwise Entanglement via ₂ Symmetry Breaking

Andreas Osterloh, Guillaume Palacios, and Simone Montangero

Published 18 December 2006
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We study the effect of symmetry breaking in a quantum phase transition on pairwise entanglement in spin-1/2 models. We give a set of conditions on correlation functions a model has to meet in order to keep the pairwise entanglement unchanged by a parity symmetry breaking. It turns out that all mean-field solvable models do meet this requirement, whereas the presence of strong correlations leads to a violation of this condition. This results in an order-induced enhancement of entanglement, and we report on two examples where this takes place.

Observation of Higher-Harmonic Helical Spin-Resonance Modes in the Chromium Spinel CdCr₂O₄

S. Kimura, M. Hagiwara, H. Ueda, Y. Narumi, K. Kindo, H. Yashiro, T. Kashiwagi, and H. Takagi

Published 18 December 2006
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High frequency ESR measurements on the chromium spinel compound CdCr2O4 have been performed. The observed ESR modes below H5.7 T can be explained well by the calculated resonance modes based on a molecular field theory assuming a helical spin structure. Other than the fundamental ones, we have succeeded in observing the higher-harmonic modes for the first time. A large change of the ESR modes above H indicate that a variation of the spin structure from the helical to the four-sublattice canted one takes place around H.

Loss Separation for Dynamic Hysteresis in Ferromagnetic Thin Films

Francesca Colaiori, Gianfranco Durin, and Stefano Zapperi

Published 19 December 2006
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We develop a theory for dynamic hysteresis in ferromagnetic thin films, on the basis of the phenomenological principle of loss separation. We observe that, remarkably, the theory of loss separation, originally derived for bulk metallic materials, is applicable to disordered magnetic systems under fairly general conditions regardless of the particular damping mechanism. We confirm our theory both by numerical simulations of a driven random-field Ising model, and by reexamining several experimental data reported in the literature on dynamic hysteresis in thin films. All the experiments examined and the simulations find a natural interpretation in terms of loss separation. The power losses' dependence on the driving field rate predicted by our theory fits satisfactorily all the data in the entire frequency range, thus reconciling the apparent lack of universality observed in different materials.

Octupolar Order in the Multiple Spin Exchange Model on a Triangular Lattice

Tsutomu Momoi, Philippe Sindzingre, and Nic Shannon

Published 20 December 2006
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We show how a gapless spin liquid with hidden octupolar order arises in an applied magnetic field, in a model applicable to thin films of ³He with competing ferromagnetic and antiferromagnetic (cyclic) exchange interactions. Evidence is also presented for nematic—i.e., quadrupolar—correlations bordering on ferromagnetism in the absence of a magnetic field.

Kagomé Ice State in the Dipolar Spin Ice Dy₂Ti₂O₇

Y. Tabata, H. Kadowaki, K. Matsuhira, Z. Hiroi, N. Aso, E. Ressouche, and B. Fåk

Published 21 December 2006
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We have investigated the kagomé ice behavior of the dipolar spin-ice compound Dy2Ti2O7 in a magnetic field along a [111] direction using neutron scattering and Monte Carlo simulations. The spin correlations show that the kagomé ice behavior predicted for the nearest-neighbor interacting model, where the field induces dimensional reduction and spins are frustrated in each two-dimensional kagomé lattice, occurs in the dipole interacting system. The spins freeze at low temperatures within the macroscopically degenerate ground states of the nearest-neighbor model.

Short Period Magnetic Coupling Oscillations in Co/Si Multilayers: Theory versus Experiment

Nader Yaacoub, Christian Meny, Olivier Bengone, and Pierre Panissod

Published 22 December 2006
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Today the magnetic properties of multilayers and nanostructures including a metal or an insulator as a nonmagnetic spacer layer are rather well understood. But they are much more controversial for semiconductor spacers. For instance, for Co/Si multilayers short period coupling oscillations are predicted by ab initio computations but have yet to be observed. Here we show in Co/Si multilayers prepared at low temperature (90 K) strong saturation field oscillations that are consistent with the predicted coupling oscillations. However, the decay length of the oscillations is much longer than the expected one and cannot be explained within the framework of available theories.

Exciton Localization of Single-Walled Carbon Nanotubes Revealed by Femtosecond Excitation Correlation Spectroscopy

Hideki Hirori, Kazunari Matsuda, Yuhei Miyauchi, Shigeo Maruyama, and Yoshihiko Kanemitsu

Published 18 December 2006
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Photoluminescence (PL) dynamics in single-walled carbon nanotubes (SWNTs) has been studied by the femtosecond excitation correlation method with a 150 fs time resolution. The SWNT samples were synthesized by different methods and suspended in gelatin films or D2O solutions. The PL dynamics of SWNTs depends on the local environment surrounding the SWNTs rather than the synthesis methods. The very weak temperature dependence of PL and the environment-dependent PL reveal that the PL relaxation process is dominated by the interplay between free excitons and weakly localized excitons.

NMR Evidence for Higher-Order Multipole Order Parameters in NpO₂

Y. Tokunaga, D. Aoki, Y. Homma, S. Kambe, H. Sakai, S. Ikeda, T. Fujimoto, R. E. Walstedt, H. Yasuoka, E. Yamamoto, A. Nakamura, and Y. Shiokawa

Published 19 December 2006
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We report a microscopic investigation of multipolar order parameters in the ordered state of NpO2 conducted via ¹⁷O NMR on a single crystal. From the angular dependence of hyperfine fields at ¹⁷O nuclei, we have obtained clear evidence for the appearance of field-induced antiferro-octupolar as well as field-induced antiferro-dipolar moments below T0=26 K. We have also observed oscillatory spin-echo decay, which is well understood in terms of small electric field gradients created by antiferro-quadrupolar ordering. This reveals that the quadrupolar order parameter is directly observable by means of NMR. The present NMR studies provide definitive support for a proposed longitudinal triple-q type octupolar-quadrupolar ordering model for NpO2.

Multiferroic Domain Dynamics in Strained Strontium Titanate

A. Vasudevarao, A. Kumar, L. Tian, J. H. Haeni, Y. L. Li, C.-J. Eklund, Q. X. Jia, R. Uecker, P. Reiche, K. M. Rabe, L. Q. Chen, D. G. Schlom, and Venkatraman Gopalan

Published 21 December 2006
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Multiferroicity can be induced in strontium titanate by applying biaxial strain. Using optical second harmonic generation, we report a transition from 4/mmm to the ferroelectric mm2 phase, followed by a transition to a ferroelastic-ferroelectric mm2 phase in a strontium titanate thin film. Piezoelectric force microscopy is used to study ferroelectric domain switching. Second harmonic generation, combined with phase-field modeling, is used to reveal the mechanism of coupled ferroelectric-ferroelastic domain wall motion. These studies have relevance to multiferroics with coupled polar and axial phenomena.

Evidence of Simultaneous Double-Electron Promotion in F⁺ Collisions with Surfaces

J. Mace, M. J. Gordon, and K. P. Giapis

Published 22 December 2006
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A high-flux beam of mass-filtered F⁺ at low energy (100–1300 eV) was scattered off Al and Si surfaces to study core-level excitations of F⁰ and F⁺. Elastic scattering behavior for F⁺ was observed at energies <300 (500) eV off Al (Si) for a 90° lab angle. However, above this energy threshold, orbital mixing in the hard collision step results in electronic excitation of F via molecular orbital promotion along the 4f (F-2p), significantly reducing the observed ion exit energy. In addition, despite the electronegativity of F, scattering at energies >450 (700) eV off Al (Si) produces F²⁺—behavior which is remarkably similar to Ne⁺ off the same surfaces. Inelasticities measured for single collision events agree well with the energy deficits required to form (doubly excited) F^** and F^+** states from F⁰ and F⁺, respectively; these excited species most likely decay to inelastic F⁺ and F²⁺ via autoionization.

Soft Matter, Biological, and Interdisciplinary Physics

Critical Scaling in Linear Response of Frictionless Granular Packings near Jamming

Wouter G. Ellenbroek, Ellák Somfai, Martin van Hecke, and Wim van Saarloos

Published 22 December 2006
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We study the origin of the scaling behavior in frictionless granular media above the jamming transition by analyzing their linear response. The response to local forcing is non-self-averaging and fluctuates over a length scale that diverges at the jamming transition. The response to global forcing becomes increasingly nonaffine near the jamming transition. This is due to the proximity of floppy modes, the influence of which we characterize by the local linear response. We show that the local response also governs the anomalous scaling of elastic constants and contact number.

Diffusion and Segmental Dynamics of Double-Stranded DNA

E. P. Petrov, T. Ohrt, R. G. Winkler, and P. Schwille

Published 18 December 2006
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Diffusion and segmental dynamics of the double-stranded -phage DNA polymer are quantitatively studied over the transition range from stiff to semiflexible chains. Spectroscopy of fluorescence fluctuations of single-end fluorescently labeled monodisperse DNA fragments unambiguously shows that double-stranded DNA in the length range of 10²–2×10⁴ base pairs behaves as a semiflexible polymer with segmental dynamics controlled by hydrodynamic interactions.

Hydra Molecular Network Reaches Criticality at the Symmetry-Breaking Axis-Defining Moment

Jordi Soriano, Cyril Colombo, and Albrecht Ott

Published 18 December 2006
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We study biological, multicellular symmetry breaking on a hollow cell sphere as it occurs during hydra regeneration from a random cell aggregate. We show that even a weak temperature gradient directs the axis of the regenerating animal—but only if it is applied during the symmetry-breaking moment. We observe that the spatial distribution of the early expressed, head-specific gene ks1 has become scale-free and fractal at that point. We suggest the self-organized critical state to reflect long range signaling, which is required for axis definition and arises from cell next-neighbor communication.

Coevolution of Strategy and Structure in Complex Networks with Dynamical Linking

Jorge M. Pacheco, Arne Traulsen, and Martin A. Nowak

Published 19 December 2006
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We introduce a model in which individuals differ in the rate at which they seek new interactions with others, making rational decisions modeled as general symmetric two-player games. Once a link between two individuals has formed, the productivity of this link is evaluated. Links can be broken off at different rates. We provide analytic results for the limiting cases where linking dynamics is much faster than evolutionary dynamics and vice versa, and show how the individual capacity of forming new links or severing inconvenient ones maps into the problem of strategy evolution in a well-mixed population under a different game. For intermediate ranges, we investigate numerically the detailed interplay determined by these two time scales and show that the scope of validity of the analytical results extends to a much wider ratio of time scales than expected.

Bistable Bacterial Growth Rate in Response to Antibiotics with Low Membrane Permeability

Johan Elf, Karin Nilsson, Tanel Tenson, and Måns Ehrenberg

Published 19 December 2006
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We demonstrate that growth rate bistability for bacterial cells growing exponentially at a fixed external antibiotic concentration can emerge when the cell wall permeability for the drug is low and the growth rate sensitivity to the intracellular drug concentration is high. Under such conditions, an initially high growth rate can remain high, due to dilution of the intracellular drug concentration by rapid cell volume increase, while an initially low growth rate can remain low, due to slow cell volume increase and insignificant drug dilution. Our findings have implications for the testing of novel antibiotics on growing bacterial strains.

Mechanisms of Molecular Response in the Optimal Control of Photoisomerization

Benjamin Dietzek, Ben Brüggemann, Torbjörn Pascher, and Arkady Yartsev

Published 20 December 2006
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We report on adaptive feedback control of photoinduced barrierless isomerization of 1,1'-diethyl-2,2'-cyanine in solution. We compare the effect of different fitness parameters and show that optimal control of the absolute yield of isomerization (photoisomer concentration versus excitation photons) can be achieved, while the relative isomerization yield (photoisomer concentration versus number of relaxed excited-state molecules) is unaffected by adaptive feedback control. The temporal structure of the optimized excitation pulses allows one to draw clear mechanistic conclusions showing the critical importance of coherent nuclear motion for the control of isomerization.

Nonlinear Screening and Gas-Liquid Separation in Suspensions of Charged Colloids

Bas Zoetekouw and René van Roij

Published 20 December 2006
258302 Full Text: PDF (168 kB) | Buy Article

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We calculate phase diagrams of charged colloidal spheres (valency Z and radius a) in a 1:1 electrolyte from multicentered nonlinear Poisson-Boltzmann theory. Our theory takes into account charge renormalization of the colloidal interactions and volume terms due to many-body effects. For valencies as small as Z=1 and as large as 10⁴ we find a gas–liquid spinodal instability in the colloid-salt phase diagram provided ZB/a24±1, where B is the Bjerrum length.

Collective Rearrangement at the Onset of Flow of a Polycrystalline Hexagonal Columnar Phase

Teresa Bauer, Julian Oberdisse, and Laurence Ramos

Published 21 December 2006
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Creep experiments on polycrystalline surfactant hexagonal columnar phases show a power law regime, followed by a drastic fluidization before reaching a final stationary flow. The scaling of the fluidization time with the shear modulus of the sample and stress applied suggests that the onset of flow involves a bulk reorganization of the material. This is confirmed by x-ray scattering under stress coupled to in situ rheology experiments, which show a collective reorientation of all crystallites at the onset of flow. The analogy with the fracture of heterogeneous materials is discussed.

COMMENTS

Comment on “Röntgen Quantum Phase Shift: A Semiclassical Local Electrodynamical Effect?”

G. Spavieri and M. Rodriguez

Published 19 December 2006
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A Comment on the Letter by S. A. R. Horsley and M. Babiker, Phys. Rev. Lett. 95, 010405 (2005). The authors of the Letter offer a Reply.

Horsley and Babiker Reply:

S. A. R. Horsley and M. Babiker

Published 19 December 2006
258902 Full Text: PDF (46 kB) | Buy Article

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A Reply to the Comment by G. Spavieri and M. Rodriguez.

Comment on “Atomic Origin of Magnetocrystalline Anisotropy in Nd₂Fe₁₄B”

M. D. Kuz'min

Published 20 December 2006
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A Comment on the Letter by D. Haskel et al., Phys. Rev. Lett. 95, 217207 (2005). The authors of the Letter offer a Reply.

Haskel and van Veenendaal Reply:

Daniel Haskel and Michel van Veenendaal

Published 20 December 2006
259702 Full Text: PDF (259 kB) | Buy Article

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A Reply to the Comment by M. D. Kuz'min.

ERRATA