2,013 research outputs found
Diffusion in Fluctuating Media: The Resonant Activation Problem
We present a one-dimensional model for diffusion in a fluctuating lattice;
that is a lattice which can be in two or more states. Transitions between the
lattice states are induced by a combination of two processes: one periodic
deterministic and the other stochastic. We study the dynamics of a system of
particles moving in that medium, and characterize the problem from different
points of view: mean first passage time (MFPT), probability of return to a
given site (), and the total length displacement or number of visited
lattice sites (). We observe a double {\it resonant activation}-like
phenomenon when we plot the MFPT and as functions of the intensity of
the transition rate stochastic component.Comment: RevTex, 15 pgs, 8 figures, submitted to Eur.Phys.J.
Bulk Mediated Surface Diffusion: The Infinite System Case
An analytical soluble model based on a Continuous Time Random Walk (CTRW)
scheme for the adsorption-desorption processes at interfaces, called
bulk-mediated surface diffusion, is presented. The time evolution of the
effective probability distribution width on the surface is calculated and
analyzed within an anomalous diffusion framework. The asymptotic behavior for
large times shows a sub-diffusive regime for the effective surface diffusion
but, depending on the observed range of time, other regimes may be obtained.
Montecarlo simulations show excellent agreement with analytical results. As an
important byproduct of the indicated approach, we present the evaluation of the
time for the first visit to the surface.Comment: 15 pages, 7 figure
Bulk Mediated Surface Diffusion: Finite System Case
We address the dynamics of adsorbed molecules (a fundamental issue in surface
physics) within the framework of a Master Equation scheme, and study the
diffusion of particles in a finite cubic lattice whose boundaries are at the
and the planes where , while the and
directions are unbounded. As we are interested in the effective diffusion
process at the interface , we calculate analytically the conditional
probability for finding the system on the plane as well as the surface
dispersion as a function of time and compare these results with Monte Carlo
simulations finding an excellent agreement.Comment: 19 pages, 8 figure
Number distributions for fermions and fermionized bosons in periodic potentials
We compute the spatial population statistics for one-dimensional fermi-gases
and for bose-gases with hard core repulsions in periodic potentials. We show
how the statistics depend on the atomic density in the ground state of the
system, and we present calculations for the dynamical turn-on of the potential.Comment: 8 pages, 4 figures, submitted to Phys. Rev.
Narrow-escape-time problem: the imperfect trapping case
We present a master equation approach to the \emph{narrow escape time} (NET)
problem, i.e. the time needed for a particle contained in a confining domain
with a single narrow opening, to exit the domain for the first time. We
introduce a finite transition probability, , at the narrow escape window
allowing the study of the imperfect trapping case. Ranging from 0 to ,
allowed the study of both extremes of the trapping process: that of a
highly deficient capture, and situations where escape is certain ("perfect
trapping" case). We have obtained analytic results for the basic quantity
studied in the NET problem, the \emph{mean escape time} (MET), and we have
studied its dependence in terms of the transition (desorption) probability over
(from) the surface boundary, the confining domain dimensions, and the finite
transition probability at the escape window. Particularly we show that the
existence of a global minimum in the NET depends on the `imperfection' of the
trapping process. In addition to our analytical approach, we have implemented
Monte Carlo simulations, finding excellent agreement between the theoretical
results and simulations.Comment: 9 page
Nova Geminorum 1912 and the Origin of the Idea of Gravitational Lensing
Einstein's early calculations of gravitational lensing, contained in a
scratch notebook and dated to the spring of 1912, are reexamined. A hitherto
unknown letter by Einstein suggests that he entertained the idea of explaining
the phenomenon of new stars by gravitational lensing in the fall of 1915 much
more seriously than was previously assumed. A reexamination of the relevant
calculations by Einstein shows that, indeed, at least some of them most likely
date from early October 1915. But in support of earlier historical
interpretation of Einstein's notes, it is argued that the appearance of Nova
Geminorum 1912 (DN Gem) in March 1912 may, in fact, provide a relevant context
and motivation for Einstein's lensing calculations on the occasion of his first
meeting with Erwin Freundlich during a visit in Berlin in April 1912. We also
comment on the significance of Einstein's consideration of gravitational
lensing in the fall of 1915 for the reconstruction of Einstein's final steps in
his path towards general relativity.Comment: 31 p
Multiplicative noise: A mechanism leading to nonextensive statistical mechanics
A large variety of microscopic or mesoscopic models lead to generic results
that accommodate naturally within Boltzmann-Gibbs statistical mechanics (based
on ). Similarly, other classes of models
point toward nonextensive statistical mechanics (based on , where the value of the entropic index depends on
the specific model). We show here a family of models, with multiplicative
noise, which belongs to the nonextensive class. More specifically, we consider
Langevin equations of the type , where
and are independent zero-mean Gaussian white noises with
respective amplitudes and . This leads to the Fokker-Planck equation
. Whenever the
deterministic drift is proportional to the noise induced one, i.e., , the stationary solution is shown to be (with and ). This distribution is
precisely the one optimizing with the constraint constant. We also
introduce and discuss various characterizations of the width of the
distributions.Comment: 3 PS figure
Dog Ecology and Dog Rabies Control
Dog populations, like other populations, depend on the availability of resources (food, water, and shelter). Humans either make available or deliberately withhold resources for varying proportions of dog populations. Dog-keeping practices and the duties of responsible ownership vary with the cultural setting. Dog populations often attain densities that allow the species to be a main host of rabies. The epidemiology of dog rabies is not well understood, despite the easy access to dog populations. Today dog rabies is predomina~t in developing countries. In addition to the high rate of exposure of humans to dogs, tradItional medical beliefs and practices are the most important cultural factors that lead to high numbers of cases of human rabies. Dog rabies control programs have been succe~sful in the past, but most are failing today. Program development should follow managenal principles and take into consideration the biology of dog populations as w~ll as. cultural constraints. Elimination of stray dogs IS not an effIcIent means of controllIng eIther the dog population or rabies, but it may create public awarenes
Disruption of a Yeast ADE6 Gene Homolog in Ustilago maydis
A putative homolog of the Sacharromyces cereviseae ADE6 and Escherichia coli purL genes is identified near a multigenic complex, which contains two genes, sid1 and sid2, involved in a siderophore biosynthetic pathway inUstilago maydis. The putative ADE6 homolog was mutated by targeted gene disruption. The resulting mutant strains demonstrated a requirement for exogenous adenine, indicating that the U. maydis ade6 homolog is required for purine biosynthesis
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