532 research outputs found
The role of the alloy structure in the magnetic behavior of granular systems
The effect of grain size, easy magnetization axis and anisotropy constant
distributions in the irreversible magnetic behavior of granular alloys is
considered. A simulated granular alloy is used to provide a realistic grain
structure for the Monte Carlo simulation of the ZFC-FC curves. The effect of
annealing and external field is also studied. The simulation curves are in good
agreement with the FC and ZFC magnetization curves measured on melt spun Cu-Co
ribbons.Comment: 13 pages, 10 figures, submitted to PR
Scaling in Late Stage Spinodal Decomposition with Quenched Disorder
We study the late stages of spinodal decomposition in a Ginzburg-Landau mean
field model with quenched disorder. Random spatial dependence in the coupling
constants is introduced to model the quenched disorder. The effect of the
disorder on the scaling of the structure factor and on the domain growth is
investigated in both the zero temperature limit and at finite temperature. In
particular, we find that at zero temperature the domain size, , scales
with the amplitude, , of the quenched disorder as with and in two
dimensions. We show that , where is the
Lifshitz-Slyosov exponent. At finite temperature, this simple scaling is not
observed and we suggest that the scaling also depends on temperature and .
We discuss these results in the context of Monte Carlo and cell dynamical
models for phase separation in systems with quenched disorder, and propose that
in a Monte Carlo simulation the concentration of impurities, , is related to
by .Comment: RevTex manuscript 5 pages and 5 figures (obtained upon request via
email [email protected]
Coupled Map Modeling for Cloud Dynamics
A coupled map model for cloud dynamics is proposed, which consists of the
successive operations of the physical processes; buoyancy, diffusion,
viscosity, adiabatic expansion, fall of a droplet by gravity, descent flow
dragged by the falling droplet, and advection. Through extensive simulations,
the phases corresponding to stratus, cumulus, stratocumulus and cumulonimbus
are found, with the change of the ground temperature and the moisture of the
air. They are characterized by order parameters such as the cluster number,
perimeter-to-area ratio of a cloud, and Kolmogorov-Sinai entropy.Comment: 9 pages, 4 figure, LaTeX, mpeg simulations available at
http://aurora.elsip.hokudai.ac.jp
Structural Stability and Renormalization Group for Propagating Fronts
A solution to a given equation is structurally stable if it suffers only an
infinitesimal change when the equation (not the solution) is perturbed
infinitesimally. We have found that structural stability can be used as a
velocity selection principle for propagating fronts. We give examples, using
numerical and renormalization group methods.Comment: 14 pages, uiucmac.tex, no figure
Coupled Maps on Trees
We study coupled maps on a Cayley tree, with local (nearest-neighbor)
interactions, and with a variety of boundary conditions. The homogeneous state
(where every lattice site has the same value) and the node-synchronized state
(where sites of a given generation have the same value) are both shown to occur
for particular values of the parameters and coupling constants. We study the
stability of these states and their domains of attraction. As the number of
sites that become synchronized is much higher compared to that on a regular
lattice, control is easier to effect. A general procedure is given to deduce
the eigenvalue spectrum for these states. Perturbations of the synchronized
state lead to different spatio-temporal structures. We find that a mean-field
like treatment is valid on this (effectively infinite dimensional) lattice.Comment: latex file (25 pages), 4 figures included. To be published in Phys.
Rev.
Extended Clausius Relation and Entropy for Nonequilibrium Steady States in Heat Conducting Quantum Systems
Recently, in their attempt to construct steady state thermodynamics (SST),
Komatsu, Nakagwa, Sasa, and Tasaki found an extension of the Clausius relation
to nonequilibrium steady states in classical stochastic processes. Here we
derive a quantum mechanical version of the extended Clausius relation. We
consider a small system of interest attached to large systems which play the
role of heat baths. By only using the genuine quantum dynamics, we realize a
heat conducting nonequilibrium steady state in the small system. We study the
response of the steady state when the parameters of the system are changed
abruptly, and show that the extended Clausius relation, in which "heat" is
replaced by the "excess heat", is valid when the temperature difference is
small. Moreover we show that the entropy that appears in the relation is
similar to von Neumann entropy but has an extra symmetrization with respect to
time-reversal. We believe that the present work opens a new possibility in the
study of nonequilibrium phenomena in quantum systems, and also confirms the
robustness of the approach by Komtatsu et al.Comment: 19 pages, 2 figure
From Quantum Dynamics to the Canonical Distribution: General Picture and a Rigorous Example
Derivation of the canonical (or Boltzmann) distribution based only on quantum
dynamics is discussed. Consider a closed system which consists of mutually
interacting subsystem and heat bath, and assume that the whole system is
initially in a pure state (which can be far from equilibrium) with small energy
fluctuation. Under the "hypothesis of equal weights for eigenstates", we derive
the canonical distribution in the sense that, at sufficiently large and typical
time, the (instantaneous) quantum mechanical expectation value of an arbitrary
operator of the subsystem is almost equal to the desired canonical expectation
value. We present a class of examples in which the above derivation can be
rigorously established without any unproven hypotheses.Comment: LaTeX, 8 pages, no figures. The title, abstract and some discussions
are modified to stress physical motivation of the work. References are added
to [2]. This version will appear in Phys. Rev. Lett. There is an accompanying
unpublished note (cond-mat/9707255
Phase Separation Kinetics in a Model with Order-Parameter Dependent Mobility
We present extensive results from 2-dimensional simulations of phase
separation kinetics in a model with order-parameter dependent mobility. We find
that the time-dependent structure factor exhibits dynamical scaling and the
scaling function is numerically indistinguishable from that for the
Cahn-Hilliard (CH) equation, even in the limit where surface diffusion is the
mechanism for domain growth. This supports the view that the scaling form of
the structure factor is "universal" and leads us to question the conventional
wisdom that an accurate representation of the scaled structure factor for the
CH equation can only be obtained from a theory which correctly models bulk
diffusion.Comment: To appear in PRE, figures available on reques
Dynamic charge density correlation function in weakly charged polyampholyte globules
We study solutions of statistically neutral polyampholyte chains containing a
large fraction of neutral monomers. It is known that, even if the quality of
the solvent with respect to the neutral monomers is good, a long chain will
collapse into a globule. For weakly charged chains, the interior of this
globule is semi-dilute. This paper considers mainly theta-solvents, and we
calculate the dynamic charge density correlation function g(k,t) in the
interior of the globules, using the quadratic approximation to the
Martin-Siggia-Rose generating functional. It is convenient to express the
results in terms of dimensionless space and time variables. Let R be the blob
size, and let T be the characteristic time scale at the blob level. Define the
dimensionless wave vector q = R k, and the dimensionless time s = t/T. We find
that for q<1, corresponding to length scales larger than the blob size, the
charge density fluctuations relax according to g(q,s) = q^2(1-s^(1/2)) at short
times s < 1, and according to g(q,s) = q^2 s^(-1/2) at intermediate times 1 < s
0.1, where
entanglements are unimportant.Comment: 12 pages RevTex, 1 figure ps, PACS 61.25.Hq, reason replacement:
Expression for dynamic corr. function g(k,t) in old version was incorrect
(though expression for Fourier transform g(k,w) was correct, so the major
part of the calculation remains.) Also major textual chang
Density mismatch in thin diblock copolymer films
Thin films of diblock copolymer subject to gravitational field are simulated
by means of a cell dynamical system model. The difference in density of the two
sides of the molecule and the presence of the field causes the formation of
lamellar patterns with orientation parallel to the confining walls even when
they are neutral. The concentration profile of those films is analyzed in the
weak segregation regime and a functional form for the profile is proposed.Comment: 9 pages and 8 figures. Needs EPSF macros. Submitted to PR
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