3,431 research outputs found
Aging, rejuvenation and memory effects in re-entrant ferromagnets
We have studied the slow dynamics of the ferromagnetic phases of the
re-entrant CdCr_{2x}In_{2-2x}S_4 system for 0.85<x<=1 by means of low frequency
ac susceptibility and magnetization measurements. Experimental procedures
widely used in the investigation of the out-of-equilibrium dynamics of spin
glasses (such as the x=0.85 compound) have been applied to search for aging,
rejuvenation and memory effects, and to test their dependence on the disorder
introduced by dilution of the magnetic ions. Whereas the rejuvenation effect is
found in all studied samples, the memory effect is clearly enhanced for
increasing dilutions. The results support a description of aging in both
ferromagnetic and re-entrant spin-glass phases in terms of hierarchical
reconformations of domain walls pinned by the disorder.Comment: Service de Physique de l'Etat Condense, DRECAM, DSM, CEA Saclay,91191
Gif sur Yvette Cedex, France, 9 pages, including 7 figures, To appear in Eur.
Phys. J. B (2002
Rheology of cholesteric blue phases
Blue phases of cholesteric liquid crystals offer a spectacular example of
naturally occurring disclination line networks. Here we numerically solve the
hydrodynamic equations of motion to investigate the response of three types of
blue phases to an imposed Poiseuille flow. We show that shear forces bend and
twist and can unzip the disclination lines. Under gentle forcing the network
opposes the flow and the apparent viscosity is significantly higher than that
of an isotropic liquid. With increased forcing we find strong shear thinning
corresponding to the disruption of the defect network. As the viscosity starts
to drop, the imposed flow sets the network into motion. Disclinations break-up
and re-form with their neighbours in the flow direction. This gives rise to
oscillations in the time-dependent measurement of the average stress.Comment: 4 pages, 4 figure
Quantum Hall effect anomaly and collective modes in the magnetic-field-induced spin-density-wave phases of quasi-one-dimensional conductors
We study the collective modes in the magnetic-field-induced spin-density-wave
(FISDW) phases experimentally observed in organic conductors of the Bechgaard
salts family. In phases that exhibit a sign reversal of the quantum Hall effect
(Ribault anomaly), the coexistence of two spin-density waves gives rise to
additional collective modes besides the Goldstone modes due to spontaneous
translation and rotation symmetry breaking. These modes strongly affect the
charge and spin response functions. We discuss some experimental consequences
for the Bechgaard salts.Comment: Final version (LaTex, 8 pages, no figure), to be published in
Europhys. Let
Aging phenomena in spin glass and ferromagnetic phases: domain growth and wall dynamics
We compare aging in a disordered ferromagnet and in a spin glass, by studying
the different phases of a reentrant system. We have measured the relaxation of
the low-frequency ac susceptibility, in both the ferromagnetic and spin-glass
phases of a CdCr_{1.9}In_{0.1}S_4 sample. A restart of aging processes when the
temperature is lowered (`chaos-like' effect) is observed in both phases. The
memory of previous aging at a higher temperature can be retrieved upon
re-heating, but in the ferromagnetic phase it can rapidly be erased by the
growth of ferromagnetic domains. We interpret the behaviour observed in the
ferromagnetic phase in terms of a combination of domain growth and pinned wall
reconformations, and suggest that aging in spin glasses is dominated by such
wall reconformation processes.Comment: SPEC, CEA Saclay, 91191 Gif sur Yvette Cedex, France, to appear in
Europhys. Lett. (2000
Holomorphic Simplicity Constraints for 4d Riemannian Spinfoam Models
Starting from the reformulation of the classical phase space of Loop Quantum
Gravity in terms of spinor variables and spinor networks, we build coherent
spin network states and show how to use them to write the spinfoam path
integral for topological BF theory in terms of Gaussian integrals in the
spinors. Finally, we use this framework to revisit the simplicity constraints
reducing topological BF theory to 4d Riemannian gravity. These holomorphic
simplicity constraints lead us to a new spinfoam model for quantum gravity
whose amplitudes are defined as the evaluation of the coherent spin networks.Comment: 4 pages. Proceedings of Loops'11, Madrid. To appear in Journal of
Physics: Conference Series (JPCS
Coupling Lattice Boltzmann and Molecular Dynamics models for dense fluids
We propose a hybrid model, coupling Lattice Boltzmann and Molecular Dynamics
models, for the simulation of dense fluids. Time and length scales are
decoupled by using an iterative Schwarz domain decomposition algorithm. The MD
and LB formulations communicate via the exchange of velocities and velocity
gradients at the interface. We validate the present LB-MD model in simulations
of flows of liquid argon past and through a carbon nanotube. Comparisons with
existing hybrid algorithms and with reference MD solutions demonstrate the
validity of the present approach.Comment: 14 pages, 5 figure
Efficient Quantum Polar Coding
Polar coding, introduced 2008 by Arikan, is the first (very) efficiently
encodable and decodable coding scheme whose information transmission rate
provably achieves the Shannon bound for classical discrete memoryless channels
in the asymptotic limit of large block sizes. Here we study the use of polar
codes for the transmission of quantum information. Focusing on the case of
qubit Pauli channels and qubit erasure channels, we use classical polar codes
to construct a coding scheme which, using some pre-shared entanglement,
asymptotically achieves a net transmission rate equal to the coherent
information using efficient encoding and decoding operations and code
construction. Furthermore, for channels with sufficiently low noise level, we
demonstrate that the rate of preshared entanglement required is zero.Comment: v1: 15 pages, 4 figures. v2: 5+3 pages, 3 figures; argumentation
simplified and improve
Hubble Space Telescope FUV Spectra of the Post-Common-Envelope Hyades Binary V471 Tauri
We have carried out an analysis of the HST STIS archival spectra of the
magnetic white dwarf in the Hyades eclipsing-spectroscopic, post-common
envelope binary V471 Tauri, time resolved on the orbit and on the X-ray
rotational phase of the magnetic white dwarf. An HST STIS spectrum obtained
during primary eclipse reveals a host of transition region/chromospheric
emission features including N V (1238, 1242), Si IV (1393, 1402), C IV (1548,
1550) and He II (1640). The spectroscopic characteristics and emission line
fluxes of the transition region/chromosphere of the very active, rapidly
rotating, K2V component of V471 Tauri, are compared with the emission
characteristics of fast rotating K dwarfs in young open clusters. We have
detected a number of absorption features associated with metals accreted onto
the photosphere of the magnetic white dwarf from which we derive radial
velocities. All of the absorption features are modulated on the 555s rotation
period of the white dwarf with maximum line strength at rotational phase 0.0
when the primary magnetic accretion region is facing the observer. The
photospheric absorption features show no clear evidence of Zeeman splitting and
no evidence of a correlation between their variations in strength and orbital
phase. We report clear evidence of a secondary accretion pole. We derive C and
Si abundances from the Si IV and C III features. All other absorption lines are
either interstellar or associated with a region above the white dwarf and/or
with coronal mass ejection events illuminated as they pass in front of the
white dwarf.Comment: The Astrophysical Journal, May 10, 2012 issue - 16 figure
Holomorphic Simplicity Constraints for 4d Spinfoam Models
Within the framework of spinfoam models, we revisit the simplicity
constraints reducing topological BF theory to 4d Riemannian gravity. We use the
reformulation of SU(2) intertwiners and spin networks in term of spinors, which
has come out from both the recently developed U(N) framework for SU(2)
intertwiners and the twisted geometry approach to spin networks and spinfoam
boundary states. Using these tools, we are able to perform a
holomorphic/anti-holomorphic splitting of the simplicity constraints and define
a new set of holomorphic simplicity constraints, which are equivalent to the
standard ones at the classical level and which can be imposed strongly on
intertwiners at the quantum level. We then show how to solve these new
holomorphic simplicity constraints using coherent intertwiner states. We
further define the corresponding coherent spin network functionals and
introduce a new spinfoam model for 4d Riemannian gravity based on these
holomorphic simplicity constraints and whose amplitudes are defined from the
evaluation of the new coherent spin networks.Comment: 27 page
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