2,610 research outputs found
Quasi-Stationary Distributions for Models of Heterogeneous Catalysis
We construct the quasi-stationary (QS) distribution for two models of
heterogeneous catalysis having two absorbing states: the ZGB model for the
oxidation of CO, and a version with noninstantaneous reactions. Using a
mean-field-like approximation, we study the quasi-stationary surface coverages,
moment ratios and the lifetime of the QS state. We also derive an improved,
consistent one-site mean-field theory for the ZGB model.Comment: 18 pages, 13 figure
Asymmetric dynamics and critical behavior in the Bak-Sneppen model
We investigate, using mean-field theory and simulation, the effect of
asymmetry on the critical behavior and probability density of Bak-Sneppen
models. Two kinds of anisotropy are investigated: (i) different numbers of
sites to the left and right of the central (minimum) site are updated and (ii)
sites to the left and right of the central site are renewed in different ways.
Of particular interest is the crossover from symmetric to asymmetric scaling
for weakly asymmetric dynamics, and the collapse of data with different numbers
of updated sites but the same degree of asymmetry. All non-symmetric rules
studied fall, independent of the degree of asymmetry, in the same universality
class. Conversely, symmetric variants reproduce the exponents of the original
model. Our results confirm the existence of two symmetry-based universality
classes for extremal dynamics.Comment: 14 pages, 8 figures, 1 tabl
Absorbing-state phase transitions: exact solutions of small systems
I derive precise results for absorbing-state phase transitions using exact
(numerically determined) quasistationary probability distributions for small
systems. Analysis of the contact process on rings of 23 or fewer sites yields
critical properties (control parameter, order-parameter ratios, and critical
exponents z and beta/nu_perp) with an accuracy of better than 0.1%; for the
exponent nu_perp the accuracy is about 0.5%. Good results are also obtained for
the pair contact process
Path-integral representation for a stochastic sandpile
We introduce an operator description for a stochastic sandpile model with a
conserved particle density, and develop a path-integral representation for its
evolution. The resulting (exact) expression for the effective action highlights
certain interesting features of the model, for example, that it is nominally
massless, and that the dynamics is via cooperative diffusion. Using the
path-integral formalism, we construct a diagrammatic perturbation theory,
yielding a series expansion for the activity density in powers of the time.Comment: 22 pages, 6 figure
Diffusive epidemic process: theory and simulation
We study the continuous absorbing-state phase transition in the
one-dimensional diffusive epidemic process via mean-field theory and Monte
Carlo simulation. In this model, particles of two species (A and B) hop on a
lattice and undergo reactions B -> A and A + B -> 2B; the total particle number
is conserved. We formulate the model as a continuous-time Markov process
described by a master equation. A phase transition between the (absorbing)
B-free state and an active state is observed as the parameters (reaction and
diffusion rates, and total particle density) are varied. Mean-field theory
reveals a surprising, nonmonotonic dependence of the critical recovery rate on
the diffusion rate of B particles. A computational realization of the process
that is faithful to the transition rates defining the model is devised,
allowing for direct comparison with theory. Using the quasi-stationary
simulation method we determine the order parameter and the survival time in
systems of up to 4000 sites. Due to strong finite-size effects, the results
converge only for large system sizes. We find no evidence for a discontinuous
transition. Our results are consistent with the existence of three distinct
universality classes, depending on whether A particles diffusive more rapidly,
less rapidly, or at the same rate as B particles.Comment: 19 pages, 5 figure
Asymptotic behavior of the order parameter in a stochastic sandpile
We derive the first four terms in a series for the order paramater (the
stationary activity density rho) in the supercritical regime of a
one-dimensional stochastic sandpile; in the two-dimensional case the first
three terms are reported. We reorganize the pertubation theory for the model,
recently derived using a path-integral formalism [R. Dickman e R. Vidigal, J.
Phys. A 35, 7269 (2002)], to obtain an expansion for stationary properties.
Since the process has a strictly conserved particle density p, the Fourier mode
N^{-1} psi_{k=0} -> p, when the number of sites N -> infinity, and so is not a
random variable. Isolating this mode, we obtain a new effective action leading
to an expansion for rho in the parameter kappa = 1/(1+4p). This requires
enumeration and numerical evaluation of more than 200 000 diagrams, for which
task we develop a computational algorithm. Predictions derived from this series
are in good accord with simulation results. We also discuss the nature of
correlation functions and one-site reduced densities in the small-kappa
(large-p) limit.Comment: 18 pages, 5 figure
Uphill migration in coupled driven particle systems
In particle systems subject to a nonuniform drive, particle migration is
observed from the driven to the non--driven region and vice--versa, depending
on details of the hopping dynamics, leading to apparent violations of Fick's
law and of steady--state thermodynamics. We propose and discuss a very basic
model in the framework of independent random walkers on a pair of rings, one of
which features biased hopping rates, in which this phenomenon is observed and
fully explained.Comment: 8 pages, 10 figure
Series expansion for a stochastic sandpile
Using operator algebra, we extend the series for the activity density in a
one-dimensional stochastic sandpile with fixed particle density p, the first
terms of which were obtained via perturbation theory [R. Dickman and R.
Vidigal, J. Phys. A35, 7269 (2002)]. The expansion is in powers of the time;
the coefficients are polynomials in p. We devise an algorithm for evaluating
expectations of operator products and extend the series to O(t^{16}).
Constructing Pade approximants to a suitably transformed series, we obtain
predictions for the activity that compare well against simulations, in the
supercritical regime.Comment: Extended series and improved analysi
A field theoretic approach to master equations and a variational method beyond the Poisson ansatz
We develop a variational scheme in a field theoretic approach to a stochastic
process. While various stochastic processes can be expressed using master
equations, in general it is difficult to solve the master equations exactly,
and it is also hard to solve the master equations numerically because of the
curse of dimensionality. The field theoretic approach has been used in order to
study such complicated master equations, and the variational scheme achieves
tremendous reduction in the dimensionality of master equations. For the
variational method, only the Poisson ansatz has been used, in which one
restricts the variational function to a Poisson distribution. Hence, one has
dealt with only restricted fluctuation effects. We develop the variational
method further, which enables us to treat an arbitrary variational function. It
is shown that the variational scheme developed gives a quantitatively good
approximation for master equations which describe a stochastic gene regulatory
network.Comment: 13 pages, 2 figure
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