395 research outputs found
Degree Correlations in a Dynamically Generated Model Food Web
We explore aspects of the community structures generated by a simple
predator-prey model of biological coevolution, using large-scale kinetic Monte
Carlo simulations. The model accounts for interspecies and intraspecies
competition for resources, as well as adaptive foraging behavior. It produces a
metastable low-diversity phase and a stable high-diversity phase. The
structures and joint indegree-outdegree distributions of the food webs
generated in the latter phase are discussed.Comment: 4 page
Field-driven solid-on-solid interfaces moving under a stochastic Arrhenius dynamic: effects of the barrier height
We present analytical results and kinetic Monte Carlo simulations for the
mobility and microscopic structure of solid-on-solid (SOS) interfaces driven
far from equilibrium by an external force, such as an applied field or
(electro)chemical potential difference. The interfaces evolve under a specific
stochastic dynamic with a local energy barrier (an Arrhenius dynamic), known as
the transition dynamics approximation (TDA). We calculate the average height of
steps on the interface, the average interface velocity, and the skewness of the
interface as functions of the driving force and the height of the energy
barrier. We find that the microscopic interface structure depends quite
strongly on the barrier height. As the barrier becomes higher, the local
interface width decreases and the skewness increases, suggesting increasing
short-range correlations between the step heights.Comment: 6 pages, 5 figs. RevTe
Computational Lattice-Gas Modeling of the Electrosorption of Small Molecules and Ions
We present two recent applications of lattice-gas modeling techniques to
electrochemical adsorption on catalytically active metal substrates: urea on
Pt(100) and (bi)sulfate on Rh(111). Both involve the specific adsorption of
small molecules or ions on well-characterized single-crystal electrodes, and
they provide a particularly good fit between the adsorbate geometry and the
substrate structure. The close geometric fit facilitates the formation of
ordered submonolayer adsorbate phases in a range of electrode potential
positive of the range in which an adsorbed monolayer of hydrogen is stable. In
both systems the ordered-phase region is separated from the adsorbed- hydrogen
region by a phase transition, signified in cyclic voltammograms by a sharp
current peak. Based on data from {\it in situ\/} radiochemical surface
concentration measurements, cyclic voltammetry, and scanning tunneling micro-
scopy, and {\it ex situ\/} Auger electron spectroscopy and low-energy electron
diffraction, we have developed specific lattice-gas models for the two systems.
These models were studied by group-theoretical ground-state calcu- lations and
numerical Monte Carlo simulations, and effective lattice-gas inter- action
parameters were determined so as to provide agreement with experiments.Comment: 17 pp. uuencoded postscript, FSU-SCRI-94C-9
Fluctuations in models of biological macroevolution
Fluctuations in diversity and extinction sizes are discussed and compared for
two different, individual-based models of biological coevolution. Both models
display power-law distributions for various quantities of evolutionary
interest, such as the lifetimes of individual species, the quiet periods
between evolutionary upheavals larger than a given cutoff, and the sizes of
extinction events. Time series of the diversity and measures of the size of
extinctions give rise to flicker noise. Surprisingly, the power-law behaviors
of the probability densities of quiet periods in the two models differ, while
the distributions of the lifetimes of individual species are the same.Comment: 7 pages, 5 figure
Discrete-Event Analytic Technique for Surface Growth Problems
We introduce an approach for calculating non-universal properties of rough
surfaces. The technique uses concepts of distinct surface-configuration
classes, defined by the surface growth rule. The key idea is a mapping between
discrete events that take place on the interface and its elementary local-site
configurations. We construct theoretical probability distributions of
deposition events at saturation for surfaces generated by selected growth
rules. These distributions are then used to compute measurable physical
quantities. Despite the neglect of temporal correlations, our approximate
analytical results are in very good agreement with numerical simulations. This
discrete-event analytic technique can be particularly useful when applied to
quantification problems, which are known to not be suited to continuum methods.Comment: 4 pages, 7 figures, published 17 Feb. 200
Magnetization Switching in Single-Domain Ferromagnets
A model for single-domain uniaxial ferromagnetic particles with high
anisotropy, the Ising model, is studied. Recent experimental observations have
been made of the probability that the magnetization has not switched. Here an
approach is described in which it is emphasized that a ferromagnetic particle
in an unfavorable field is in fact a metastable system, and the switching is
accomplished through the nucleation and subsequent growth of localized
droplets. Nucleation theory is applied to finite systems to determine the
coercivity as a function of particle size and to calculate the probability of
not switching. Both of these quantities are modified by different boundary
conditions, magnetostatic interactions, and quenched disorder.Comment: 4 pages, LaTeX, 2 figures, documentstyle{elsart} More fits and
Mathematica notebook at http://www.scri.fsu.edu/~novotny/magnetism.html To
appear in J.Mag.Mag.Mater. Conference Proceedings of 7th International
Conference on Magnetism Cairns, Australia, August, 199
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