650 research outputs found
Single-Particle Diffusion-Coefficient on Surfaces with Ehrlich-Schwoebel-Barriers
The diffusion coefficient of single particles in the presence of
Ehrlich-Schwoebel barriers (ESB)is considered. An exact expression is given for
the diffusion coefficient on linear chains with random arrangements of ESB. The
results are extended to surfaces having ESB with uniform extension in one or
both directions. All results are verified by Monte Carlo simulations.Comment: 11 pages, LaTeX2e, 6 eps-figure
General technique of calculating drift velocity and diffusion coefficient in arbitrary periodic systems
We develop a practical method of computing the stationary drift velocity V
and the diffusion coefficient D of a particle (or a few particles) in a
periodic system with arbitrary transition rates. We solve this problem both in
a physically relevant continuous-time approach as well as for models with
discrete-time kinetics, which are often used in computer simulations. We show
that both approaches yield the same value of the drift, but the difference
between the diffusion coefficients obtained in each of them equals V*V/2.
Generalization to spaces of arbitrary dimension and several applications of the
method are also presented.Comment: 12 pages + 2 figures, RevTeX. Submitted to J. Phys. A: Math. Ge
Hopping motion of lattice gases through nonsymmetric potentials under strong bias conditions
The hopping motion of lattice gases through potentials without
mirror-reflection symmetry is investigated under various bias conditions. The
model of 2 particles on a ring with 4 sites is solved explicitly; the resulting
current in a sawtooth potential is discussed. The current of lattice gases in
extended systems consisting of periodic repetitions of segments with sawtooth
potentials is studied for different concentrations and values of the bias.
Rectification effects are observed, similar to the single-particle case. A
mean-field approximation for the current in the case of strong bias acting
against the highest barriers in the system is made and compared with numerical
simulations. The particle-vacancy symmetry of the model is discussed.Comment: 8 pages (incl. 6 eps figures); RevTeX 3.
Relaxation at late stages in an entropy barrier model for glassy systems
The ground state dynamics of an entropy barrier model proposed recently for
describing relaxation of glassy systems is considered. At stages of evolution
the dynamics can be described by a simple variant of the Ehrenfest urn model.
Analytical expression for the relaxation times from an arbitrary state to the
ground state is derived. Upper and lower bounds for the relaxation times as a
function of system size are obtained.Comment: 9 pages no figures. to appear in J.Phys. A: Math. and Ge
Mean-Field Treatment of the Many-Body Fokker-Planck Equation
We review some properties of the stationary states of the Fokker - Planck
equation for N interacting particles within a mean field approximation, which
yields a non-linear integrodifferential equation for the particle density.
Analytical results show that for attractive long range potentials the steady
state is always a precipitate containing one cluster of small size. For
arbitrary potential, linear stability analysis allows to state the conditions
under which the uniform equilibrium state is unstable against small
perturbations and, via the Einstein relation, to define a critical temperature
Tc separating two phases, uniform and precipitate. The corresponding phase
diagram turns out to be strongly dependent on the pair-potential. In addition,
numerical calculations reveal that the transition is hysteretic. We finally
discuss the dynamics of relaxation for the uniform state suddenly cooled below
Tc.Comment: 13 pages, 8 figure
Continuum theory of vacancy-mediated diffusion
We present and solve a continuum theory of vacancy-mediated diffusion (as
evidenced, for example, in the vacancy driven motion of tracers in crystals).
Results are obtained for all spatial dimensions, and reveal the strongly
non-gaussian nature of the tracer fluctuations. In integer dimensions, our
results are in complete agreement with those from previous exact lattice
calculations. We also extend our model to describe the vacancy-driven
fluctuations of a slaved flux line.Comment: 25 Latex pages, subm. to Physical Review
Critical dimensions for random walks on random-walk chains
The probability distribution of random walks on linear structures generated
by random walks in -dimensional space, , is analytically studied
for the case . It is shown to obey the scaling form
, where is
the density of the chain. Expanding in powers of , we find that
there exists an infinite hierarchy of critical dimensions, ,
each one characterized by a logarithmic correction in . Namely, for
, ; for ,
; for , ; for , ; for , , {\it etc.\/} In particular, for
, this implies that the temporal dependence of the probability density of
being close to the origin .Comment: LATeX, 10 pages, no figures submitted for publication in PR
"A Decentralized Operations Concept for the European Payloads on the International Space Station"
The European Module Columbus of the International Space Station (ISS) is planned to be launched 2004. For its exploitation phase as well as for the early utilisation of the Space Station starting from 2003 onwards the operations procedures are now being defined in detail and the implementation of specific infrastructure has started. A decentralised operations concept will allow the investigators to perform their experiments using the telescience technique of remote experiment operations whenever feasible. User Support and Operation Centres (USOCs) will act as Facility Responsible Centres (FRC) performing the operations for multi user experiment facilities. The Columbus Control Centre (COL-CC) will perform the Columbus system operations, co-ordinate the European payload operations and provide the European Communications network. This paper gives an overview on the operations concepts and the tasks and set up of the involved sites
Generalized model for dynamic percolation
We study the dynamics of a carrier, which performs a biased motion under the
influence of an external field E, in an environment which is modeled by dynamic
percolation and created by hard-core particles. The particles move randomly on
a simple cubic lattice, constrained by hard-core exclusion, and they
spontaneously annihilate and re-appear at some prescribed rates. Using
decoupling of the third-order correlation functions into the product of the
pairwise carrier-particle correlations we determine the density profiles of the
"environment" particles, as seen from the stationary moving carrier, and
calculate its terminal velocity, V_c, as the function of the applied field and
other system parameters. We find that for sufficiently small driving forces the
force exerted on the carrier by the "environment" particles shows a
viscous-like behavior. An analog Stokes formula for such dynamic percolative
environments and the corresponding friction coefficient are derived. We show
that the density profile of the environment particles is strongly
inhomogeneous: In front of the stationary moving carrier the density is higher
than the average density, , and approaches the average value as an
exponential function of the distance from the carrier. Past the carrier the
local density is lower than and the relaxation towards may
proceed differently depending on whether the particles number is or is not
explicitly conserved.Comment: Latex, 32 pages, 4 ps-figures, submitted to PR
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