Physical Review Letters -- March 3, 2006

Effective Theory of Excitations in a Feshbach-Resonant Superfluid

Published 28 February 2006

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A strongly interacting Fermi gas, such as that of cold atoms operative near a Feshbach resonance, is difficult to study by perturbative many-body theory to go beyond mean-field approximation. Here I develop an effective field theory for the resonant superfluid based on broken symmetry. The theory retains both fermionic quasiparticles and superfluid phonons, the interaction between them being derived nonperturbatively. The theory converges and can be improved order by order, in a manner governed by a low energy expansion rather than by a coupling constant. I apply the effective theory to calculate the specific heat and discuss the theory with a recent heat capacity experiment.

Direct Experimental Evidence of Free-Fermion Antibunching

M. Iannuzzi, A. Orecchini, F. Sacchetti, P. Facchi, and S. Pascazio

Published 1 March 2006

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Fermion antibunching was observed on a beam of free noninteracting neutrons. A monochromatic beam of thermal neutrons was first split by a graphite single crystal, then fed to two detectors, displaying a reduced coincidence rate. The result is a fermionic complement to the Hanbury Brown and Twiss effect for photons.

Casimir Interaction between a Plate and a Cylinder

T. Emig, R. L. Jaffe, M. Kardar, and A. Scardicchio

Published 1 March 2006

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We find the exact Casimir force between a plate and a cylinder, a geometry intermediate between parallel plates, where the force is known exactly, and the plate sphere, where it is known at large separations. The force has an unexpectedly weak decay ~L/[H³ln(H/R)] at large plate-cylinder separations H (L and R are the cylinder length and radius), due to transverse magnetic modes. Path integral quantization with a partial wave expansion additionally gives a qualitative difference for the density of states of electric and magnetic modes, and corrections at finite temperatures.

Bosonization, Pairing, and Superconductivity of the Fermionic Tonks-Girardeau Gas

M. D. Girardeau and A. Minguzzi

Published 2 March 2006

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We determine some exact static and time-dependent properties of the fermionic Tonks-Girardeau (FTG) gas, a spin-aligned one-dimensional Fermi gas with infinitely strongly attractive zero-range odd-wave interactions. We show that its two-particle reduced density matrix exhibits superconductive off-diagonal long-range order, and on a ring an FTG gas with an even number of atoms has a highly degenerate ground state with quantization of Coriolis rotational flux and high sensitivity to rotation and to external fields and accelerations. For a gas initially under harmonic confinement, we show that during an expansion the momentum distribution undergoes a "dynamical bosonization," approaching that of an ideal Bose gas without violating the Pauli exclusion principle.

Einstein–de Haas Effect in Dipolar Bose-Einstein Condensates

Yuki Kawaguchi, Hiroki Saito, and Masahito Ueda

Published 3 March 2006

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The general properties of the order parameter for a dipolar spinor Bose-Einstein condensate are discussed based on symmetries of interactions. An initially spin-polarized dipolar condensate is shown to dynamically generate a nonsingular vortex via spin-orbit interactions—a phenomenon reminiscent of the Einstein–de Haas effect in ferromagnets.

Entanglement Reciprocation between Qubits and Continuous Variables

Jinhyoung Lee, M. Paternostro, M. S. Kim, and S. Bose

Published 28 February 2006

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We investigate how entanglement can be transferred between qubits and continuous-variable (CV) systems. We find that one ebit borne in maximally entangled qubits can be fully transferred to two CV systems which are initially prepared in a pure separable Gaussian field with high excitation. We show that it is possible to retrieve the entanglement back to qubits from the entangled CV systems. The deposition of multiple ebits from qubits to the initially separable CV systems is also pointed out. We show that the entanglement transfer and retrieval are done at a quasisteady state.

Extremality of Gaussian Quantum States

Michael M. Wolf, Geza Giedke, and J. Ignacio Cirac

Published 2 March 2006

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We investigate Gaussian quantum states in view of their exceptional role within the space of all continuous variables states. A general method for deriving extremality results is provided and applied to entanglement measures, secret key distillation and the classical capacity of bosonic quantum channels. We prove that for every given covariance matrix the distillable secret key rate and the entanglement, if measured appropriately, are minimized by Gaussian states. This result leads to a clearer picture of the validity of frequently made Gaussian approximations. Moreover, it implies that Gaussian encodings are optimal for the transmission of classical information through bosonic channels, if the capacity is additive.

D=5 Einstein-Maxwell-Chern-Simons Black Holes

Jutta Kunz and Francisco Navarro-Lérida

Published 27 February 2006

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Five-dimensional Einstein-Maxwell-Chern-Simons theory with a Chern-Simons coefficient lambda

=1 has supersymmetric black holes with a vanishing horizon angular velocity but finite angular momentum. Here supersymmetry is associated with a borderline between stability and instability, since for lambda

>1 a rotational instability arises, where counterrotating black holes appear, whose horizon rotates in the opposite sense to the angular momentum. For lambda

>2 black holes are no longer uniquely characterized by their global charges, and rotating black holes with vanishing angular momentum appear.

Leptogenesis from Gravity Waves in Models of Inflation

Stephon H. S. Alexander, Michael E. Peskin, and M. M. Sheikh-Jabbari

Published 27 February 2006

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We present a new mechanism for creating the observed cosmic matter-antimatter asymmetry which satisfies all three Sakharov conditions from one common thread, gravitational waves. We generate lepton number through the gravitational anomaly in the lepton number current. The source term comes from elliptically polarized gravity waves that are produced during inflation if the inflaton field contains a CP-odd component. The amount of matter asymmetry generated in our model can be of realistic size for the parameters within the range of some inflationary scenarios and grand unified theories.

Locking Information in Black Holes

John A. Smolin and Jonathan Oppenheim

Published 28 February 2006

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We show that a central presumption in the debate over black-hole information loss is incorrect. Ensuring that information not escape during evaporation does not require that it all remain trapped until the final stage of the process. Using the recent quantum information-theoretic result of locking, we show that the amount of information that must remain can be very small, even as the amount already radiated is negligible. Information need not be additive: A small system can lock a large amount of information, making it inaccessible. Only if the set of initial states is restricted can information leak.

J^-- Glueballs and a Low Odderon Intercept

Felipe J. Llanes-Estrada, Pedro Bicudo, and Stephen R. Cotanch

Published 1 March 2006

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We report an odderon Regge trajectory emerging from a field theoretical Coulomb gauge QCD model for the odd signature J^PC (P=C=-1) glueball states. The trajectory intercept is clearly smaller than the Pomeron and even the omega

trajectory's intercept which provides an explanation for the nonobservation of the odderon in high energy scattering data. To further support this result we compare to glueball lattice data and also perform calculations with an alternative model based upon an exact Hamiltonian diagonalization for three constituent gluons.

Topological Mass Generation in Four Dimensions

Gia Dvali, R. Jackiw, and So-Young Pi

Published 2 March 2006

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We show that in a large class of physically interesting systems the mass-generation phenomenon can be understood in terms of topological structures, without requiring a detailed knowledge of the underlying dynamics. This is first demonstrated by showing that Schwinger's mechanism for mass generation relies on topological structures of a two-dimensional gauge theory. In the same manner, corresponding four-dimensional topological entities give rise to topological mass generation in four dimensions. This formulation offers a unified topological description of some seemingly unrelated phenomena, such as two-dimensional superconductivity, and the generation of eta

and axion masses by QCD, and possibly by gravity.

Neutron–Mirror-Neutron Oscillations: How Fast Might They Be?

Zurab Berezhiani and Luís Bento

Published 27 February 2006

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We discuss the phenomenological implications of the neutron (n) oscillation into the mirror neutron (n), a hypothetical particle exactly degenerate in mass with the neutron but sterile to normal matter. We show that the present experimental data allow a maximal n-n oscillation in vacuum with a characteristic time tau

much shorter than the neutron lifetime, in fact as small as 1 sec. This phenomenon may manifest in neutron disappearance and regeneration experiments perfectly accessible to present experimental capabilities and may also have interesting astrophysical consequences, in particular, for the propagation of ultra high energy cosmic rays.

New Measurements of Cabibbo-Suppressed Decays of D Mesons with the CLEO-c Detector

P. Rubin et al. (CLEO Collaboration)

Published 28 February 2006

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Using 281 pb^-1 of data collected with the CLEO-c detector, we report on first observations and measurements of Cabibbo-suppressed decays of D mesons in the following six decay modes:

⁺

^-

⁰

⁰,

⁺

^-

⁰,

⁺

⁰

⁰,

⁺

^-

⁰,

⁰, and

⁺

^-. Improved branching fraction measurements in eight other multipion decay modes are also presented. The measured D -->

rates allow us to extract the ratio of isospin amplitudes A( Delta

I=3/2)/A(

I=1/2)=0.420±0.014(stat)±0.016(syst) and the strong phase shift of delta

I=(86.4±2.8±3.3)°, which is quite large and now more precisely determined.

Search for Higgs Bosons Decaying to b and Produced in Association with W Bosons in p Collisions at =1.96 TeV

A. Abulencia et al. (CDF Collaboration)

Published 1 March 2006

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We present a search for Higgs bosons decaying into b [overline b]

and produced in association with W bosons in p [overline p]

collisions at sqrt(s)

=1.96 TeV. This search uses 320 pb^-1 of the data set accumulated by the upgraded Collider Detector at Fermilab. Events are selected that have a high-transverse momentum electron or muon, missing transverse energy, and two jets, at least one of which is consistent with the hadronization of a b quark. Both the number of events and the dijet mass distribution are consistent with standard model background expectations, and we set 95% confidence level upper limits on the production cross section times branching ratio for the Higgs boson or any new particle with similar decay kinematics. These upper limits range from 10 pb for mH=110 GeV/c² to 3 pb for mH=150 GeV/c².

Probing Small x Parton Densities in Ultraperipheral AA and pA Collisions at the CERN Large Hadron Collider

Mark Strikman, Ramona Vogt, and Sebastian White

Published 27 February 2006

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We calculate photoproduction rates for several hard processes in ultraperipheral proton-lead and lead-lead collisions at the CERN Large Hadron Collider (LHC) with sqrt(s[sub NN])

=8.8 and 5.5 TeV, respectively, which could be triggered in the large LHC detectors. We use ATLAS as an example. The lead ion is treated as a source of (coherently produced) photons with energies and intensities greater than those of equivalent ep collisions at the DESY collider HERA. We find very large rates for both inclusive and diffractive production that will extend the HERA x range by nearly an order of magnitude for similar virtualities. We demonstrate that it is possible to reach the kinematic regime where nonlinear effects are larger than at HERA.

Evidence for the Exclusive Decay BJ/^± and Measurement of the Mass of the B Meson

A. Abulencia et al. (CDF Collaboration)

Published 28 February 2006

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We report the first evidence for a fully reconstructed decay mode of the B c[plus-minus]

meson in the channel B c[plus-minus]

J/

^±, with J/ psi

µ⁺µ^-. The analysis is based on an integrated luminosity of 360 pb^-1 in p [overline p]

collisions at 1.96 TeV center of mass energy collected by the Collider Detector at Fermilab. We observe 14.6±4.6 signal events with a background of 7.1±0.9 events, and a fit to the J/ psi

^± mass spectrum yields a B c[plus-minus]

mass of 6285.7±5.3(stat)±1.2(syst) MeV/c². The probability of a peak of this magnitude occurring by random fluctuation in the search region is estimated as 0.012%.

Observation of a Candidate in D Production at Belle

S. Uehara et al. (Belle Collaboration)

Published 1 March 2006

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We report on a search for new resonant states in the process gamma

D

. A candidate C-even charmonium state is observed in the vicinity of 3.93 GeV/c². The production rate and the angular distribution in the gamma

center-of-mass frame suggest that this state is the previously unobserved chi

, the 2³P2 charmonium state.

Observation of (3770)J/ and Measurement of _ee[(2S)]

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

Published 3 March 2006

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We observe signals for the decays psi

(3770)

XJ/

from data acquired with the CLEO detector operating at the CESR e⁺e^- collider with sqrt(s)

=3773 MeV. We measure the following branching fractions [script B]

(

(3770)

XJ/

) and significances: (189±20±20)×10^-5 (11.6 sigma

) for X=

⁺

^-, (80±25±16)×10^-5 (3.4 sigma

) for X=

⁰

⁰, and (87±33±22)×10^-5 (3.5 sigma

) for X=

, where the errors are statistical and systematic, respectively. The radiative return process e⁺e^- -->

(2S) populates the same event sample and is used to measure Gamma

ee[

(2S)]=(2.54±0.03±0.11) keV.

Measurement of Two- and Three-Nucleon Short-Range Correlation Probabilities in Nuclei

K. S. Egiyan et al. (CLAS Collaboration)

Published 1 March 2006

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The ratios of inclusive electron scattering cross sections of ⁴He, ¹²C, and ⁵⁶Fe to ³He have been measured at 1<xB<3. At Q²>1.4 GeV², the ratios exhibit two separate plateaus, at 1.5<xB<2 and at xB>2.25. This pattern is predicted by models that include 2- and 3-nucleon short-range correlations (SRC). Relative to A=3, the per-nucleon probabilities of 3-nucleon SRC are 2.3, 3.1, and 4.4 times larger for A=4, 12, and 56. This is the first measurement of 3-nucleon SRC probabilities in nuclei.

Magnetic Field-Induced Spectroscopy of Forbidden Optical Transitions with Application to Lattice-Based Optical Atomic Clocks

A. V. Taichenachev, V. I. Yudin, C. W. Oates, C. W. Hoyt, Z. W. Barber, and L. Hollberg

Published 1 March 2006

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We develop a method of spectroscopy that uses a weak static magnetic field to enable direct optical excitation of forbidden electric-dipole transitions that are otherwise prohibitively weak. The power of this scheme is demonstrated using the important application of optical atomic clocks based on neutral atoms confined to an optical lattice. The simple experimental implementation of this method—a single clock laser combined with a dc magnetic field—relaxes stringent requirements in current lattice-based clocks (e.g., magnetic field shielding and light polarization), and could therefore expedite the realization of the extraordinary performance level predicted for these clocks. We estimate that a clock using alkaline-earth-like atoms such as Yb could achieve a fractional frequency uncertainty of well below 10^-17 for the metrologically preferred even isotopes.

Direct Excitation of the Forbidden Clock Transition in Neutral ¹⁷⁴Yb Atoms Confined to an Optical Lattice

Z. W. Barber, C. W. Hoyt, C. W. Oates, L. Hollberg, A. V. Taichenachev, and V. I. Yudin

Published 1 March 2006

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We report direct single-laser excitation of the strictly forbidden (6s²)¹S0 <-->

(6s6p)³P0 clock transition in ¹⁷⁴Yb atoms confined to a 1D optical lattice. A small (~1.2 mT) static magnetic field was used to induce a nonzero electric dipole transition probability between the clock states at 578.42 nm. Narrow resonance linewidths of 20 Hz (FWHM) with high contrast were observed, demonstrating a resonance quality factor of 2.6×10¹³. The previously unknown ac Stark shift-canceling (magic) wavelength was determined to be 759.35±0.02 nm. This method for using the metrologically superior even isotope can be easily implemented in current Yb and Sr lattice clocks and can create new clock possibilities in other alkaline-earth-like atoms such as Mg and Ca.

Limits to the Polarization for Spin-Exchange Optical Pumping of ³He

E. Babcock, B. Chann, T. G. Walker, W. C. Chen, and T. R. Gentile

Published 3 March 2006

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Based on measurements of the temperature dependence of ³He relaxation in a wide range of spin-exchange optical pumping cells, we report evidence for a previously unrecognized surface relaxation process. The relaxation rate was found to be linearly proportional to the alkali-metal density with a slope that exceeds the spin-exchange rate, which limits the polarization for current applications, including neutron spin filters, polarized targets, and polarized gas magnetic resonance imaging. We find that the magnitude of this excess relaxation can vary widely between cells, and that the variation is larger for cells of higher surface to volume ratio. We have observed ³He polarization as high as 81%, but further improvements require understanding the origin of this relaxation.

Filtering Out Photonic Fock States

Kaoru Sanaka, Kevin J. Resch, and Anton Zeilinger

Published 27 February 2006

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Unprecedented optical nonlinearities can be generated probabilistically in simple linear-optical networks conditioned on specific measurement outcomes. We describe a highly controllable quantum filter for photon number states, which takes advantage of such a measurement-induced amplitude nonlinearity. The basis for this filter is multiphoton nonclassical interference which we demonstrate for one- and two-photon states over a wide range of beam splitter reflectivities. Specifically, we show that the transmission probability, conditional on a specific measurement outcome, can be larger for a two-photon state than a one-photon state; this is not possible with linear optics alone.

Using High-Power Lasers for Detection of Elastic Photon-Photon Scattering

E. Lundström, G. Brodin, J. Lundin, M. Marklund, R. Bingham, J. Collier, J. T. Mendonça, and P. Norreys

Published 2 March 2006

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The properties of four-wave interaction via the nonlinear quantum vacuum is investigated. The effect of the quantum vacuum is to generate photons with new frequencies and wave vectors, due to elastic photon-photon scattering. An expression for the number of generated photons is derived, and using state-of-the-art laser data it is found that the number of photons can reach detectable levels. In particular, the prospect of using the high-repetition Astra Gemini system at the Rutherford Appleton Laboratory is discussed. The problem of noise sources is reviewed, and it is found that the noise level can be reduced well below the signal level. Thus, detection of elastic photon-photon scattering may for the first time be achieved.

Conclusive Evidence of an Attosecond Pulse Train Observed with the Mode-Resolved Autocorrelation Technique

Yasuo Nabekawa, Toshihiko Shimizu, Tomoya Okino, Kentaro Furusawa, Hirokazu Hasegawa, Kaoru Yamanouchi, and Katsumi Midorikawa

Published 27 February 2006

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We report on the direct observation of an attosecond pulse train with a mode-resolved autocorrelation technique. The chirp among the three harmonic fields is specified by analyzing two-photon above-threshold ionization spectra of electrons, resulting in a pulse duration that should be shorter than 450 as, which is, to our knowledge, the first determination of the chirp in the attosecond pulse train with an autocorrelation technique. These results will open the way to full characterization of an attosecond pulse train with its envelope.

Generation and Weak Beam Control of Two-Dimensional Multicolored Arrays in a Quadratic Nonlinear Medium

Heping Zeng, Jian Wu, Han Xu, and Kun Wu

Published 2 March 2006

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We report on the first experimental observation of 2D multicolored transverse arrays in a quadratic nonlinear medium under the pump of two crossly overlapped femtosecond beams. The 2D reproducible patterns are caused by cascaded noncollinear quadratic nonlinear couplings between the input pulses and quadratic spatial solitons originated from spatial breakup of one of the input beams with spatial ellipticity. A seed supercontinuum pulse is then diffracted and amplified with phase preservation, resulting in the formation of up-converted multicolor 2D transverse arrays. By seeding with weak second harmonic pulses, the 2D multicolored transverse patterns are suppressed due to weak beam control of the induced quadratic spatial solitons.

Dynamical Control: Comparison of Map and Continuous-Flow Approaches

I. A. Khovanov, N. A. Khovanova, E. V. Grigorieva, D. G. Luchinsky, and P. V. E. McClintock

Published 3 March 2006

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Continuous and pulsed forms of control of a multistable system are compared directly, both theoretically and numerically, taking as an example the switching of a periodically driven class-B laser between its stable and unstable pulsing regimes. It is shown that continuous control is the more energy efficient. This result is illuminated by making use of the close correspondence that exists between the problems of energy-optimal control and the stability of a steady state.

Observation of Discrete Solitons and Soliton Rotation in Optically Induced Periodic Ring Lattices

Xiaosheng Wang, Zhigang Chen, and P. G. Kevrekidis

Published 3 March 2006

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We report the first experimental demonstration of ring-shaped photonic lattices by optical induction and the formation of discrete solitons in such radially symmetric lattices. The transition from discrete diffraction to single-channel guidance or nonlinear self-trapping of a probe beam is achieved by fine-tuning the lattice potential or the focusing nonlinearity. In addition to solitons trapped in the lattice center and in different lattice rings, we demonstrate controlled soliton rotation in the Bessel-like ring lattices.

Self-Optimization of Optical Confinement in an Ultraviolet Photonic Crystal Slab Laser

A. Yamilov, X. Wu, X. Liu, R. P. H. Chang, and H. Cao

Published 3 March 2006

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We studied numerically and experimentally the effects of structural disorder on the performance of ultraviolet photonic crystal slab lasers. Optical gain selectively amplifies the high-quality modes of the passive system. For these modes, the in-plane and out-of-plane leakage rates may be automatically balanced in the presence of disorder. The spontaneous optimization of in-plane and out-of-plane confinement of light in a photonic crystal slab may lead to a reduction of the lasing threshold.

High-Rayleigh-Number Convection in a Vertical Channel

M. Gibert, H. Pabiou, F. Chillà, and B. Castaing

Published 28 February 2006

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We measure the relation between convective heat flux and temperature gradient in a vertical channel filled with water, the average vertical mass flux being zero. Compared to the classical Rayleigh-Bénard case, this situation has the advantage of avoiding plates and, thus, their neighborhood, in which is usually concentrated most of the temperature gradient. Consequently, inertial processes should control the convection, with poor influence of the viscosity. This idea gives a good account of our observations, if we consider that a natural vertical length, different from the channel width, appears. Our results also suggest that heat fluxes can be deduced from velocity measurements in free convective flows. This confers to our results a wide range of applications.

Physical Mechanism of the Two-Dimensional Inverse Energy Cascade

Shiyi Chen, Robert E. Ecke, Gregory L. Eyink, Michael Rivera, Minping Wan, and Zuoli Xiao

Published 28 February 2006

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We study the physical mechanisms of the two-dimensional inverse energy cascade using theory, numerics, and experiment. Kraichnan's prediction of a -5/3 spectrum with constant, negative energy flux is verified in our simulations of 2D Navier-Stokes equations. We observe a similar but shorter range of inverse cascade in laboratory experiments. Our theory predicts, and the data confirm, that inverse cascade results mainly from turbulent stress proportional to small-scale strain rotated by 45°. This "skew-Newtonian" stress is explained by the elongation and thinning of small-scale vortices by large-scale strain which weakens their velocity and transfers their energy upscale.

Microevaporators for Kinetic Exploration of Phase Diagrams

Jacques Leng, Barbara Lonetti, Patrick Tabeling, Mathieu Joanicot, and Armand Ajdari

Published 1 March 2006

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We use pervaporation-based microfluidic devices to concentrate species in aqueous solutions with spatial and temporal control of the process. Using experiments and modeling, we quantitatively describe the advection-diffusion behavior of the concentration field of various solutions (electrolytes, colloids, etc.) and demonstrate the potential of these devices as universal tools for the kinetic exploration of the phases and textures that form upon concentration.

Consistent Theory of Turbulent Transport in Two-Dimensional Magnetohydrodynamics

Eun-jin Kim

Published 3 March 2006

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A theory of turbulent transport is presented in two-dimensional magnetohydrodynamics with background shear and magnetic fields. We provide theoretical predictions for the transport of magnetic flux, momentum, and particles and turbulent intensities, which show stronger reduction compared with the hydrodynamic case, with different dependences on shearing rate, magnetic field, and values of viscosity, Ohmic diffusion, and particle diffusivity. In particular, particle transport is more severely suppressed than momentum transport, effectively leading to a more efficient momentum transport. The role of magnetic fields in quenching transport without altering the amplitude of flow velocity and in inhibiting the generation of shear flows is elucidated. Implications of the results are discussed.

Harmonic Lasing in a Free-Electron-Laser Amplifier

B. W. J. McNeil, G. R. M. Robb, M. W. Poole, and N. R. Thompson

Published 2 March 2006

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A method is demonstrated that allows a planar wiggler high-gain Free-Electron-Laser (FEL) amplifier to lase so that the interaction with an odd harmonic of the radiation field dominates that of the fundamental. This harmonic lasing of the FEL is achieved by disrupting the electron interaction with the usually dominant fundamental while allowing that of a harmonic interaction to evolve unhindered. The disruption is achieved by a series of relative phase changes between the electrons and the ponderomotive potentials of both the fundamental and harmonic fields. Such phase changes are relatively easy to implement and some current FEL designs would require little or no structural modification to test the scheme.

Transient Electrostatic Fields and Related Energetic Proton Generation with a Plasma Fiber

Z. L. Chen, G. R. Kumar, Z. M. Sheng, T. Matsuoka, Y. Sentoku, M. Tampo, K. A. Tanaka, T. Tsutsumi, T. Yabuuchi, and R. Kodama

Published 3 March 2006

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We observe a hollow structure and a fine ring in the proton images from a petawatt scale laser interaction with a "cone-fiber" target. The protons related to the hollow structure are accelerated from the cone-tip surface and deflected later by a radial electric field surrounding the fiber. Those associated with the fine ring are accelerated from the fiber surface by this radial electric field. This field is found to decay exponentially within 3 ps from about 5×10¹² V/m. Two-dimensional particle-in-cell simulations produce similar proton angular distributions.

Impurity Pinch from a Ratchet Process

M. Vlad, F. Spineanu, and S. Benkadda

Published 28 February 2006

085001 Full Text: PDF (164 kB) | Buy Article

+ -	Show Abstract A ratchet-type average velocity is shown to appear for test particles moving in a stochastic potential and a magnetic field that is space dependent. This is a possible explanation for impurity behavior in tokamak plasmas.

Production of Dense Plasmas with sub-10-fs Laser Pulses

J. Osterholz, F. Brandl, T. Fischer, D. Hemmers, M. Cerchez, G. Pretzler, O. Willi, and S. J. Rose

Published 28 February 2006

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Close to solid state density plasmas with peak electron temperatures of about 190 eV have been generated with sub-10-fs laser pulses incident on solid targets. Extreme ultraviolet (XUV) spectroscopy is used to investigate the K shell emission from the plasma. In the spectra, a series limit for the H- and He-like resonance lines becomes evident which is explained by pressure ionization in the dense plasma. The spectra are consistent with computer simulations calculating the XUV emission and the expansion of the plasma.

An Exact Magnetic-Moment Invariant of Charged-Particle Gyromotion

Hong Qin and Ronald C. Davidson

Published 3 March 2006

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For the motion of a charged particle in a uniform, time-dependent axial magnetic field B(t)ez, it is shown that there is an exact magnetic-moment invariant of the particle dynamics M, to which the adiabatic magnetic-moment invariant µ=mv [perpendicular]2

/2B is asymptotic when the time scale of the magnetic field variation is much slower than the gyroperiod. The connection between the exact invariant M and the adiabatic invariant µ enables us to characterize in detail the robustness of the adiabatic magnetic-moment invariant µ.

Vortex Multiplication in Applied Flow: A Precursor to Superfluid Turbulence

A. P. Finne, V. B. Eltsov, G. Eska, R. Hänninen, J. Kopu, M. Krusius, E. V. Thuneberg, and M. Tsubota

Published 27 February 2006

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A surface-mediated process is identified in ³He-B which generates vortices at a roughly constant rate. It precedes a faster form of turbulence where intervortex interactions dominate. This precursor becomes observable when vortex loops are introduced in low-velocity rotating flow at sufficiently low mutual friction dissipation at temperatures below 0.5Tc. Our measurements indicate that the formation of new loops is associated with a single vortex interacting in the applied flow with the sample boundary. Numerical calculations show that the single-vortex instability arises when a helical Kelvin wave expands from a reconnection kink at the wall and then intersects again with the wall.

Hardness of Covalent and Ionic Crystals: First-Principle Calculations

Antonín Šimůnek and Jiří Vackář

Published 3 March 2006

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A new concept, the strength of bond, and a new form expressing the hardness of covalent and ionic crystals are presented. Hardness is expressed by means of quantities inherently coupled to the atomistic structure of matter, and, therefore, hardness can be determined by first-principles calculations. Good agreement between theory and experiment is observed in the range of 2 orders of magnitude. It is shown that a lower coordination number of atoms results in higher hardness, contrary to common opinion presented in general literature.

Nonequilibrium Steady States in Sheared Binary Fluids

P. Stansell, K. Stratford, J.-C. Desplat, R. Adhikari, and M. E. Cates

Published 2 March 2006

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We simulate by lattice Boltzmann the steady shearing of a binary fluid mixture undergoing phase separation with full hydrodynamics in two dimensions. Contrary to some theoretical scenarios, a dynamical steady state is attained with finite domain lengths Lx,y in the directions (x,y) of velocity and velocity gradient. Apparent scaling exponents are estimated as Lx~ gamma-dot

^-2/3 and Ly~ gamma-dot

^-3/4. We discuss the relative roles of diffusivity and hydrodynamics in attaining steady state.

Nonlinear Ripple Dynamics on Amorphous Surfaces Patterned by Ion Beam Sputtering

Javier Muñoz-García, Mario Castro, and Rodolfo Cuerno

Published 27 February 2006

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Erosion by ion-beam sputtering (IBS) of amorphous targets at off-normal incidence frequently produces a (nanometric) rippled surface pattern, strongly resembling macroscopic ripples on aeolian sand dunes. A suitable generalization of continuum descriptions of the latter allows us to describe theoretically for the first time the main nonlinear features of ripple dynamics by IBS, namely, wavelength coarsening and nonuniform translation velocity, that agree with similar results in experiments and discrete models. These properties are seen to be the anisotropic counterparts of in-plane ordering and (interrupted) pattern coarsening in IBS experiments on rotating substrates and at normal incidence.

Orientation of Ordered Structures of Cytosine and Cytidine 5-Monophosphate Adsorbed at Au(110)/Liquid Interfaces

P. Weightman, G. J. Dolan, C. I. Smith, M. C. Cuquerella, N. J. Almond, T. Farrell, D. G. Fernig, C. Edwards, and D. S. Martin

Published 1 March 2006

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It is demonstrated using reflection anisotropy spectroscopy that the adsorption of cytosine and cytidine 5-monophosphate at the Au(110) 1×2/electrolyte interface gives rise to ordered structures in which the base is oriented vertical to the surface and parallel to the [1 [overline 1]

0] axis of the Au(110) plane.

High-Resolution Resonance Measurements of the Physisorption Interaction

P. Linde and S. Andersson

Published 1 March 2006

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Narrow selective adsorption resonances, with line widths given by the natural lifetime of the intermediate resonance state, have been observed directly in the angular distributions of Ne nozzle beams diffracted from a 20 K Cu(110) surface. Using beams of ²⁰Ne and ²²Ne isotopes we show that this high-resolution mode of resonance detection permits, even in a case of a small isotope effect, unique assignment of bound level sequences compatible with a single gas-surface potential curve.

Bimodal Growth of Au on SrTiO₃(001)

Fabien Silly and Martin R. Castell

Published 2 March 2006

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We have investigated the vapor phase growth of Au on SrTiO3(001)-(2×1) substrates by UHV scanning tunneling microscopy. Submonolayer (ML) coverages below 300 °C wet the surface as disordered metastable 2D islands. Beyond 0.75 ML fcc nanocrystals with a (111) interface are nucleated and ripen by dewetting the surrounding layer. Some multiply twinned fivefold symmetric clusters are also created. Above 400 °C dewetting occurs for all coverages and the surface is only populated by nanocrystals and fivefold clusters. A planar ground state configuration for small Au clusters and a higher interface energy for crystals than for wetted 2D ML films explains these results.

Oscillatory Dissipation of a Simple Confined Liquid

Abdelhamid Maali, Touria Cohen-Bouhacina, Gérard Couturier, and Jean-Pierre Aimé

Published 3 March 2006

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We present a sensitive measurement of the dissipation and the effective viscosity of a simple confined liquid (octamethylcyclotetrasiloxane) using an atomic force microscope. The experimental data show that the damping and the effective viscosity increase and present oscillations as the gap between the cantilever tip and the surface is diminished. To our knowledge, the damping and the viscosity modulation are reported here with such good accuracy for the first time. Such an experimental result is different from what has been reported earlier where only a continuous increase of the damping and the viscosity are observed.

Self-Assembled Nanofold Network Formation on Layered Crystal Surfaces during Metal Intercalation

E. Spiecker, A. K. Schmid, A. M. Minor, U. Dahmen, S. Hollensteiner, and W. Jäger

Published 27 February 2006

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We study the formation of planar network nanostructures, which develop during metal deposition on initially smooth surfaces of layered compounds. Using in situ low-energy electron microscopy for dynamic observation and high-resolution transmission electron microscopy for structure analysis, we have observed the rapid formation of hexagonal networks of linear "nanofolds" with prismatic cavities on top of layered VSe2 crystals. Their formation results from relaxation of compressive strains which build up during Cu intercalation into a thin surface layer.

Self-Localization of Composite Spin-Lattice Polarons

Peter Prelov

ek, Roland Zeyher, and Peter Horsch

Published 1 March 2006

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Self-localization of holes in the Holstein t-J model is studied in the adiabatic limit using exact diagonalization and the retraceable path approximation. It is shown that the critical electron-phonon coupling lambda

c decreases with increasing J and that this behavior is determined mainly by the incoherent rather than by the coherent motion of the hole. The obtained spin correlation functions in the localized region can be understood within a percolation picture where antiferromagnetic order can persist up to a substantial hole doping. These results restrict the possibility of self-localization of holes in lightly doped cuprates.

Giant Anisotropy of Zeeman Splitting of Quantum Confined Acceptors in Si/Ge

K.-M. Haendel, R. Winkler, U. Denker, O. G. Schmidt, and R. J. Haug

Published 1 March 2006

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Shallow acceptor levels in Si/Ge/Si quantum well heterostructures are characterized by resonant-tunneling spectroscopy in the presence of high magnetic fields. In a perpendicular magnetic field we observe a linear Zeeman splitting of the acceptor levels. In an in-plane field, on the other hand, the Zeeman splitting is strongly suppressed. This anisotropic Zeeman splitting is shown to be a consequence of the huge light-hole–heavy-hole splitting caused by a large biaxial strain and a strong quantum confinement in the Ge quantum well.

Can Competition between the Crystal Field and the Kondo Effect Cause Non-Fermi-Liquid-Like Behavior?

Frithjof B. Anders and Thomas Pruschke

Published 1 March 2006

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The recently reported unusual behavior of the static and dynamical magnetic susceptibility as well as the specific heat in Ce1-xLaxNi9Ge4 has raised the question of a possible non-Fermi-liquid ground state in this material. We argue that for a consistent physical picture the crystal-field splitting of two low-lying magnetic doublets of the Ce 4f-shell must be taken into account. Furthermore, we show that for a splitting of the order of the low temperature scale T^* of the system a crossover behavior between an SU(4) and an SU(2) Kondo effect is found. The screening of the two doublets occurs on different temperature scales leading to a different behavior of the magnetic susceptibility and the specific heat at low temperatures. The experimentally accessible temperature regime down to 50 mK still lies in the extended crossover regime into a strong-coupling Fermi-liquid fixed point.

Ab Initio Prediction of Conduction Band Spin Splitting in Zinc Blende Semiconductors

Athanasios N. Chantis, Mark van Schilfgaarde, and Takao Kotani

Published 2 March 2006

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We use a recently developed self-consistent GW approximation to present systematic ab initio calculations of the conduction band spin splitting in III-V and II-VI zinc blende semiconductors. The spin-orbit interaction is taken into account as a perturbation to the scalar relativistic Hamiltonian. These are the first calculations of conduction band spin splittings based on a quasiparticle approach; and because the self-consistent GW scheme accurately reproduces the relevant band parameters, it is expected to be a reliable predictor of spin splittings. The results are compared to the few available experimental data and a previous calculation based on a model one-particle potential. We also briefly address the widely used k·p parametrization in the context of these results.

Spontaneous Formation of Vanadium "Molecules" in a Geometrically Frustrated Crystal: AlV₂O₄

Y. Horibe, M. Shingu, K. Kurushima, H. Ishibashi, N. Ikeda, K. Kato, Y. Motome, N. Furukawa, S. Mori, and T. Katsufuji

Published 3 March 2006

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We investigated the charge-ordered (CO) state in spinel AlV2O4 by electron diffraction, synchrotron x-ray diffraction, and magnetic measurements. It was found that the CO structure appearing below 700 K is characterized by the formation of V clusters (heptamers), each of which is consisting of 7 vanadium atoms and is in a spin-singlet state as a total. Theoretical consideration indicates that this unique molecularlike V heptamer is stabilized by a strong bonding of vanadium t2g orbitals.

Doped Spin Liquid: Luttinger Sum Rule and Low Temperature Order

R. M. Konik, T. M. Rice, and A. M. Tsvelik

Published 3 March 2006

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We analyze a model of two-leg Hubbard ladders weakly coupled by interladder tunneling. At half filling a semimetallic state with small Fermi pockets is induced beyond a threshold tunneling strength. The sign changes in the single electron Green's function relevant for the Luttinger sum rule now take place at surfaces with both zeros and infinities with important consequences for the interpretation of angle-resolved photoemission spectroscopy experiments. Residual interactions between electron and holelike quasiparticles cause a transition to long range order at low temperatures. The theory can be extended to small doping leading to superconducting order.

Charge-Transport Regime of Crystalline Organic Semiconductors: Diffusion Limited by Thermal Off-Diagonal Electronic Disorder

Alessandro Troisi and Giorgio Orlandi

Published 3 March 2006

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We propose that the electron transport in crystalline organic semiconductors at room temperature (RT) is neither polaronic nor a combination of thermally activated hopping and polaronic transport, as previously thought. Thermal molecular motions cause large fluctuations in the intermolecular transfer integrals that, in turn, localize the charge carrier. This effect destroys the translational symmetry of the electronic Hamiltonian and makes the band description inadequate for RT organic crystals. We used a one-dimensional semiclassical model to compute the (temperature dependent) charge carrier mobility in the presence of thermal fluctuations of the electronic Hamiltonian. This transport mechanism explains several contrasting experimental observations pointing sometimes to a delocalized "bandlike" transport and sometimes to the existence of strongly localized charge carriers.

Microwave-Induced Dephasing in One-Dimensional Metal Wires

J. Wei, S. Pereverzev, and M. E. Gershenson

Published 1 March 2006

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We report on the effect of monochromatic microwave (MW) radiation on the weak-localization corrections to the conductivity of quasi-one-dimensional silver wires. Because of the improved electron cooling in the wires, the MW-induced dephasing is observed without a concomitant overheating of electrons over wide ranges of the MW power PMW and frequency f. The observed dependences of the conductivity and MW-induced dephasing rate on PMW and f are in agreement with the theory by Altshuler, Aronov, and Khmelnitsky [Solid State Commun. 39, 619 (1981)]. Our results suggest that in the low-temperature experiments with 1D wires, saturation of the temperature dependence of the dephasing time can be caused by an MW electromagnetic noise with a sub-pW power.

Resonant Intrinsic Spin Hall Effect in p-Type GaAs Quantum Well Structure

Xi Dai, Zhong Fang, Yu-Gui Yao, and Fu-Chun Zhang

Published 1 March 2006

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We study intrinsic spin Hall effect in p-type GaAs quantum well structure described by Luttinger Hamiltonian and a Rashba spin-orbit coupling arising from the structural inversion symmetry breaking. The Rashba term induces an energy level crossing in the lowest heavy hole subband, which gives rise to a resonant spin Hall conductance. The resonance may be used to identify the intrinsic spin Hall effect in experiments.

Full Counting Statistics for a Single-Electron Transistor: Nonequilibrium Effects at Intermediate Conductance

Yasuhiro Utsumi, Dmitri S. Golubev, and Gerd Schön

Published 1 March 2006

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We evaluate the current distribution for a single-electron transistor with intermediate strength tunnel conductance. Using the Schwinger-Keldysh approach and the drone (Majorana) fermion representation, we account for the renormalization of system parameters. Nonequilibrium effects induce a lifetime broadening of the charge-state levels, which suppress large current fluctuations.

Local Pressure-Induced Metallization of a Semiconducting Carbon Nanotube in a Crossed Junction

L. Vitali, M. Burghard, P. Wahl, M. A. Schneider, and K. Kern

Published 3 March 2006

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The electronic and vibrational density of states of a semiconducting carbon nanotube in a crossed junction was investigated by elastic and inelastic scanning tunneling spectroscopy. The strong radial compression of the nanotube at the junction induces local metallization spatially confined to a few nanometers. The local electronic modifications are correlated with the observed changes in the radial breathing and G band phonon modes, which react very sensitively to local mechanical deformation. In addition, the experiments reveal the crucial contribution of the image charges to the contact potential at nanotube-metal interfaces.

Landau-Level Degeneracy and Quantum Hall Effect in a Graphite Bilayer

Edward McCann and Vladimir I. Fal'ko

Published 3 March 2006

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We derive an effective two-dimensional Hamiltonian to describe the low-energy electronic excitations of a graphite bilayer, which correspond to chiral quasiparticles with a parabolic dispersion exhibiting Berry phase 2

. Its high-magnetic-field Landau-level spectrum consists of almost equidistant groups of fourfold degenerate states at finite energy and eight zero-energy states. This can be translated into the Hall conductivity dependence on carrier density, sigma

xy(N), which exhibits plateaus at integer values of 4e²/h and has a double 8e²/h step between the hole and electron gases across zero density, in contrast to (4n+2)e²/h sequencing in a monolayer.

Uniform Mixing of High-T_c Superconductivity and Antiferromagnetism on a Single CuO₂ Plane of a Hg-Based Five-Layered Cuprate

H. Mukuda, M. Abe, Y. Araki, Y. Kitaoka, K. Tokiwa, T. Watanabe, A. Iyo, H. Kito, and Y. Tanaka

Published 28 February 2006

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We report a site selective Cu-NMR study on underdoped Hg-based five-layered high-Tc cuprate HgBa2Ca4Cu5O12+ delta

with a Tc=72 K. Antiferromagnetism (AFM) has been found to take place at TN=290 K, exhibiting a large antiferromagnetic moment of 0.67–0.69µB at three inner planes (IP). This value is comparable to the values reported for nondoped cuprates, suggesting that the IP may be in a nearly nondoped regime. Most surprisingly, the AFM order is also detected with MAFM(OP)=0.1µB even at two outer planes (OP) that are responsible for the onset of superconductivity (SC). The high-Tc SC at Tc=72 K can uniformly coexist on a microscopic level with the AFM at OP's. This is the first microscopic evidence for the uniform mixed phase of AFM and SC on a single CuO2 plane in a simple environment without any vortex lattice and/or stripe order.

Nucleation and Collapse of the Superconducting Phase in Type-I Superconducting Films

C. Gourdon, V. Jeudy, and A. C

bers

Published 28 February 2006

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The phase transition between the intermediate and normal states in type-I superconducting films is investigated using magneto-optical imaging. Magnetic hysteresis with different transition fields for collapse and nucleation of superconducting domains is found. This is accompanied by topological hysteresis characterized by the collapse of circular domains and the appearance of lamellar domains. Magnetic hysteresis is shown to arise from supercooled and superheated states. Domain-shape instability resulting from long-range magnetic interaction accounts well for topological hysteresis.

First-Order Metal-Insulator Transitions in Manganites: Are They Universal?

Qing' An Li, K. E. Gray, S. Nyborg Ancona, H. Zheng, S. Rosenkranz, R. Osborn, and J. F. Mitchell

Published 28 February 2006

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Conductivity data for La2-2xSr1+2xMn2O7 (x=0.6) show a first-order transition from an orbital- or charge-ordered insulator to a metal as the temperature falls below ~160 K. The change in conductivity is 100 times larger than that seen previously in any single-phase manganite in zero field. The metallic low-temperature state is similar to x=0.58, but x=0.58 shows no evidence of orbital or charge order. This result supports a conclusion that strongly coupled magnetic-conductive transitions are first order.

Unified Picture of Ferromagnetism, Quasi-Long-Range Order, and Criticality in Random-Field Models

Matthieu Tissier and Gilles Tarjus

Published 1 March 2006

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By applying the recently developed nonperturbative functional renormalization group (FRG) approach, we study the interplay between ferromagnetism, quasi-long-range order (QLRO), and criticality in the d-dimensional random-field O(N) model in the whole (N,d) diagram. Even though the "dimensional reduction" property breaks down below some critical line, the topology of the phase diagram is found similar to that of the pure O(N) model, with, however, no equivalent of the Kosterlitz-Thouless transition. In addition, we obtain that QLRO, namely, a topologically ordered "Bragg glass" phase, is absent in the 3-dimensional random-field XY model. The nonperturbative results are supplemented by a perturbative FRG analysis to two loops around d=4.

Magnon Decay in Gapped Quantum Spin Systems

Alexei Kolezhuk and Subir Sachdev

Published 2 March 2006

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In the continuum O(3) sigma

-model description of gapped spin systems, S=1 magnons can only decay into three lower energy magnons. We argue that the symmetry of the quantum spin Hamiltonian often allows decay into two magnons, and compute this decay rate in model systems. Two-magnon decay is present in Haldane gap S=1 spin chains, even though it cannot be induced by any allowed term written in powers and gradients of the sigma

-model field. We compare our results with recent measurements of Stone et al. on a two-dimensional spin system.

Metallic Spin-Liquid Behavior of the Geometrically Frustrated Kondo Lattice Pr₂Ir₂O₇

S. Nakatsuji, Y. Machida, Y. Maeno, T. Tayama, T. Sakakibara, J. van Duijn, L. Balicas, J. N. Millican, R. T. Macaluso, and Julia Y. Chan

Published 3 March 2006

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Strongly frustrated magnetism of the metallic pyrochlore oxide Pr2Ir2O7 has been revealed by single crystal study. While Pr 4f moments have an antiferromagnetic RKKY interaction energy scale of |T^*|=20 K mediated by Ir 5d-conduction electrons, no magnetic long-range order is found except for partial spin freezing at 120 mK. Instead, the Kondo effect, including a lnT dependence in the resistivity, emerges and leads to a partial screening of the moments below |T^*|. Our results indicate that the underscreened moments show spin-liquid behavior below a renormalized correlation scale of 1.7 K.

Origin of Ferromagnetism and its Pressure and Doping Dependence in Tl₂Mn₂O₇

T. Saha-Dasgupta, Molly De Raychaudhury, and D. D. Sarma

Published 3 March 2006

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Using the Nth order muffin-tin obital downfolding technique, we investigate the origin of ferromagnetism in pyrochlore Tl2Mn2O7. It is found to be driven by a hybridization induced spin polarization of delocalized charge carriers derived from Tl-s and O-p states. The mean-field estimate of the ferromagnetic transition temperature Tc estimated using computed exchange integrals are found to be in good agreement with measurements. We find an enhancement of Tc for moderate doping with nonmagnetic Sb and a suppression of Tc upon application of pressure, both in agreement with experimental findings.

Electron Dynamics of Silicon Surface States: Second-Harmonic Hole Burning on Si(111)-(7×7)

John A. McGuire, Markus B. Raschke, and Y. Ron Shen

Published 28 February 2006

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We report the first all-optical study of homogeneous linewidths of surface excitations by the spectral-hole-burning technique with surface-specific second-harmonic generation as a probe. Measurement of transient spectral holes induced by a 100 fs pump pulse in excitations of the surface dangling-bond states of Si(111)-(7×7) led to a pump-fluence-dependent homogeneous linewidth as broad as ~100 meV or a dephasing time as short as 15 fs. The hole-burning spectra also revealed a strong coupling between the localized dangling-bond states and the associated surface phonon mode at 570 cm^-1. Carrier-carrier scattering was responsible for the linear dependence of the dephasing rate on pump fluence, and the carrier screening effect appeared to be weak.

Density Matrix Tomography through Sequential Coherent Optical Rotations of an Exciton Qubit in a Single Quantum Dot

Yanwen Wu, Xiaoqin Li, L. M. Duan, D. G. Steel, and D. Gammon

Published 28 February 2006

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We demonstrate single qubit density matrix tomography in a single semiconductor quantum dot system through consecutive phase sensitive rotations of the qubit via ultrafast coherent optical excitations. The result is important for quantifying gate operations in quantum information processing in the quantum dot systems as well as demonstrating consecutive arbitrary qubit rotations.

Mechanisms of High-Order Perturbative Photoemission from Cu(001)

F. Bisio, M. Nývlt, J. Franta, H. Petek, and J. Kirschner

Published 27 February 2006

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We observed high-order 2- to 4-photon photoemission and above threshold photoemission (ATP) processes with 3.07 eV light from the Cu(001) surface. The intensity of 3-photon photoemission via excitation through the n=1 image potential state significantly exceeded that of the 2-photon process. The ATP occurs either via single photon transitions from the image potential resonances above the vacuum level or by multiphoton transitions from image potential states below the vacuum level. The experimental ratio of the m- to (m+1)-photon process yields is sensitive to the electronic band structure of the solid.

Velocity Autocorrelation Functions of Hard-Sphere Fluids: Long-Time Tails upon Undercooling

Stephen R. Williams, G. Bryant, I. K. Snook, and W. van Megen

Published 28 February 2006

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Molecular dynamics simulations are employed to obtain the velocity autocorrelation function (VAF) for hard spheres, spanning a wide range of volume fractions from dilute to high-density metastable fluids. For all volume fractions below freezing, Alder's classical positive 3/2 long-time tail is observed. For volume fractions from 0.45 to 0.48 the VAF becomes negative, before becoming positive and decaying with the positive 3/2 long-time tail. At the freezing volume fraction (0.494) the Alder 3/2 tail is not observed. At higher volume fractions a negative tail with an exponent of 5/2 emerges, which coincides with the long-time tail of a Lorentz gas.

Coupling Hydrophobicity, Dispersion, and Electrostatics in Continuum Solvent Models

J. Dzubiella, J. M. J. Swanson, and J. A. McCammon

Published 3 March 2006

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An implicit solvent model is presented that couples hydrophobic, dispersion, and electrostatic solvation energies by minimizing the system Gibbs free energy with respect to the solvent volume exclusion function. The solvent accessible surface is the output of the theory. The method is illustrated with the solvation of simple solutes on different length scales and captures the sensitivity of hydration to the particular form of the solute-solvent interactions in agreement with recent computer simulations.

Light Scattering and Phase Behavior of Lysozyme-Poly(Ethylene Glycol) Mixtures

J. Bloustine, T. Virmani, G. M. Thurston, and S. Fraden

Published 3 March 2006

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Measurements of liquid-liquid phase transition temperatures (cloud points) of mixtures of a protein (lysozyme) and a polymer, poly(ethylene glycol) (PEG) show that the addition of low molecular weight PEG stabilizes the mixture whereas high molecular weight PEG was destabilizing. We demonstrate that this behavior is inconsistent with an entropic lysozyme-PEG depletion interaction and suggest that an energetic lysozyme-PEG attraction is responsible. In order to independently characterize the lysozyme-PEG interactions, light scattering experiments on the same mixtures were performed to measure second and third virial coefficients. These measurements indicate that PEG induces repulsion between lysozyme molecules, contrary to the depletion prediction. Furthermore, it is shown that third virial terms must be included in the mixture's free energy in order to qualitatively capture our data.

Anomalous Vibrational Dispersion in Holographically Trapped Colloidal Arrays

Marco Polin, David G. Grier, and Stephen R. Quake

Published 1 March 2006

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Colloidal spheres localized in an array of harmonic wells form a thermally excited, viscously damped dynamical system capable of supporting propagating elastic waves. Experimentally realized with micrometer-scale polystyrene spheres localized in a line of holographic optical traps, the hydrodynamically coupled array's behavior is quantitatively explained by a model based on the Oseen superposition approximation. The spheres' purely dissipative coupling is predicted to mediate a crossover to a regime of underdamped propagating elastic waves with uniformly negative group velocities that has yet to be verified experimentally.

Stress-Dependent Elasticity of Composite Actin Networks as a Model for Cell Behavior

M. L. Gardel, F. Nakamura, J. Hartwig, J. C. Crocker, T. P. Stossel, and D. A. Weitz

Published 3 March 2006

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Networks of filamentous actin cross-linked with the actin-binding protein filamin A exhibit remarkable strain stiffening leading to an increase in differential elastic modulus by several orders of magnitude over the linear value. The variation of the frequency dependence of the differential elastic and loss moduli as a function of prestress is consistent with that observed in living cells, suggesting that cell elasticity is always measured in the nonlinear regime, and that prestress is an essential control parameter.

Interface Instability in Shear-Banding Flow

S. Lerouge, M. Argentina, and J. P. Decruppe

Published 28 February 2006

088301 Full Text: PDF (666 kB) | Buy Article

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We report on the spatiotemporal dynamics of the interface in shear-banding flow of a wormlike micellar system (cetyltrimethylammonium bromide and sodium nitrate in water) during a start-up experiment. Using the scattering properties of the induced structures, we demonstrate the existence of an instability of the interface between bands along the vorticity direction. Different regimes of spatiotemporal dynamics of the interface are identified along the stress plateau. We build a model based on the flow symmetry which qualitatively describes the observed patterns.

Optimal Cell Approach to Osmotic Properties of Finite Stiff-Chain Polyelectrolytes

Dmytro Antypov and Christian Holm

Published 2 March 2006

088302 Full Text: PDF (202 kB) | Buy Article

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We propose a self-consistent geometry optimized cell model approach to study osmotic properties of stiff-chain polyelectrolyte solutions. In contrast with the usual monotonic Poisson-Boltzmann prediction, the cell model predicts the correct nonmonotonic dependence of the osmotic coefficient on concentration. A lower degree of polymerization is found to reduce significantly the counterion condensation in a typical dilute strong polyelectrolyte. The results agree quantitatively with simulations of a corresponding many-body bulk system up to a dense semidilute regime.

Cyclic Motion of a Grafted Polymer under Shear Flow

Rafael Delgado-Buscalioni

Published 2 March 2006

088303 Full Text: PDF (388 kB) | Buy Article

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Physical Review Letters

3 March 2006

Volume 96, Number 8 , Articles (08xxxx)

Articles published 25 February - 3 March 2006

LETTERS

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

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