495 research outputs found
Ising Dynamics with Damping
We show for the Ising model that is possible construct a discrete time
stochastic model analogous to the Langevin equation that incorporates an
arbitrary amount of damping. It is shown to give the correct equilibrium
statistics and is then used to investigate nonequilibrium phenomena, in
particular, magnetic avalanches. The value of damping can greatly alter the
shape of hysteresis loops, and for small damping and high disorder, the
morphology of large avalanches can be drastically effected. Small damping also
alters the size distribution of avalanches at criticality.Comment: 8 pages, 8 figures, 2 colum
Critical Hysteresis in Random Field XY and Heisenberg Models
We study zero-temperature hysteresis in random-field XY and Heisenberg models
in the zero-frequency limit of a cyclic driving field. We consider three
distributions of the random field and present exact solutions in the mean field
limit. The results show a strong effect of the form of disorder on critical
hysteresis as well as the shape of hysteresis loops. A discrepancy with an
earlier study based on the renormalization group is resolved.Comment: 10 pages, 6 figures; this is published version (added some text and
references
Universal Pulse Shape Scaling Function and Exponents: A Critical Test for Avalanche Models applied to Barkhausen Noise
In order to test if the universal aspects of Barkhausen noise in magnetic
materials can be predicted from recent variants of the non-equilibrium zero
temperature Random Field Ising Model (RFIM), we perform a quantitative study of
the universal scaling function derived from the
Barkhausen pulse shape in simulations and experiment. Through data collapses
and scaling relations we determine the critical exponents and
in both simulation and experiment. Although we find agreement
in the critical exponents, we find differences between theoretical and
experimental pulse shape scaling functions as well as between different
experiments.Comment: 19 pages (in preprint format), 5 figures, 1 tabl
Training-induced criticality in martensites
We propose an explanation for the self-organization towards criticality
observed in martensites during the cyclic process known as `training'. The
scale-free behavior originates from the interplay between the reversible phase
transformation and the concurrent activity of lattice defects. The basis of the
model is a continuous dynamical system on a rugged energy landscape, which in
the quasi-static limit reduces to a sandpile automaton. We reproduce all the
principal observations in thermally driven martensites, including power-law
statistics, hysteresis shakedown, asymmetric signal shapes, and correlated
disorder.Comment: 5 pages, 4 figure
Hysteresis in Random Field XY and Heisenberg Models: Mean Field Theory and Simulations at Zero Temperature
We examine zero temperature hysteresis in random field XY and Heisenberg
models in the zero frequency limit of a cyclic driving field. Exact expressions
for hysteresis loops are obtained in the mean field approximation. These show
rather unusual features. We also perform simulations of the two models on a
simple cubic lattice and compare them with the predictions of the mean field
theory.Comment: replaced by the published versio
Gas adsorption/desorption in silica aerogels: a theoretical study of scattering properties
We present a numerical study of the structural correlations associated to gas
adsorption/desorption in silica aerogels in order to provide a theoretical
interpretation of scattering experiments. Following our earlier work, we use a
coarse-grained lattice-gas description and determine the nonequilibrium
behavior of the adsorbed gas within a local mean-field analysis.
We focus on the differences between the adsorption and desorption mechanisms
and their signature in the fluid-fluid and gel-fluid structure factors as a
function of temperature. At low temperature, but still in the regime where the
isotherms are continuous, we find that the adsorbed fluid density, during both
filling and draining, is correlated over distances that may be much larger than
the gel correlation length. In particular, extended fractal correlations may
occur during desorption, indicating the existence of a ramified cluster of
vapor filled cavities. This also induces an important increase of the
scattering intensity at small wave vectors. The similarity and differences with
the scattering of fluids in other porous solids such as Vycor are discussed.Comment: 16 pages, 15 figure
Inferring processes underlying B-cell repertoire diversity
We quantify the VDJ recombination and somatic hypermutation processes in
human B-cells using probabilistic inference methods on high-throughput DNA
sequence repertoires of human B-cell receptor heavy chains. Our analysis
captures the statistical properties of the naive repertoire, first after its
initial generation via VDJ recombination and then after selection for
functionality. We also infer statistical properties of the somatic
hypermutation machinery (exclusive of subsequent effects of selection). Our
main results are the following: the B-cell repertoire is substantially more
diverse than T-cell repertoires, due to longer junctional insertions; sequences
that pass initial selection are distinguished by having a higher probability of
being generated in a VDJ recombination event; somatic hypermutations have a
non-uniform distribution along the V gene that is well explained by an
independent site model for the sequence context around the hypermutation site.Comment: acknowledgement adde
A statistical mechanical description of metastable states and hysteresis in the 3D soft-spin random-field model at T=0
We present a formalism for computing the complexity of metastable states and
the zero-temperature magnetic hysteresis loop in the soft-spin random-field
model in finite dimensions. The complexity is obtained as the Legendre
transform of the free-energy associated to a certain action in replica space
and the hysteresis loop above the critical disorder is defined as the curve in
the field-magnetization plane where the complexity vanishes; the nonequilibrium
magnetization is therefore obtained without having to follow the dynamical
evolution. We use approximations borrowed from condensed-matter theory and
based on assumptions on the structure of the direct correlation functions (or
proper vertices), such as a local approximation for the self-energies, to
calculate the hysteresis loop in three dimensions, the correlation functions
along the loop, and the second moment of the avalanche-size distribution.Comment: 28 pages, 12 figure
Polarons as Nucleation Droplets in Non-Degenerate Polymers
We present a study of the nucleation mechanism that allows the decay of the
metastable phase (trans-cisoid) to the stable phase
(cis-transoid) in quasi one-dimensional non-degenerate polymers within the
continuum electron-phonon model. The electron-phonon configurations that lead
to the decay, i.e. the critical droplets (or transition state), are identified
as polarons of the metastable phase. We obtain an estimate for the decay rate
via thermal activation within a range of parameters consistent with
experimental values for the gap of the cis-configuration. It is pointed out
that, upon doping, the activation barriers of the excited states are quite
smaller and the decay rate is greatly enhanced. Typical activation energies for
electron or hole polarons are eV and the typical size for a
critical droplet (polaron) is about . Decay via quantum nucleation is
also studied and it is found that the crossover temperature between quantum
nucleation and thermal activation is of order . Metastable
configurations of non-degenerate polymers may provide examples for mesoscopic
quantum tunneling.Comment: REVTEX 3.0, 28 PAGES, 3 FIGURES AVAILABLE UPON REQUEST, PITT 94-0
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