5,769 research outputs found
The ferroelectric Mott-Hubbard phase of organic (TMTTF)2X conductors
We present experimental evidences for a ferro-electric transition in the
family of quasi one- dimensional conductors (TMTTF)2X. We interpret this new
transition in the frame of the combined Mott-Hubbard state taking into account
the double action of the spontaneous charge disproportionation on the TMTTF
molecular stacks and of the X anionic potentials
Molecular transport and flow past hard and soft surfaces: Computer simulation of model systems
The properties of polymer liquids on hard and soft substrates are
investigated by molecular dynamics simulation of a coarse-grained bead-spring
model and dynamic single-chain-in-mean-field (SCMF) simulations of a soft,
coarse-grained polymer model. Hard, corrugated substrates are modelled by an
FCC Lennard-Jones solid while polymer brushes are investigated as a
prototypical example of a soft, deformable surface. From the molecular
simulation we extract the coarse-grained parameters that characterise the
equilibrium and flow properties of the liquid in contact with the substrate:
the surface and interface tensions, and the parameters of the hydrodynamic
boundary condition. The so-determined parameters enter a continuum description
like the Stokes equation or the lubrication approximation.Comment: 41 pages, 13 figure
Superconductivity and Antiferromagnetism in Quasi-one-dimensional Organic Conductors
We review the current understanding of superconductivity in the
quasi-one-dimensional organic conductors of the Bechgaard and Fabre salt
families. We discuss the interplay between superconductivity,
antiferromagnetism, and charge-density-wave fluctuations. The connection to
recent experimental observations supporting unconventional pairing and the
possibility of a triplet-spin order parameter for the superconducting phase is
also presented.Comment: (v1) 30 pages, 13 figures; Review article for the 20th anniversary of
high-Tc superconductivity, to appear in J. Low Temp. Phys. (v2) 1 Ref. adde
Time series prediction via aggregation : an oracle bound including numerical cost
We address the problem of forecasting a time series meeting the Causal
Bernoulli Shift model, using a parametric set of predictors. The aggregation
technique provides a predictor with well established and quite satisfying
theoretical properties expressed by an oracle inequality for the prediction
risk. The numerical computation of the aggregated predictor usually relies on a
Markov chain Monte Carlo method whose convergence should be evaluated. In
particular, it is crucial to bound the number of simulations needed to achieve
a numerical precision of the same order as the prediction risk. In this
direction we present a fairly general result which can be seen as an oracle
inequality including the numerical cost of the predictor computation. The
numerical cost appears by letting the oracle inequality depend on the number of
simulations required in the Monte Carlo approximation. Some numerical
experiments are then carried out to support our findings
Random noise in Diffusion Tensor Imaging, its Destructive Impact and Some Corrections
The empirical origin of random noise is described, its influence on DTI variables is illustrated by a review of numerical and in vivo studies supplemented by new simulations investigating high noise levels. A stochastic model of noise propagation is presented to structure noise impact in DTI. Finally, basics of voxelwise and spatial denoising procedures are presented. Recent denoising procedures are reviewed and consequences of the stochastic model for convenient denoising strategies are discussed
Finite-Temperature Charge-Ordering Transition and Fluctuation Effects in Quasi-One-Dimensional Electron Systems at Quarter Filling
Finite-temperature charge-ordering phase transition in quasi one-dimensional
(1D) molecular conductors is investigated theoretically, based on a quasi 1D
extended Hubbard model at quarter filling with interchain Coulomb repulsion
. The interchain term is treated within mean-field approximation
whereas the 1D fluctuations in the chains are fully taken into account by the
bosonization theory. Three regions are found depending on how the charge
ordered state appears at finite temperature when is introduced:
(i) weak-coupling region where the system transforms from a metal to a charge
ordered insulator with finite transition temperature at a finite critical value
of ,
(ii) an intermediate region where this transition occurs by infinitesimal
due to the stability of inherent 1D fluctuation, and
(iii) strong-coupling region where the charge ordered state is realized
already in the purely 1D case, of which the transition temperature becomes
finite with infinitesimal . Analytical formula for the
dependence of the transition temperature is derived for each region.Comment: 4 pages, submitted to J. Phys. Soc. Jp
Possible Triplet Electron Pairing and an Anisotropic Spin Susceptibility in Organic Superconductors (TMTSF)_2 X
We argue that (TMTSF)_2 PF_6 compound under pressure is likely a triplet
superconductor with a vector order parameter d(k) \equiv (d_a(k) \neq 0, d_c(k)
= ?, d_{b'}(k) = 0); |d_a(k)| > |d_c(k)|. It corresponds to an anisotropic spin
susceptibility at T=0: \chi_{b'} = \chi_0, \chi_a \ll \chi_0, where \chi_0 is
its value in a metallic phase. [The spin quantization axis, z, is parallel to a
so-called b'-axis]. We show that the suggested order parameter explains why the
upper critical field along the b'-axis exceeds all paramagnetic limiting
fields, including that for a nonuniform superconducting state, whereas the
upper critical field along the a-axis (a \perp b') is limited by the Pauli
paramagnetic effects [I. J. Lee, M. J. Naughton, G. M. Danner and P. M.
Chaikin, Phys. Rev. Lett. 78, 3555 (1997)]. The triplet order parameter is in
agreement with the recent Knight shift measurements by I. J. Lee et al. as well
as with the early results on a destruction of superconductivity by nonmagnetic
impurities and on the absence of the Hebel-Slichter peak in the NMR relaxation
rate.Comment: 4 pages, 1 eps figur
Spin-wave scattering at low temperatures in manganite films
The temperature and magnetic field dependence of the resistivity
has been measured for LaSrMnO (y=0 and 0.128)
films grown on (100) SrTiO substrates. The low-temperature in the
ferromagnetic metallic region follows well with being the residual resistivity. We attribute the second and third term to
small-polaron and spin-wave scattering, respectively. Our analysis based on
these scattering mechanisms also gives the observed difference between the
metal-insulator transition temperatures of the films studied. Transport
measurements in applied magnetic field further indicate that spin-wave
scattering is a key transport mechanism at low temperatures.Comment: 5 pages, 4 figures. to appear in Phys. Rev.
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