820 research outputs found
Vacancy diffusion in the triangular lattice dimer model
We study vacancy diffusion on the classical triangular lattice dimer model,
sub ject to the kinetic constraint that dimers can only translate, but not
rotate. A single vacancy, i.e. a monomer, in an otherwise fully packed lattice,
is always localized in a tree-like structure. The distribution of tree sizes is
asymptotically exponential and has an average of 8.16 \pm 0.01 sites. A
connected pair of monomers has a finite probability of being delocalized. When
delocalized, the diffusion of monomers is anomalous:Comment: 15 pages, 27 eps figures. submitted to Physical Review
Kosterlitz-Thouless transition of the quasi two-dimensional trapped Bose gas
We present Quantum Monte Carlo calculations with up to N=576000 interacting
bosons in a quasi two-dimensional trap geometry closely related to recent
experiments with atomic gases. The density profile of the gas and the
non-classical moment of inertia yield intrinsic signatures for the
Kosterlitz--Thouless transition temperature T_KT. From the reduced one-body
density matrix, we compute the condensate fraction, which is quite large for
small systems. It decreases slowly with increasing system sizes, vanishing in
the thermodynamic limit. We interpret our data in the framework of the
local-density approximation, and point out the relevance of our results for the
analysis of experiments.Comment: 4 pages, 4 figure
Selective-pivot sampling of radial distribution functions in asymmetric liquid mixtures
We present a Monte Carlo algorithm for selectively sampling radial
distribution functions and effective interaction potentials in asymmetric
liquid mixtures. We demonstrate its efficiency for hard-sphere mixtures, and
for model systems with more general interactions, and compare our simulations
with several analytical approximations. For interaction potentials containing a
hard-sphere contribution, the algorithm yields the contact value of the radial
distribution function.Comment: 5 pages, 5 figure
A Rapid Dynamical Monte Carlo Algorithm for Glassy Systems
In this paper we present a dynamical Monte Carlo algorithm which is
applicable to systems satisfying a clustering condition: during the dynamical
evolution the system is mostly trapped in deep local minima (as happens in
glasses, pinning problems etc.). We compare the algorithm to the usual Monte
Carlo algorithm, using as an example the Bernasconi model. In this model, a
straightforward implementation of the algorithm gives an improvement of several
orders of magnitude in computational speed with respect to a recent, already
very efficient, implementation of the algorithm of Bortz, Kalos and Lebowitz.Comment: RevTex 7 pages + 4 figures (uuencoded) appended; LPS preprin
Damage spreading and coupling in Markov chains
In this paper, we relate the coupling of Markov chains, at the basis of
perfect sampling methods, with damage spreading, which captures the chaotic
nature of stochastic dynamics. For two-dimensional spin glasses and hard
spheres we point out that the obstacle to the application of perfect-sampling
schemes is posed by damage spreading rather than by the survey problem of the
entire configuration space. We find dynamical damage-spreading transitions
deeply inside the paramagnetic and liquid phases, and show that critical values
of the transition temperatures and densities depend on the coupling scheme. We
discuss our findings in the light of a classic proof that for arbitrary Monte
Carlo algorithms damage spreading can be avoided through non-Markovian coupling
schemes.Comment: 6 pages, 8 figure
Creep dynamics of elastic manifolds via exact transition pathways
We study the steady state of driven elastic strings in disordered media below
the depinning threshold. In the low-temperature limit, for a fixed sample, the
steady state is dominated by a single configuration, which we determine exactly
from the transition pathways between metastable states. We obtain the dynamical
phase diagram in this limit. At variance with a thermodynamic phase transition,
the depinning transition is not associated with a divergent length scale of the
steady state below threshold, but only of the transient dynamics. We discuss
the distribution of barrier heights, and check the validity of the dynamic
phase diagram at small but finite temperatures using Langevin simulations. The
phase diagram continues to hold for broken statistical tilt symmetry. We point
out the relevance of our results for experiments of creep motion in elastic
interfaces.Comment: 14 pages, 18 figure
Equilibrium equation of state of a hard sphere binary mixture at very large densities using replica exchange Monte-Carlo simulations
We use replica exchange Monte-Carlo simulations to measure the equilibrium
equation of state of the disordered fluid state for a binary hard sphere
mixture up to very large densities where standard Monte-Carlo simulations do
not easily reach thermal equilibrium. For the moderate system sizes we use (up
to N=100), we find no sign of a pressure discontinuity near the location of
dynamic glass singularities extrapolated using either algebraic or simple
exponential divergences, suggesting they do not correspond to genuine
thermodynamic glass transitions. Several scenarios are proposed for the fate of
the fluid state in the thermodynamic limit.Comment: 10 pages, 8 fig
Depinning exponents of the driven long-range elastic string
We perform a high-precision calculation of the critical exponents for the
long-range elastic string driven through quenched disorder at the depinning
transition, at zero temperature. Large-scale simulations are used to avoid
finite-size effects and to enable high precision. The roughness, growth, and
velocity exponents are calculated independently, and the dynamic and
correlation length exponents are derived. The critical exponents satisfy known
scaling relations and agree well with analytical predictions.Comment: 6 pages, 5 figure
Phase diagram of the bose Hubbard model
The first reliable analytic calculation of the phase diagram of the bose gas
on a -dimensional lattice with on-site repulsion is presented. In one
dimension, the analytic calculation is in excellent agreement with the
numerical Monte Carlo results. In higher dimensions, the deviations from the
Monte Carlo calculations are larger, but the correct shape of the Mott
insulator lobes is still obtained. Explicit expressions for the energy of the
Mott and the ``defect'' phase are given in a strong-coupling expansion.Comment: RevTeX 3.
- …