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Physical Review Letters

18 January 2008

Volume 100, Number 2 , Articles (02xxxx)

Articles published 12 January - 18 January 2008


Atomic configuration illustrating nucleation (blue) at the site of a partial dislocation in the surface layer of a square copper nanowire (yellow and green) under uniaxial stress.

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EDITORIALS, ESSAYS, AND ANNOUNCEMENTS

Essay: Physical Review Letters; Sam Goudsmit's Vision
Robert K. Adair
Published 18 January 2008
020001  Full Text: PDF (161 kB) 
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Sam Goudsmit implemented his vision of converting the Letters section of Physical Review into a distinct journal fifty years ago. Physical Review Letters was designed to publish “only papers that really deserve rapid communication.” The new journal became so successful with physicists throughout the world that Physical Review Letters now publishes 3500 Letters per year.

LETTERS

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

Ground-State Properties of a One-Dimensional System of Hard Rods
F. Mazzanti, G. E. Astrakharchik, J. Boronat, and J. Casulleras
Published 17 January 2008
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A quantum Monte Carlo simulation of a system of bosonic hard rods in one dimension is presented and discussed. The calculation is exact since the analytical form of the wave function is known and is in excellent agreement with predictions obtained from asymptotic expansions valid at large distances. The analysis of the static structure factor and the pair distribution function indicates that a solidlike and a gaslike phases exist at high and low densities, respectively. The one-body density matrix decays following a power law at large distances and produces a divergence in the low density momentum distribution at k=0 which can be identified as a quasicondensate.

Recurrence and Pólya Number of Quantum Walks
M. Štefaňák, I. Jex, and T. Kiss
Published 14 January 2008
020501  Full Text: PDF (110 kB)  | Buy Article
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We analyze the recurrence probability (Pólya number) for d-dimensional unbiased quantum walks. A sufficient condition for a quantum walk to be recurrent is derived. As a by-product we find a simple criterion for localization of quantum walks. In contrast with classical walks, where the Pólya number is characteristic for the given dimension, the recurrence probability of a quantum walk depends in general on the topology of the walk, choice of the coin and the initial state. This allows us to change the character of the quantum walk from recurrent to transient by altering the initial state.

Robust Quantum Error Correction via Convex Optimization
Robert L. Kosut, Alireza Shabani, and Daniel A. Lidar
Published 16 January 2008
020502  Full Text: PDF (200 kB)  | Buy Article
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We present a semidefinite program optimization approach to quantum error correction that yields codes and recovery procedures that are robust against significant variations in the noise channel. Our approach allows us to optimize the encoding, recovery, or both, and is amenable to approximations that significantly improve computational cost while retaining fidelity. We illustrate our theory numerically for optimized 5-qubit codes, using the standard [5,1,3] code as a benchmark. Our optimized encoding and recovery yields fidelities that are uniformly higher by 1–2 orders of magnitude against random unitary weight-2 errors compared to the [5,1,3] code with standard recovery.

Local Distinguishability of Multipartite Unitary Operations
Runyao Duan, Yuan Feng, and Mingsheng Ying
Published 17 January 2008
020503  Full Text: PDF (121 kB)  | Buy Article
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We show that any two different unitary operations acting on an arbitrary multipartite quantum system can be perfectly distinguished by local operations and classical communication when a finite number of runs is allowed. Intuitively, this result indicates that the lost identity of a nonlocal unitary operation can be recovered locally. No entanglement between distant parties is required.

Failure of the Work-Hamiltonian Connection for Free-Energy Calculations
Jose M. G. Vilar and J. Miguel Rubi
Published 16 January 2008
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Extensions of statistical mechanics are routinely being used to infer free energies from the work performed over single-molecule nonequilibrium trajectories. A key element of this approach is the ubiquitous expression dW/dt=[partial-derivative]H(x,t)/[partial-derivative]t, which connects the microscopic work W performed by a time-dependent force on the coordinate x with the corresponding Hamiltonian H(x,t) at time t. Here we show that this connection, as pivotal as it is, cannot be used to estimate free-energy changes. We discuss the implications of this result for single-molecule experiments and atomistic molecular simulations and point out possible avenues to overcome these limitations.

New Duality Relations for Classical Ground States
S. Torquato and F. H. Stillinger
Published 17 January 2008
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We derive new duality relations that link the energy of configurations associated with a class of soft pair potentials to the corresponding energy of the dual (Fourier-transformed) potential. We apply them by showing how information about the classical ground states of short-ranged potentials can be used to draw new conclusions about the nature of the ground states of long-ranged potentials and vice versa. They also lead to bounds on the T=0 system energies in density intervals of phase coexistence, the identification of a one-dimensional system that exhibits an infinite number of “phase transitions,” and a conjecture regarding the ground states of purely repulsive monotonic potentials.

Well-Tempered Metadynamics: A Smoothly Converging and Tunable Free-Energy Method
Alessandro Barducci, Giovanni Bussi, and Michele Parrinello
Published 18 January 2008
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We present a method for determining the free-energy dependence on a selected number of collective variables using an adaptive bias. The formalism provides a unified description which has metadynamics and canonical sampling as limiting cases. Convergence and errors can be rigorously and easily controlled. The parameters of the simulation can be tuned so as to focus the computational effort only on the physically relevant regions of the order parameter space. The algorithm is tested on the reconstruction of an alanine dipeptide free-energy landscape.

Gravitation and Astrophysics

Flavor Evolution of the Neutronization Neutrino Burst From an O-Ne-Mg Core-Collapse Supernova
Huaiyu Duan, George M. Fuller, J. Carlson, and Yong-Zhong Qian
Published 18 January 2008
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We present results of 3-neutrino flavor evolution simulations for the neutronization burst from an O-Ne-Mg core-collapse supernova. We find that nonlinear neutrino self-coupling engineers a single spectral feature of stepwise conversion in the inverted neutrino mass hierarchy case and in the normal mass hierarchy case, a superposition of two such features corresponding to the vacuum neutrino mass-squared differences associated with solar and atmospheric neutrino oscillations. These neutrino spectral features offer a unique potential probe of the conditions in the supernova environment and may allow us to distinguish between O-Ne-Mg and Fe core-collapse supernovae.

Lorentz Violation for Photons and Ultrahigh-Energy Cosmic Rays
Matteo Galaverni and Günter Sigl
Published 18 January 2008
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Lorentz symmetry breaking at very high energies may lead to photon dispersion relations of the form omega2=k2+xink2(k/MPl)n with new terms suppressed by a power n of the Planck mass MPl. We show that first and second order terms of size |xi1|>~10-14 and xi2<~-10-6, respectively, would lead to a photon component in cosmic rays above 1019 eV that should already have been detected, if corresponding terms for e± are significantly smaller. This suggests that LI breaking suppressed up to second order in the Planck scale is unlikely to be phenomenologically viable for photons.

Fitting Cosmic Microwave Background Data with Cosmic Strings and Inflation
Neil Bevis, Mark Hindmarsh, Martin Kunz, and Jon Urrestilla
Published 14 January 2008
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We perform a multiparameter likelihood analysis to compare measurements of the cosmic microwave background (CMB) power spectra with predictions from models involving cosmic strings. Adding strings to the standard case of a primordial spectrum with power-law tilt ns, we find a 2sigma detection of strings: f10=0.11±0.05, where f10 is the fractional contribution made by strings in the temperature power spectrum (at [script-l]=10). CMB data give moderate preference to the model ns=1 with cosmic strings over the standard zero-strings model with variable tilt. When additional non-CMB data are incorporated, the two models become on a par. With variable ns and these extra data, we find that f10<0.11, which corresponds to Gµ<0.7×10-6 (where µ is the string tension and G is the gravitational constant).

Simultaneous Solution to Dark Matter and Flavor Problems of Supersymmetry
Jonathan L. Feng, Bryan T. Smith, and Fumihiro Takayama
Published 15 January 2008
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Neutralino dark matter is well motivated, but also suffers from two shortcomings: it requires gravity-mediated supersymmetry breaking, which generically violates flavor constraints, and its thermal relic density Omega is typically too large. We propose a simple solution to both problems: neutralinos freeze-out with Omega~10–100, but then decay to ~1 GeV gravitinos, which are simultaneously light enough to satisfy flavor constraints and heavy enough to be all of dark matter. This scenario is naturally realized in high-scale gauge-mediation models, ameliorates small scale structure problems, and implies that “cosmologically excluded” models may, in fact, be cosmologically preferred.

First Results from the XENON10 Dark Matter Experiment at the Gran Sasso National Laboratory
J. Angle et al. (XENON Collaboration)
Published 17 January 2008
021303  Full Text: PDF (1331 kB)  | Buy Article
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The XENON10 experiment at the Gran Sasso National Laboratory uses a 15 kg xenon dual phase time projection chamber to search for dark matter weakly interacting massive particles (WIMPs). The detector measures simultaneously the scintillation and the ionization produced by radiation in pure liquid xenon to discriminate signal from background down to 4.5 keV nuclear-recoil energy. A blind analysis of 58.6 live days of data, acquired between October 6, 2006, and February 14, 2007, and using a fiducial mass of 5.4 kg, excludes previously unexplored parameter space, setting a new 90% C.L. upper limit for the WIMP-nucleon spin-independent cross section of 8.8×10-44 cm2 for a WIMP mass of 100 GeV/c2, and 4.5×10-44 cm2 for a WIMP mass of 30 GeV/c2. This result further constrains predictions of supersymmetric models.

Information Theory, Spectral Geometry, and Quantum Gravity
Achim Kempf and Robert Martin
Published 18 January 2008
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We show that there exists a deep link between the two disciplines of information theory and spectral geometry. This allows us to obtain new results on a well-known quantum gravity motivated natural ultraviolet cutoff which describes an upper bound on the spatial density of information. Concretely, we show that, together with an infrared cutoff, this natural ultraviolet cutoff beautifully reduces the path integral of quantum field theory on curved space to a finite number of ordinary integrations. We then show, in particular, that the subsequent removal of the infrared cutoff is safe.

Elementary Particles and Fields

Monte Carlo Studies of Supersymmetric Matrix Quantum Mechanics with Sixteen Supercharges at Finite Temperature
Konstantinos N. Anagnostopoulos, Masanori Hanada, Jun Nishimura, and Shingo Takeuchi
Published 15 January 2008
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We present the first Monte Carlo results for supersymmetric matrix quantum mechanics with 16 supercharges at finite temperature. The recently proposed nonlattice simulation enables us to include the effects of fermionic matrices in a transparent and reliable manner. The internal energy nicely interpolates the weak coupling behavior obtained by the high temperature expansion, and the strong coupling behavior predicted from the dual black-hole geometry. The Polyakov line asymptotes at low temperature to a characteristic behavior for a deconfined theory, suggesting the absence of a phase transition. These results provide highly nontrivial evidence for the gauge-gravity duality.

Time-Dependent CP-Violating Asymmetry in B0-->rho0gamma Decays
Y. Ushiroda et al. (Belle Collaboration)
Published 18 January 2008
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We report the first measurement of CP-violation parameters in B0-->rho0gamma decays based on a data sample of 657×106B[overline B] pairs collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. We obtain the time-dependent and direct CP-violating parameters, [script S]rho0gamma=-0.83±0.65(stat)±0.18(syst) and [script A]rho0gamma=-0.44±0.49(stat)±0.14(syst), respectively.

Observation of the Semileptonic Decays B-->D*tau-[overline nu ]tau and Evidence for B-->Dtau-[overline nu ]tau
B. Aubert et al. (BABAR Collaboration)
Published 15 January 2008
021801  Full Text: PDF (259 kB)  | Buy Article
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We present measurements of the semileptonic decays B--->D0tau-[overline nu ]tau, B--->D*0tau-[overline nu ]tau, [overline B]0-->D+tau-[overline nu ]tau, and [overline B]0-->D*+tau-[overline nu ]tau, which are potentially sensitive to non-standard model amplitudes. The data sample comprises 232×106 Upsilon(4S)-->B[overline B] decays collected with the BABAR detector. From a combined fit to B- and [overline B]0 channels, we obtain the branching fractions [script B](B-->Dtau-[overline nu ]tau)=(0.86±0.24±0.11±0.06)% and [script B](B-->D*tau-[overline nu ]tau)=(1.62±0.31±0.10±0.05)% (normalized for the [overline B]0), where the uncertainties are statistical, systematic, and normalization-mode-related.

Electroweak Sudakov Corrections using Effective Field Theory
Jui-yu Chiu, Frank Golf, Randall Kelley, and Aneesh V. Manohar
Published 15 January 2008
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Electroweak Sudakov corrections of the form alphanlogms/MW,Z<sup>2</sup> are summed using renormalization group evolution in soft-collinear effective theory. Results are given for the scalar, vector, and tensor form factors for fermion and scalar particles. The formalism for including massive gauge bosons in soft-collinear effective theory is developed.

First Observation of the Decay B<sub>s</sub><sup>0</sup>-->D<sub>s</sub><sup>-</sup>D<sub>s</sub><sup>+</sup> and Measurement of Its Branching Ratio
T. Aaltonen et al. (CDF Collaboration)
Published 16 January 2008
021803  Full Text: PDF (284 kB)  | Buy Article
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We report the observation of the exclusive decay Bs<sup>0</sup>-->Ds<sup>-</sup>Ds<sup>+</sup> at the 7.5 standard deviation level using 355 pb-1 of data collected by the CDF II detector in p[overline p] collisions at sqrt(s)=1.96 TeV at the Fermilab Tevatron. We measure the relative branching ratio [script B](Bs<sup>0</sup>-->Ds<sup>-</sup>Ds<sup>+</sup>)/[script B](B0-->D-Ds<sup>+</sup>)=1.44-0.44<sup>+0.48</sup>. Using the world average value for [script B](B0-->D-Ds<sup>+</sup>), we find [script B](Bs<sup>0</sup>-->Ds<sup>-</sup>Ds<sup>+</sup>)=(9.4-4.2<sup>+4.4</sup>)×10-3. This provides a lower bound DeltaGammas<sup>CP</sup>/Gammas>=2[script B](Bs<sup>0</sup>-->Ds<sup>-</sup>Ds<sup>+</sup>)>1.2×10-2 at 95% C.L.

Nuclear Physics

Mass Shift and Width Broadening of J/psi in Hot Gluonic Plasma from QCD Sum Rules
Kenji Morita and Su Houng Lee
Published 14 January 2008
022301  Full Text: PDF (259 kB)  | Buy Article
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We investigate possible mass shift and width broadening of J/psi in hot gluonic matter using QCD sum rules. Input values of gluon condensates at finite temperature are extracted from lattice QCD data for the energy density and pressure. Although stability of the moment ratio is achieved only up to T/Tc~=1.05, the gluon condensates cause a decrease of the moment ratio, which results in a change of the spectral properties. Using the Breit-Wigner form for the phenomenological side, we find that the mass shift of J/psi just above Tc can reach maximally 200 MeV and the width can broaden to dozens of MeV.

Evidence for Radial Flow of Thermal Dileptons in High-Energy Nuclear Collisions
R. Arnaldi et al. (NA60 Collaboration)
Published 16 January 2008
022302  Full Text: PDF (218 kB)  | Buy Article
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The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158A GeV In-In collisions. An excess of pairs above the known meson decays has been reported before. We now present precision results on the associated transverse momentum spectra. The slope parameter Teff extracted from the spectra rises with dimuon mass up to the rho, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline signals a transition to an emission source with much smaller flow. This may well represent the first direct evidence for thermal radiation of partonic origin in nuclear collisions.

Nuclear Enhancement of Universal Dynamics of High Parton Densities
H. Kowalski, T. Lappi, and R. Venugopalan
Published 17 January 2008
022303  Full Text: PDF (254 kB)  | Buy Article
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We show that the enhancement of the saturation scale in large nuclei relative to the proton is significantly influenced by the effects of quantum evolution and the impact parameter dependence of dipole cross sections in high energy QCD. We demonstrate that there is a strong A dependence in diffractive deeply inelastic scattering and discuss its sensitivity to the measurement of the recoil nucleus.

Lightest Isotope of Bh Produced via the 209Bi(52Cr,n)260Bh Reaction
S. L. Nelson, K. E. Gregorich, I. Dragojević, M. A. Garcia, J. M. Gates, R. Sudowe, and H. Nitsche
Published 14 January 2008
022501  Full Text: PDF (203 kB)  | Buy Article
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The lightest isotope of Bh was produced in the new 209Bi(52Cr,n)260Bh reaction at the Lawrence Berkeley National Laboratory's 88-Inch Cyclotron. Positive identification was made by observation of eight correlated alpha particle decay chains in the focal plane detector of the Berkeley Gas-Filled Separator. 260Bh decays with a 35-9<sup>+19</sup> ms half-life by alpha particle emission mainly by a group at 10.16 MeV. The measured cross section of 59-20<sup>+29</sup> pb is compared to model predictions. The influence of the N=152 and Z=108 shells on alpha decay properties is discussed.

Coincidence Measurement of Residues and Light Particles in the Reaction 56Fe+p at 1 GeV per Nucleon with the Spallation Reactions Setup SPALADIN
E. Le Gentil, T. Aumann, C. O. Bacri, J. Benlliure, S. Bianchin, M. Böhmer, A. Boudard, J. Brzychczyk, E. Casarejos, M. Combet, L. Donadille, J. E. Ducret, M. Fernandez-Ordoñez, R. Gernhäuser, H. Johansson, K. Kezzar, T. Kurtukian-Nieto, A. Lafriakh, F. Lavaud, A. Le Fèvre, S. Leray, J. Lühning, J. Lukasik, U. Lynen, W. F. J. Müller, P. Pawlowski, S. Pietri, F. Rejmund, C. Schwarz, C. Sfienti, H. Simon, W. Trautmann, C. Volant, and O. Yordanov
Published 16 January 2008
022701  Full Text: PDF (284 kB)  | Buy Article
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The spallation of 56Fe in collisions with hydrogen at 1A GeV has been studied in inverse kinematics with the large-aperture setup SPALADIN at GSI. Coincidences of residues with low-center-of-mass kinetic energy light particles and fragments have been measured allowing the decomposition of the total reaction cross section into the different possible deexcitation channels. Detailed information on the evolution of these deexcitation channels with excitation energy has also been obtained. The comparison of the data with predictions of several deexcitation models coupled to the INCL4 intranuclear cascade model shows that only GEMINI can reasonably account for the bulk of collected results, indicating that in a light system with no compression and little angular momentum, multifragmentation might not be necessary to explain the data.

Atomic, Molecular, and Optical Physics

Experimental Determination of the Helium 2 3P1–1 1S0 Transition Rate
R. G. Dall, K. G. H. Baldwin, L. J. Byron, and A. G. Truscott
Published 15 January 2008
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See Also: Publisher's Note
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We present the first experimental determination of the 2 3P1–1 1S0 transition rate in helium and compare this measurement with theoretical quantum-electrodynamic predictions. The experiment exploits the very long (~1 minute) confinement times obtained for atoms magneto-optically trapped in an apparatus used to create a Bose-Einstein condensate of metastable (2 3S1) helium. The 2 3P1–1 1S0 transition rate is measured directly from the decay rate of the cold atomic cloud following 1083 nm laser excitation from the 2 3S1 to the 2 3P1 state, and from accurate knowledge of the 2 3P1 population. The value obtained is 177±8 s-1, which agrees very well with theoretical predictions, and has an accuracy that compares favorably with measurements for the same transition in heliumlike ions higher in the isoelectronic sequence.

In Situ Measurement of Three-Dimensional Ion Densities in Focused Femtosecond Pulses
J. Strohaber and C. J. G. J. Uiterwaal
Published 16 January 2008
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We image spatial distributions of Xeq+ ions in the focus of a laser beam of ultrashort, intense pulses in all three dimensions, with a resolution of ~3 µm and ~12 µm in the two transverse directions. This allows for studying ionization processes without spatially averaging ion yields. Our in situ ion imaging is also useful to analyze focal intensity profiles and to investigate the transverse modal purity of tightly focused beams of complex light. As an example, the intensity profile of a Hermite-Gaussian beam mode HG1,0 recorded with ions is found to be in good agreement with optical images.

Using Molecules to Measure Nuclear Spin-Dependent Parity Violation
D. DeMille, S. B. Cahn, D. Murphree, D. A. Rahmlow, and M. G. Kozlov
Published 17 January 2008
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Nuclear spin-dependent parity violation arises from weak interactions between electrons and nucleons and from nuclear anapole moments. We outline a method to measure such effects, using a Stark-interference technique to determine the mixing between opposite-parity rotational/hyperfine levels of ground-state molecules. The technique is applicable to nuclei over a wide range of atomic number, in diatomic species that are theoretically tractable for interpretation. This should provide data on anapole moments of many nuclei and on previously unmeasured neutral weak couplings.

Quantum Interference Spectroscopy of Rubidium-Helium Exciplexes Formed on Helium Nanodroplets
M. Mudrich, F. Stienkemeier, G. Droppelmann, P. Claas, and C. P. Schulz
Published 15 January 2008
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Femtosecond multiphoton pump-probe photoionization is applied to helium nanodroplets doped with rubidium (Rb). The yield of Rb+ ions features pronounced quantum interference (QI) fringes demonstrating the coherence of a superposition of electronic states on a time scale of tens of picoseconds. Furthermore, we observe QI in the yield of formed RbHe exciplex molecules. The quantum interferogram allows us to determine the vibrational structure of these unstable molecules. From a sliced Fourier analysis one cannot only extract the population dynamics of vibrational states but also follow their energetic evolution during the RbHe formation.

Electro-Optic Quantum Memory for Light Using Two-Level Atoms
G. Hétet, J. J. Longdell, A. L. Alexander, P. K. Lam, and M. J. Sellars
Published 16 January 2008
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We present a simple quantum memory scheme that allows for the storage of a light field in an ensemble of two-level atoms. The technique is analogous to the NMR gradient echo for which the imprinting and recalling of the input field are performed by controlling a linearly varying broadening. Our protocol is perfectly efficient in the limit of high optical depths and the output pulse is emitted in the forward direction. We provide a numerical analysis of the protocol together with an experiment performed in a solid state system. In close agreement with our model, the experiment shows a total efficiency of up to 15%, and a recall efficiency of 26%. We suggest simple realizable improvements for the experiment to surpass the no-cloning limit.

Nonlinear Dynamics, Fluid Dynamics, Classical Optics, etc.

Guiding and Focusing of Electromagnetic Fields with Wedge Plasmon Polaritons
Esteban Moreno, Sergio G. Rodrigo, Sergey I. Bozhevolnyi, L. Martín-Moreno, and F. J. García-Vidal
Published 14 January 2008
023901  Full Text: PDF (1585 kB)  | Buy Article
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We study theoretically electromagnetic modes guided by metallic wedges at telecom wavelengths. These modes are found to exhibit superior confinement while showing similar propagation loss as compared to other subwavelength guiding configurations. It is also shown that mode focusing can be realized by gradual modification of the wedge geometry along the mode propagation direction.

One-Way Electromagnetic Waveguide Formed at the Interface between a Plasmonic Metal under a Static Magnetic Field and a Photonic Crystal
Zongfu Yu, Georgios Veronis, Zheng Wang, and Shanhui Fan
Published 17 January 2008
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We demonstrate theoretically the existence of one-way electromagnetic modes in a waveguide formed between a semi-infinite photonic crystal structure and a semi-infinite metal region under a static magnetic field. Such a waveguide provides a frequency range where only one propagating direction is allowed. In this frequency range, disorder-induced scattering is completely suppressed. Such a waveguide also modifies the basic properties of waveguide-cavity interaction.

Experimental Demonstration of Electromagnetic Tunneling Through an Epsilon-Near-Zero Metamaterial at Microwave Frequencies
Ruopeng Liu, Qiang Cheng, Thomas Hand, Jack J. Mock, Tie Jun Cui, Steven A. Cummer, and David R. Smith
Published 18 January 2008
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Silveirinha and Engheta have recently proposed that electromagnetic waves can tunnel through a material with an electric permittivity (epsilon) near zero (ENZ). An ENZ material of arbitrary geometry can thus serve as a perfect coupler between incoming and outgoing waveguides with identical cross-sectional area, so long as one dimension of the ENZ is electrically small. In this Letter we present an experimental demonstration of microwave tunneling between two planar waveguides separated by a thin ENZ channel. The ENZ channel consists of a planar waveguide in which complementary split ring resonators are patterned on the lower surface. A tunneling passband is found in transmission measurements, while a two-dimensional spatial map of the electric field distribution reveals a uniform phase variation across the channel—both measurements in agreement with theory and numerical simulations.

When Can One Observe Good Hydrodynamic Lyapunov Modes?
Hong-liu Yang and Günter Radons
Published 14 January 2008
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Inspired by recent results on differences in fluctuations of finite-time Lyapunov exponents between hard-core and soft-potential systems we surmise that partial domination of the Oseledec splitting (DOS) with respect to subspaces associated with near-zero Lyapunov exponents is essential for observing good hydrodynamic Lyapunov modes (HLMs). Numerical results for coupled map lattices are presented to show the importance of DOS for observing good HLMs. This is achieved by relating splitting parameters to the maximum value of the Lyapunov mode structure factor.

Loss Enhanced Phase Locking in Coupled Oscillators
Vardit Eckhouse, Moti Fridman, Nir Davidson, and Asher A. Friesem
Published 15 January 2008
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Phase locking, which is achieved by transferring some energy from one oscillator to the others, strongly depends on the coupling strength between the oscillators. Typically, the coupling strength must be above a certain threshold in order to achieve phase locking. Here we show how this threshold can be significantly reduced when phase-dependent losses are introduced into the oscillators. Specifically, the coupling strength can be reduced by at least an order of magnitude, thereby substantially decreasing the needed transfer of energy between oscillators. The resulting enhancement of phase locking does not only influence the laser research area, but also affects many other areas that involve coupled ensembles.

Experimental Realization of Quantum-Resonance Ratchets at Arbitrary Quasimomenta
I. Dana, V. Ramareddy, I. Talukdar, and G. S. Summy
Published 17 January 2008
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Quantum-resonance ratchets associated with the kicked particle are experimentally realized for arbitrary quasimomentum using a Bose-Einstein condensate (BEC) exposed to a pulsed standing light wave. The ratchet effect for general quasimomentum arises even though both the standing-wave potential and the initial state of the BEC have a point symmetry. The experimental results agree well with theoretical ones which take into account the finite quasimomentum width of the BEC. In particular, this width is shown to cause a suppression of the ratchet acceleration for exactly resonant quasimomentum, leading to a saturation of the directed current.
Scattering Theory Derivation of a 3D Acoustic Cloaking Shell
Steven A. Cummer, Bogdan-Ioan Popa, David Schurig, David R. Smith, John Pendry, Marco Rahm, and Anthony Starr
Published 14 January 2008
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Through acoustic scattering theory we derive the mass density and bulk modulus of a spherical shell that can eliminate scattering from an arbitrary object in the interior of the shell—in other words, a 3D acoustic cloaking shell. Calculations confirm that the pressure and velocity fields are smoothly bent and excluded from the central region as for previously reported electromagnetic cloaking shells. The shell requires an anisotropic mass density with principal axes in the spherical coordinate directions and a radially dependent bulk modulus. The existence of this 3D cloaking shell indicates that such reflectionless solutions may also exist for other wave systems that are not isomorphic with electromagnetics.

Transfer of Angular Momentum to Matter from Acoustical Vortices in Free Space
Karen Volke-Sepúlveda, Arturo O. Santillán, and Ricardo R. Boullosa
Published 16 January 2008
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An experimental demonstration of the mechanical transfer of orbital angular momentum to matter from acoustical vortices in free field is presented. Vortices with topological charges l=±1 and l=±2 were generated and a torsion pendulum was used to study the angular momentum transfer to hanging disks of several sizes. This allowed us to make a comparative study of the effective acoustical torque in terms of topological charge of the vortex, the disk radius, and its position along the main propagation axis. A theoretical discussion of the generated sound fields is also provided.

Molecular Dynamics Study of Contact Mechanics: Contact Area and Interfacial Separation from Small to Full Contact
C. Yang and B. N. J. Persson
Published 16 January 2008
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We report a molecular dynamics study of the contact between a rigid solid with a randomly rough surface and an elastic block with a flat surface. We study the contact area and the interfacial separation from small contact (low load) to full contact (high load). For small load the contact area varies linearly with the load and the interfacial separation depends logarithmically on the load. For high load the contact area approaches the nominal contact area (i.e., complete contact), and the interfacial separation approaches zero. The present results may be very important for soft solids, e.g., rubber, or for very smooth surfaces, where complete contact can be reached at moderate high loads without plastic deformation of the solids.
focus

Decompressing Emulsion Droplets Favors Coalescence
Nicolas Bremond, Abdou R. Thiam, and Jérôme Bibette
Published 15 January 2008
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The destabilization process of an emulsion under flow is investigated in a microfluidic device. The experimental approach enables us to generate a periodic train of droplet pairs, and thus to isolate and analyze the basic step of the destabilization, namely, the coalescence of two droplets which collide. We demonstrate a counterintuitive phenomenon: coalescence occurs during the separation phase and not during the impact. Separation induces the formation of two facing nipples in the contact area that hastens the connection of the interfaces prior to fusion. Moreover, droplet pairs initially stabilized by surfactants can be destabilized by forcing the separation. Finally, we note that the fusion mechanism is responsible for a cascade of coalescence events in a compact system of droplets where the separation is driven by surface tension.

Plasma and Beam Physics

Effective-Viscosity Approach for Nonlocal Electron Kinetics in Inductively Coupled Plasmas
G. J. M. Hagelaar
Published 15 January 2008
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In inductively coupled plasmas, nonlocal electron kinetics lead to the anomalous skin effect. We show that this can be approximately described through a fluid equation for electron momentum including a viscosity term with an effective-viscosity coefficient. The solution of this momentum equation coupled with the Maxwell equations is in good agreement with results from a particle-in-cell simulation over a wide range of conditions, reproducing the nonmonotonic structure of the anomalous skin with sometimes local negative power absorption.

Artificial Collimation of Fast-Electron Beams with Two Laser Pulses
A. P. L. Robinson, M. Sherlock, and P. A. Norreys
Published 17 January 2008
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A scheme for artificially collimating fast-electron beams produced in high intensity (>1019 W cm-2) laser-solid interactions is proposed. The scheme uses a laser pulse at the relativistic threshold ([approximate]1018 W cm-2) that precedes the high intensity pulse to pregenerate a collimating magnetic field. This concept is supported by analytical calculations and numerical calculations performed using a novel hybrid-Vlasov-Fokker-Planck code called LEDA. This scheme may be highly useful for fast ignition inertial confinement fusion.

Heat Transport in a Two-Dimensional Complex (Dusty) Plasma at Melting Conditions
V. Nosenko, S. Zhdanov, A. V. Ivlev, G. Morfill, J. Goree, and A. Piel
Published 17 January 2008
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The heat transport in a two-dimensional complex (dusty) plasma undergoing a phase transition was studied experimentally. A single layer of highly charged polymer microspheres was suspended in a plasma sheath. A part of this lattice was heated by two counterpropagating focused laser beams that moved rapidly around in the lattice and provided short intense random kicks to the particles. Above a threshold, the lattice locally melted. The spatial profiles of the particle kinetic temperature were analyzed to find a thermal conductivity, which did not depend on temperature.

Mitigation of the Plasma-Implosion Inhomogeneity in Starlike Wire-Array Z Pinches
V. V. Ivanov, V. I. Sotnikov, A. Haboub, A. P. Shevelko, A. L. Astanovitskiy, A. Morozov, E. D. Kazakov, and S. D. Altemara
Published 18 January 2008
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Implosions in starlike triple and quadruple wire arrays were investigated in a 1 MA Zebra generator. Implosion in these loads is directed along the rays of the star and cascades from wire to wire to the center. Shadowgraphy shows improved homogeneity of imploding plasma and mitigation of instabilities. Despite the low azimuthal symmetry, starlike wire arrays produce a stable x-ray pulse with the highest peak power of >0.4 TW and the shortest duration of 8–12 ns among different types of tested loads. This can be linked to stabilization of instabilities due to the multiple nesting.

Multiscaling Dynamics of Impurity Transport in Drift-Wave Turbulence
S. Futatani, S. Benkadda, Y. Nakamura, and K. Kondo
Published 18 January 2008
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Intermittency effects and the associated multiscaling spectrum of exponents are investigated for impurities advection in tokamak edge plasmas. The two-dimensional Hasagawa-Wakatani model of resistive drift-wave turbulence is used as a paradigm to describe edge tokamak turbulence. Impurities are considered as a passive scalar advected by the plasma turbulent flow. The use of the extended self-similarity technique shows that the structure function relative scaling exponent of impurity density and vorticity follows the She-Leveque model. This confirms the intermittent character of the impurities advection in the turbulent plasma flow and suggests that impurities are advected by vorticity filaments.

Condensed Matter: Structure, etc.

Detection and Imaging of He2 Molecules in Superfluid Helium
W. G. Rellergert, S. B. Cahn, A. Garvan, J. C. Hanson, W. H. Lippincott, J. A. Nikkel, and D. N. McKinsey
Published 15 January 2008
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We present data that show a cycling transition can be used to detect and image metastable He2 triplet molecules in superfluid helium. We demonstrate that limitations on the cycling efficiency due to the vibrational structure of the molecule can be mitigated by the use of repumping lasers. Images of the molecules obtained using the method are also shown. This technique gives rise to a new kind of ionizing radiation detector. The use of He2 triplet molecules as tracer particles in the superfluid promises to be a powerful tool for visualization of both quantum and classical turbulence in liquid helium.

NMR Measurement of the Magnetic Field Correlation Function in Porous Media
H. Cho and Yi-Qiao Song
Published 15 January 2008
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The structure factor provides a fundamental characterization of porous and granular materials as it is the key for solid crystals via measurements of x-ray and neutron scattering. Here, we demonstrate that the structure factor of the granular and porous media can be approximated by the pair correlation function of the inhomogeneous internal magnetic field, which arises from the susceptibility difference between the pore filling liquid and the solid matrix. In-depth understanding of the internal field is likely to contribute to further development of techniques to study porous and granular media.

Temperature and Strain-Rate Dependence of Surface Dislocation Nucleation
Ting Zhu, Ju Li, Amit Samanta, Austin Leach, and Ken Gall
Published 15 January 2008
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Dislocation nucleation is essential to the plastic deformation of small-volume crystalline solids. The free surface may act as an effective source of dislocations to initiate and sustain plastic flow, in conjunction with bulk sources. Here, we develop an atomistic modeling framework to address the probabilistic nature of surface dislocation nucleation. We show the activation volume associated with surface dislocation nucleation is characteristically in the range of 1–10b3, where b is the Burgers vector. Such small activation volume leads to sensitive temperature and strain-rate dependence of the nucleation stress, providing an upper bound to the size-strength relation in nanopillar compression experiments.

Critical Role of Inelastic Interactions in Quantitative Electron Microscopy
K. A. Mkhoyan, S. E. Maccagnano-Zacher, M. G. Thomas, and J. Silcox
Published 17 January 2008
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A semiquantitative correlation between experimental observations and theoretical prediction in electron microscopy is achieved. Experiments conducted on amorphous silicon in the convergent beam electron diffraction mode provide measurements of the reduction of the central-disk intensity. In addition to elastic scattering the effects of multiple inelastic scattering of the probe electrons were incorporated into the theory describing beam propagation through the specimen. With incorporation of the dominant plasmon scattering a better than 10% match of the theory with experiment is observed indicating the critical role of multiple inelastic scattering in quantitative electron diffraction and imaging.

Nanoscale Imaging of Buried Structures with Elemental Specificity Using Resonant X-Ray Diffraction Microscopy
Changyong Song, Raymond Bergstrom, Damien Ramunno-Johnson, Huaidong Jiang, David Paterson, Martin D. de Jonge, Ian McNulty, Jooyoung Lee, Kang L. Wang, and Jianwei Miao
Published 18 January 2008
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We report the first demonstration of resonant x-ray diffraction microscopy for element specific imaging of buried structures with a pixel resolution of ~15 nm by exploiting the abrupt change in the scattering cross section near electronic resonances. We performed nondestructive and quantitative imaging of buried Bi structures inside a Si crystal by directly phasing coherent x-ray diffraction patterns acquired below and above the Bi M5 edge. We anticipate that resonant x-ray diffraction microscopy will be applied to element and chemical state specific imaging of a broad range of systems including magnetic materials, semiconductors, organic materials, biominerals, and biological specimens.

Bond-Counting Rule for Carbon and its Application to the Roughness of Diamond (001)
H. X. Yang, L. F. Xu, Z. Fang, C. Z. Gu, and S. B. Zhang
Published 14 January 2008
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Despite that carbon is tetravalent identical to silicon, first-principles calculations reveal that stable step structures on diamond (001) are entirely different from those on silicon. Moreover, pristine Si(001) is flat; pristine diamond (001) could be rough due to negative step formation energies. A generic bond-counting rule is established, which should apply to most carbon structures where sp2 and sp3 hybrids coexist: e.g., it provides a qualitative account of the step energy order without detailed calculation. Our findings agree with experimental observations.

Formation Mechanism of One-Dimensional Si Islands on a H/Si(001) Surface
Jun Nara, Hiroshi Kajiyama, Tomihiro Hashizume, Yuji Suwa, Seiji Heike, Shinobu Matsuura, Taro Hitosugi, and Takahisa Ohno
Published 14 January 2008
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The formation mechanism of one-dimensional Si islands on a H/Si(001)-(2×1) surface is studied using scanning tunneling microscopy/spectroscopy and first-principles calculations. We observed that one-dimensional islands that are made from dimer chains are formed at the initial growth stages similar to the bare Si(001) surface. It is found that the number of odd-numbered dimer chains is larger than that of even-numbered dimer chains. We propose the growth processes of the two types of growth edges to explain the observation.

Structural Origin of the Sn 4d Core Level Line Shape in Sn/Ge(111)-(3×3)
A. Tejeda, R. Cortés, J. Lobo-Checa, C. Didiot, B. Kierren, D. Malterre, E. G. Michel, and A. Mascaraque
Published 15 January 2008
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High-resolution photoemission of the Sn 4d core level of Sn/Ge(111)-(3×3) resolves three main components in the line shape, which are assigned to each of the three Sn atoms that form the unit cell. The line shape found is in agreement with an initial state picture and supports that the two down atoms are inequivalent. In full agreement with these results, scanning tunnel microscopy images directly show that the two down atoms are at slightly different heights in most of the surface, giving rise to an inequivalent-down-atoms (3×3) structure. These results solve a long-standing controversy on the interpretation of the Sn 4d core-level line shape and the structure of Sn/Ge(111)-(3×3).

Polar- and Azimuth-Angle-Dependent Rotational and Vibrational Excitation of Desorbing Product CO2 in CO Oxidation on Palladium Surfaces
Toshiro Yamanaka and Tatsuo Matsushima
Published 17 January 2008
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Clear polar and azimuth angle dependencies were found in rotational and vibrational energies of product CO2 in CO oxidation on Pd surfaces. On Pd(110)-(1×1), with increases in polar angle, both energies decreased in the [001] direction but remained constant in [1[overline 1]0]. On the Pd(110) with missing rows, both energies increased in [001] but decreased in [1[overline 1]0], indicating that the transition state changes with the geometry of the substrate. On Pd(111), the rotational energy greatly increased, but the vibrational energy decreased. Such angular dependence of internal energy provides new dimensions in surface reaction dynamics.

Condensed Matter: Electronic Properties, etc.

Hydrogen Bonding in Sodium Alanate: A Muon Spin Rotation Study
R. Kadono, K. Shimomura, K. H. Satoh, S. Takeshita, A. Koda, K. Nishiyama, E. Akiba, R. M. Ayabe, M. Kuba, and C. M. Jensen
Published 15 January 2008
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We have detected the occurrence of hydrogen bonding involving an interstitial positive muon situated between hydrogen atoms of two independent alanate anions in sodium alanate (NaAlH4). Ti doping, which is known to dramatically improve the hydrogen cycling performance of NaAlH4, reduces the kinetic barrier of the transition of the muon from the muon-dialanate state to a mobile interstitial state. This observation strongly suggests that hydrogen bonding is the primary bottleneck for hydrogen release or uptake in sodium alanate, which might be common to other complex hydrides.

In Situ Photoemission Study of Pr1-xCaxMnO3 Epitaxial Thin Films with Suppressed Charge Fluctuations
H. Wadati, A. Maniwa, A. Chikamatsu, I. Ohkubo, H. Kumigashira, M. Oshima, A. Fujimori, M. Lippmaa, M. Kawasaki, and H. Koinuma
Published 15 January 2008
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We have performed an in situ photoemission study of Pr1-xCaxMnO3 (PCMO) thin films grown on LaAlO3 (001) substrates and observed the effect of epitaxial strain on the electronic structure. We found that the chemical potential shifted monotonically with doping, unlike bulk PCMO, implying the disappearance of incommensurate charge fluctuations of bulk PCMO. In the valence-band spectra, we found a doping-induced energy shift toward the Fermi level (EF) but there was no spectral weight transfer, which was observed in bulk PCMO. The gap at EF was clearly seen in the experimental band dispersions determined by angle-resolved photoemission spectroscopy and could not be explained by the metallic band structure of the C-type antiferromagnetic state, probably due to localization of electrons along the ferromagnetic chain direction or due to another type of spin-orbital ordering.

Scaling and Enhanced Symmetry at the Quantum Critical Point of the Sub-Ohmic Bose-Fermi Kondo Model
Stefan Kirchner and Qimiao Si
Published 16 January 2008
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We consider the finite-temperature scaling properties of a Kondo-destroying quantum critical point in the Ising-anisotropic Bose-Fermi Kondo model (BFKM). A cluster-updating Monte Carlo approach is used, in order to reliably access a wide temperature range. The scaling function for the two-point spin correlator is found to have the form dictated by a boundary conformal field theory, even though the underlying Hamiltonian lacks conformal invariance. Similar conclusions are reached for all multipoint correlators of the spin-isotropic BFKM in a dynamical large-N limit. Our results suggest that the quantum critical local properties of the sub-Ohmic BFKM are those of an underlying boundary conformal field theory.

Strongly Correlated Superconductivity in Rh17S15
H. R. Naren, A. Thamizhavel, A. K. Nigam, and S. Ramakrishnan
Published 16 January 2008
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In this Letter, we report resistivity, susceptibility, heat capacity, and upper critical field studies on a polycrystalline Rh17S15 sample which exhibits superconductivity below 5.4 K. Detailed studies suggest that the superconductivity in this compound arises from strongly correlated charge carriers presumably due to the high density of states of Rh d bands at the Fermi level. Moreover, the Hall coefficient shows a sign change and increases at low temperature before the sample becomes a superconductor below 5.4 K.

Magnetic State around Cation Vacancies in II–VI Semiconductors
T. Chanier, I. Opahle, M. Sargolzaei, R. Hayn, and M. Lannoo
Published 17 January 2008
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The magnetic properties of neutral cation vacancies in II–VI semiconductors are examined using first principles calculations and group theory. A molecular cluster model of a single vacancy in II–VI semiconductors is developed to explain the observed chemical trend. We show that a single Zn vacancy in ZnO yields a total spin ST=1 in agreement with experiments. But for the other less ionic Zn-based II–VI semiconductors ZnA (A=S, Se, Te) this spin triplet state is nearly degenerate with a nonmagnetic spin singlet state.

Direct Observation of t2g Orbital Ordering in Magnetite
J. Schlappa, C. Schüßler-Langeheine, C. F. Chang, H. Ott, A. Tanaka, Z. Hu, M. W. Haverkort, E. Schierle, E. Weschke, G. Kaindl, and L. H. Tjeng
Published 17 January 2008
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Using soft-x-ray diffraction at the site-specific resonances in the Fe L2,3 edge, we find clear evidence for orbital and charge ordering in magnetite below the Verwey transition. The spectra show directly that the (00(1/2)) diffraction peak (in cubic notation) is caused by t2g orbital ordering at octahedral Fe2+ sites and the (001) by a spatial modulation of the t2g occupation.

Direct Link between Low-Temperature Magnetism and High-Temperature Sodium Order in NaxCoO2
T. F. Schulze, P. S. Häfliger, Ch. Niedermayer, K. Mattenberger, S. Bubenhofer, and B. Batlogg
Published 18 January 2008
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We prove the direct link between low-temperature (T) magnetism and high-T Na+ ordering in NaxCoO2 using the example of a so far unreported magnetic transition at 8 K which involves a weak ferromagnetic moment. The 8 K feature is characterized in detail and its dependence on a diffusive Na+ rearrangement around 200 K is demonstrated. Applying muons as local probes this process is shown to result in a reversible phase separation into distinct magnetic phases that can be controlled by specific cooling protocols. Thus the impact of ordered Na+ Coulomb potential on the CoO2 physics is evidenced opening new ways to experimentally revisit the NaxCoO2 phase diagram.

Universality of Liquid-Gas Mott Transitions at Finite Temperatures
Stefanos Papanikolaou, Rafael M. Fernandes, Eduardo Fradkin, Philip W. Phillips, Joerg Schmalian, and Rastko Sknepnek
Published 18 January 2008
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We explain, in a consistent manner, the set of seemingly conflicting experiments on the finite temperature Mott critical point, and demonstrate that the Mott transition is in the Ising universality class. We show that, even though the thermodynamic behavior of the system near such critical point is described by an Ising order parameter, the global conductivity can depend on other singular observables and, in particular, on the energy density. Finally, we show that in the presence of weak disorder the dimensionality of the system has crucial effects on the size of the critical region that is probed experimentally.

Coupling between an Optical Phonon and the Kondo Effect
K. S. Burch, Elbert E. M. Chia, D. Talbayev, B. C. Sales, D. Mandrus, A. J. Taylor, and R. D. Averitt
Published 18 January 2008
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We explore the ultrafast optical response of Yb14MnSb11, providing further evidence that this compound is the first d-electron, ferromagnetic, underscreened Kondo lattice. These results also provide the first demonstration of coupling between an optical phonon mode and the Kondo effect.

Dynamics of Quantum Noise in a Tunnel Junction under ac Excitation
J. Gabelli and B. Reulet
Published 16 January 2008
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We report the first measurement of the dynamical response of shot noise (measured at frequency omega) of a tunnel junction to an ac excitation at frequency omega0. The experiment is performed in the quantum regime, [h-bar]omega~[h-bar]omega0>>kBT at very low temperature T=35 mK and high frequency omega0/2pi=6.2 GHz. We observe that the noise responds in phase with the excitation, but not adiabatically. The results are in very good agreement with a prediction based on a new current-current correlator.

Phase Diagram of the Spin Hall Effect
E. M. Hankiewicz and G. Vignale
Published 17 January 2008
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We obtain analytic formulas for the frequency-dependent spin-Hall conductivity of a two-dimensional electron gas (2DEG) in the presence of impurities, linear spin-orbit Rashba interaction, and external magnetic field perpendicular to the 2DEG. We show how different mechanisms (skew scattering, side jump, and spin precession) can be brought in or out of focus by changing controllable parameters such as frequency, magnetic field, and temperature. We find, in particular, that the dc spin-Hall conductivity vanishes in the absence of a magnetic field, while a magnetic field restores the skew-scattering and side jump contributions proportionally to the ratio of magnetic and Rashba fields.
Shock Waves in Nanomechanical Resonators
Florian W. Beil, Achim Wixforth, Werner Wegscheider, Dieter Schuh, Max Bichler, and Robert H. Blick
Published 15 January 2008
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We study the formation of shock waves in a nanomechanical resonator with an embedded two-dimensional electron gas using surface acoustic waves. The mechanical displacement of the nanoresonator is read out via the induced acoustoelectric current. Applying acoustical standing waves, we are able to determine the so-called anomalous acoustocurrent. This current is found only in the regime of shock wave formation.

Localized States at Zigzag Edges of Bilayer Graphene
Eduardo V. Castro, N. M. R. Peres, J. M. B. Lopes dos Santos, A. H. Castro Neto, and F. Guinea
Published 15 January 2008
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We report the existence of zero-energy surface states localized at zigzag edges of bilayer graphene. Working within the tight-binding approximation we derive the analytic solution for the wave functions of these peculiar surface states. It is shown that zero-energy edge states in bilayer graphene can be divided into two families: (i) states living only on a single plane, equivalent to surface states in monolayer graphene and (ii) states with a finite amplitude over the two layers, with an enhanced penetration into the bulk. The bulk and surface (edge) electronic structure of bilayer graphene nanoribbons is also studied, both in the absence and in the presence of a bias voltage between planes.

Electronic Transport in Phosphorus-Doped Silicon Nanocrystal Networks
A. R. Stegner, R. N. Pereira, K. Klein, R. Lechner, R. Dietmueller, M. S. Brandt, M. Stutzmann, and H. Wiggers
Published 16 January 2008
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We have investigated the role of doping and paramagnetic states on the electronic transport of networks assembled from freestanding Si nanocrystals (Si-NCs). Electrically detected magnetic resonance (EDMR) studies on Si-NCs films, which show a strong increase of conductivity with doping of individual Si-NCs, reveal that P donors and Si dangling bonds contribute to dark conductivity via spin-dependent hopping, whereas in photoconductivity, these states act as spin-dependent recombination centers of photogenerated electrons and holes. Comparison between EDMR and conventional electron paramagnetic resonance shows that different subsets of P-doped nanocrystals contribute to the different transport processes.

Weak Values and the Leggett-Garg Inequality in Solid-State Qubits
Nathan S. Williams and Andrew N. Jordan
Published 16 January 2008
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An implementation of weak values is investigated in solid-state qubits. We demonstrate that a weak value can be nonclassical if and only if a Leggett-Garg inequality can also be violated. Generalized weak values are described in which post-selection occurs on a range of weak measurement results. Imposing classical weak values permits the derivation of Leggett-Garg inequalities for bounded operators. Our analysis is presented in terms of kicked quantum nondemolition measurements on a quantum double-dot charge qubit.

Coulomb Drag at Zero Temperature
Alex Levchenko and Alex Kamenev
Published 16 January 2008
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We show that the Coulomb drag effect exhibits saturation at small temperatures, when calculated to the third order in the interlayer interactions. The zero-temperature transresistance is of the order h/(e2g3), where g is the dimensionless sheet conductance. The effect is therefore the strongest in low mobility samples. This behavior should be contrasted with the conventional (second order) prediction that the transresistance scales as a certain power of temperature and is (almost) mobility independent. The result demonstrates that the zero-temperature drag is not an unambiguous signature of a strongly coupled state in double-layer systems.

Role of Hydrogen in Giant Spin Polarization Observed on Magnetic Nanostructures
Werner A. Hofer, Krisztián Palotás, Stefano Rusponi, Tristan Cren, and Harald Brune
Published 17 January 2008
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We demonstrate that the giant spin contrast observed by scanning tunneling microscopy for double-layer Coislands on Pt(111) is caused by adsorbates at the apex of the Cr-coated W tip. The most likely candidate, in ab initio simulations, is hydrogen. Here, the electron charge is highly polarized by the adjacent Cr layers. The hydrogen adsorption site is shown to change from hollow to on top due to the electric field at the tip apex, created by the tunnel voltage.

Kondo Effect from a Tunable Bound State within a Quantum Wire
F. Sfigakis, C. J. B. Ford, M. Pepper, M. Kataoka, D. A. Ritchie, and M. Y. Simmons
Published 17 January 2008
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We investigate the conductance of quantum wires with a variable open quantum dot geometry, displaying an exceptionally strong Kondo effect and most of the 0.7 structure characteristics. Our results indicate that the 0.7 structure is not a manifestation of the singlet Kondo effect. However, specific similarities between our devices and many of the clean quantum wires reported in the literature suggest a weakly bound state is often present in real quantum wires.

Testable Signatures of Quantum Nonlocality in a Two-Dimensional Chiral p-Wave Superconductor
Sumanta Tewari, Chuanwei Zhang, S. Das Sarma, Chetan Nayak, and Dung-Hai Lee
Published 17 January 2008
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A class of topological excitations—the odd-winding number vortices—in a spinless 2D chiral p-wave (px+ipy) superconductor traps Majorana fermion states in the vortex cores. For a dilute gas of such vortices, the lowest energy fermionic eigenstates are intrinsically nonlocal. We predict two testable signatures of this unusual quantum nonlocality in quasiparticle tunneling experiments. We discuss why the associated teleportationlike phenomenon does not imply the violation of causality.

Non-Abelian Superconducting Pumps
Valentina Brosco, Rosario Fazio, F. W. J. Hekking, and Alain Joye
Published 17 January 2008
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Cooper pair pumping is a coherent process. We derive a general expression for the adiabatic pumped charge in superconducting nanocircuits in the presence of level degeneracy and relate it to non-Abelian holonomies of Wilczek and Zee. We discuss an experimental system where the non-Abelian structure of the adiabatic evolution manifests in the pumped charge.

Stability of Nodal Quasiparticles in Superconductors with Coexisting Orders
E. Berg, C-C. Chen, and S. A. Kivelson
Published 18 January 2008
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We establish a condition for the perturbative stability of zero energy nodal points in the quasiparticle spectrum of superconductors in the presence of coexisting commensurate orders. The nodes are found to be stable if the Hamiltonian is invariant under time reversal followed by a lattice translation. The principle is demonstrated with a few examples. Some experimental implications of various types of assumed order are discussed in the context of the cuprate superconductors.

Smectic Vortex Phase in Optimally Doped YBa2Cu3O7 Thin Films
S. A. Baily, B. Maiorov, H. Zhou, F. F. Balakirev, M. Jaime, S. R. Foltyn, and L. Civale
Published 18 January 2008
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Angular dependent resistivity measurements of optimally doped YBa2Cu3O7 films in fields H pulsed to 50 T are presented. Up to the highest H, the vortex melting field Hm increases and vortex motion is reduced for H aligned with the correlated pinning centers along the main crystalline axes, otherwise 3D anisotropic scaling describes the vortex dynamics. For H||ab, the rapid increase in Hm at low temperatures and a critical exponent analysis near Hm confirm the presence of the liquid-crystalline smectic phase predicted for layered superconductors.