282 research outputs found
Outgroup helping as a tool to communicate ingroup warmth
Article / Letter to editorInstituut Psychologi
Influence of local carrying capacity restrictions on stochastic predator-prey models
We study a stochastic lattice predator-prey system by means of Monte Carlo
simulations that do not impose any restrictions on the number of particles per
site, and discuss the similarities and differences of our results with those
obtained for site-restricted model variants. In accord with the classic
Lotka-Volterra mean-field description, both species always coexist in two
dimensions. Yet competing activity fronts generate complex, correlated
spatio-temporal structures. As a consequence, finite systems display transient
erratic population oscillations with characteristic frequencies that are
renormalized by fluctuations. For large reaction rates, when the processes are
rendered more local, these oscillations are suppressed. In contrast with
site-restricted predator-prey model, we observe species coexistence also in one
dimension. In addition, we report results on the steady-state prey age
distribution.Comment: Latex, IOP style, 17 pages, 9 figures included, related movies
available at http://www.phys.vt.edu/~tauber/PredatorPrey/movies
Three-fold way to extinction in populations of cyclically competing species
Species extinction occurs regularly and unavoidably in ecological systems.
The time scales for extinction can broadly vary and inform on the ecosystem's
stability. We study the spatio-temporal extinction dynamics of a paradigmatic
population model where three species exhibit cyclic competition. The cyclic
dynamics reflects the non-equilibrium nature of the species interactions. While
previous work focusses on the coarsening process as a mechanism that drives the
system to extinction, we found that unexpectedly the dynamics to extinction is
much richer. We observed three different types of dynamics. In addition to
coarsening, in the evolutionary relevant limit of large times, oscillating
traveling waves and heteroclinic orbits play a dominant role. The weight of the
different processes depends on the degree of mixing and the system size. By
analytical arguments and extensive numerical simulations we provide the full
characteristics of scenarios leading to extinction in one of the most
surprising models of ecology
Novel universality classes of coupled driven diffusive systems
Motivated by the phenomenologies of dynamic roughening of strings in random
media and magnetohydrodynamics, we examine the universal properties of driven
diffusive system with coupled fields. We demonstrate that cross-correlations
between the fields lead to amplitude-ratios and scaling exponents varying
continuosly with the strength of these cross-correlations. The implications of
these results for experimentally relevant systems are discussed.Comment: To appear in Phys. Rev. E (Rapid Comm.) (2003
Reaction-controlled diffusion: Monte Carlo simulations
We study the coupled two-species non-equilibrium reaction-controlled
diffusion model introduced by Trimper et al. [Phys. Rev. E 62, 6071 (2000)] by
means of detailed Monte Carlo simulations in one and two dimensions. Particles
of type A may independently hop to an adjacent lattice site provided it is
occupied by at least one B particle. The B particle species undergoes
diffusion-limited reactions. In an active state with nonzero, essentially
homogeneous B particle saturation density, the A species displays normal
diffusion. In an inactive, absorbing phase with exponentially decaying B
density, the A particles become localized. In situations with algebraic decay
rho_B(t) ~ t^{-alpha_B}, as occuring either at a non-equilibrium continuous
phase transition separating active and absorbing states, or in a power-law
inactive phase, the A particles propagate subdiffusively with mean-square
displacement ~ t^{1-alpha_A}. We find that within the accuracy of
our simulation data, \alpha_A = \alpha_B as predicted by a simple mean-field
approach. This remains true even in the presence of strong spatio-temporal
fluctuations of the B density. However, in contrast with the mean-field
results, our data yield a distinctly non-Gaussian A particle displacement
distribution n_A(x,t) that obeys dynamic scaling and looks remarkably similar
for the different processes investigated here. Fluctuations of effective
diffusion rates cause a marked enhancement of n_A(x,t) at low displacements
|x|, indicating a considerable fraction of practically localized A particles,
as well as at large traversed distances.Comment: Revtex, 19 pages, 27 eps figures include
Cluster mean-field study of the parity conserving phase transition
The phase transition of the even offspringed branching and annihilating
random walk is studied by N-cluster mean-field approximations on
one-dimensional lattices. By allowing to reach zero branching rate a phase
transition can be seen for any N <= 12.The coherent anomaly extrapolations
applied for the series of approximations results in and
.Comment: 6 pages, 5 figures, 1 table included, Minor changes, scheduled for
pubication in PR
Kinetics of phase-separation in the critical spherical model and local scale-invariance
The scaling forms of the space- and time-dependent two-time correlation and
response functions are calculated for the kinetic spherical model with a
conserved order-parameter and quenched to its critical point from a completely
disordered initial state. The stochastic Langevin equation can be split into a
noise part and into a deterministic part which has local scale-transformations
with a dynamical exponent z=4 as a dynamical symmetry. An exact reduction
formula allows to express any physical average in terms of averages calculable
from the deterministic part alone. The exact spherical model results are shown
to agree with these predictions of local scale-invariance. The results also
include kinetic growth with mass conservation as described by the
Mullins-Herring equation.Comment: Latex2e with IOP macros, 28 pp, 2 figures, final for
Nonequilibrium relaxation and scaling properties of the two-dimensional Coulomb glass in the aging regime
We employ Monte Carlo simulations to investigate the two-time density
autocorrelation function for the two-dimensional Coulomb glass. We find that
the nonequilibrium relaxation properties of this highly correlated disordered
system can be described by a full aging scaling ansatz. The scaling exponents
are non-universal, and depend on temperature and charge density.Comment: 6 pages, 3 figures included; revised version: corrected exponents,
and some additional explanations and references added; to appear in EP
Nonequilibrium steady states of driven magnetic flux lines in disordered type-II superconductors
We investigate driven magnetic flux lines in layered type-II superconductors
subject to various configurations of strong point or columnar pinning centers
by means of a three-dimensional elastic line model and Metropolis Monte Carlo
simulations. We characterize the resulting nonequilibrium steady states by
means of the force-velocity / current-voltage curve, static structure factor,
mean vortex radius of gyration, number of double-kink and half-loop
excitations, and velocity / voltage noise spectrum. We compare the results for
the above observables for randomly distributed point and columnar defects, and
demonstrate that the three-dimensional flux line structures and their
fluctuations lead to a remarkable variety of complex phenomena in the
steady-state transport properties of bulk superconductors.Comment: 23 pages, IOP style, 18 figures include
Coarsening of Disordered Quantum Rotors under a Bias Voltage
We solve the dynamics of an ensemble of interacting rotors coupled to two
leads at different chemical potential letting a current flow through the system
and driving it out of equilibrium. We show that at low temperature the
coarsening phase persists under the voltage drop up to a critical value of the
applied potential that depends on the characteristics of the electron
reservoirs. We discuss the properties of the critical surface in the
temperature, voltage, strength of quantum fluctuations and coupling to the bath
phase diagram. We analyze the coarsening regime finding, in particular, which
features are essentially quantum mechanical and which are basically classical
in nature. We demonstrate that the system evolves via the growth of a coherence
length with the same time-dependence as in the classical limit, -- the scalar curvature driven universality class. We obtain the
scaling function of the correlation function at late epochs in the coarsening
regime and we prove that it coincides with the classical one once a prefactor
that encodes the dependence on all the parameters is factorized. We derive a
generic formula for the current flowing through the system and we show that,
for this model, it rapidly approaches a constant that we compute.Comment: 53 pages, 12 figure
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