3,798 research outputs found
Effect of phonon-phonon interactions on localization
We study the heat current J in a classical one-dimensional disordered chain
with on-site pinning and with ends connected to stochastic thermal reservoirs
at different temperatures. In the absence of anharmonicity all modes are
localized and there is a gap in the spectrum. Consequently J decays
exponentially with system size N. Using simulations we find that even a small
amount of anharmonicity leads to a J~1/N dependence, implying diffusive
transport of energy.Comment: 4 pages, 2 figures, Published versio
Heat transport and phonon localization in mass-disordered harmonic crystals
We investigate the steady state heat current in two and three dimensional
disordered harmonic crystals in a slab geometry, connected at the boundaries to
stochastic white noise heat baths at different temperatures.The disorder causes
short wavelength phonon modes to be localized so the heat current in this
system is carried by the extended phonon modes which can be either diffusive or
ballistic. Using ideas both from localization theory and from kinetic theory we
estimate the contribution of various modes to the heat current and from this we
obtain the asymptotic system size dependence of the current. These estimates
are compared with results obtained from a numerical evaluation of an exact
formula for the current, given in terms of a frequency transmission function,
as well as from direct nonequilibrium simulations. These yield a strong
dependence of the heat flux on boundary conditions. Our analytical arguments
show that for realistic boundary conditions the conductivity is finite in three
dimensions but we are not able to verify this numerically, except in the case
where the system is subjected to an external pinning potential. This case is
closely related to the problem of localization of electrons in a random
potential and here we numerically verify that the pinned three dimensional
system satisfies Fourier's law while the two dimensional system is a heat
insulator. We also investigate the inverse participation ratio of different
normal modes.Comment: 30 pages, 28 figures (Revised and improved version
Heat conduction in disordered harmonic lattices with energy conserving noise
We study heat conduction in a harmonic crystal whose bulk dynamics is
supplemented by random reversals (flips) of the velocity of each particle at a
rate . The system is maintained in a nonequilibrium stationary
state(NESS) by contacts with Langevin reservoirs at different temperatures. We
show that the one-body and pair correlations in this system are the same (after
an appropriate mapping of parameters) as those obtained for a model with
self-consistent reservoirs. This is true both for the case of equal and
random(quenched) masses. While the heat conductivity in the NESS of the ordered
system is known explicitly, much less is known about the random mass case. Here
we investigate the random system, with velocity flips. We improve the bounds on
the Green-Kubo conductivity obtained by C.Bernardin. The conductivity of the 1D
system is then studied both numerically and analytically. This sheds some light
on the effect of noise on the transport properties of systems with localized
states caused by quenched disorder.Comment: 19 pages, 8 figure
Thermoelectric properties of -FeSi
We investigate the thermoelectric properties of -FeSi
using first principles electronic structure and Boltzmann transport
calculations. We report a high thermopower for both \textit{p}- and
\textit{n}-type -FeSi over a wide range of carrier
concentration and in addition find the performance for \textit{n}-type to be
higher than for the \textit{p}-type. Our results indicate that, depending upon
temperature, a doping level of 3 - 2
cm may optimize the thermoelectric performance
Normal discrimination of spatial frequency and contrast across visual hemifields in left-onset Parkinson’s disease: evidence against perceptual hemifield biases
Individuals with Parkinson's disease (PD) with symptom onset on the left side of the body (LPD) show a mild type of left-sided visuospatial neglect, whereas those with right-onset (RPD) generally do not. The functional mechanisms underlying these observations are unknown. Two hypotheses are that the representation of left-space in LPD is either compressed or reduced in salience. We tested these hypotheses psychophysically. Participants were 31 non-demented adults with PD (15 LPD, 16 RPD) and 17 normal control adults (NC). The spatial compression hypothesis was tested by showing two sinusoidal gratings, side by side. One grating's spatial frequency (SF) was varied across trials, following a staircase procedure, whereas the comparison grating was held at a constant SF. While fixating on a central target, participants estimated the point at which they perceived the two gratings to be equal in SF. The reduced salience hypothesis was tested in a similar way, but by manipulating the contrast of the test grating rather than its SF. There were no significant differences between groups in the degree of bias across hemifields for SF discrimination or for contrast discrimination. Results did not support either the spatial compression hypothesis or the reduced salience hypothesis. Instead, they suggest that at this perceptual level, LPD do not have a systematically biased way of representing space in the left hemifield that differs from healthy individuals, nor do they perceive stimuli on the left as less salient than stimuli on the right. Neglect-like syndrome in LPD instead presumably arises from dysfunction of higher-order attention.Published versio
Comment on ``Can Disorder Induce a Finite Thermal Conductivity in 1D Lattices?''
In a recent paper [Phys. Rev. Lett. 86, 63 (2001)], Li et al have reported
that the nonequilibrium heat conducting steady state of a disordered harmonic
chain is not unique. In this comment we point out that for a large class of
stochastic heat baths the uniqueness of the steady state can be proved, and
therefore the findings of Li et al could be either due to their use of
deterministic heat baths or insufficient equilibration times in the
simulations. We give a simple example where the uniquness of the steady state
can be explicitly demonstrated.Comment: 1 page, 1 figure, accepted for publication in Phys. Rev. Let
Application of importance sampling to the computation of large deviations in non-equilibrium processes
We present an algorithm for finding the probabilities of rare events in
nonequilibrium processes. The algorithm consists of evolving the system with a
modified dynamics for which the required event occurs more frequently. By
keeping track of the relative weight of phase-space trajectories generated by
the modified and the original dynamics one can obtain the required
probabilities. The algorithm is tested on two model systems of steady-state
particle and heat transport where we find a huge improvement from direct
simulation methods.Comment: 5 pages, 4 figures; some modification
Work distribution functions for hysteresis loops in a single-spin system
We compute the distribution of the work done in driving a single Ising spin
with a time-dependent magnetic field. Using Glauber dynamics we perform
Monte-Carlo simulations to find the work distributions at different driving
rates. We find that in general the work-distributions are broad with a
significant probability for processes with negative dissipated work. The
special cases of slow and fast driving rates are studied analytically. We
verify that various work fluctuation theorems corresponding to equilibrium
initial states are satisfied while a steady state version is not.Comment: 9 pages, 15 figure
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