85 research outputs found
Flux reversal in a simple random walk model on a fluctuating symmetric lattice
A rather simple random walk model on a one-dimensional lattice is put
forward. The lattice as a whole switches randomly between two possible states
which are spatially symmetric. Both lattice states are identical, but
translated by one site with respect to each other, and consist of infinite
arrays of absorbing sites separated by two non-absorbing sites. Exact explicit
expressions for the long-time velocity and the effective diffusion coefficient
are obtained and discussed. In particular, it is shown that the direction of
the steady motion can be reversed by conveniently varying the values of either
the mean residence times in the lattice states or the transition rates to the
absorbing and non-absorbing sites.Comment: 6 pages, 3 Figures, (to appear in Physical Review E
Directed motion of spheres induced by unbiased driving forces in viscous fluids beyond the Stokes' law regime
The emergence of directed motion is investigated in a system consisting of a sphere immersed in a viscous fluid and subjected to time-periodic forces of zero average. The directed motion arises from the combined action of a nonlinear drag force and the applied driving forces, in the absence of any periodic substrate potential. Necessary conditions for the existence of such directed motion are obtained and an analytical expression for the average terminal velocity is derived within the adiabatic approximation. Special attention is paid to the case of two mutually perpendicular forces with sinusoidal time dependence, one with twice the period of the other. It is shown that, although neither of these two forces induces directed motion when acting separately, when added together, the resultant force generates directed motion along the direction of the force with the shortest period. The dependence of the average terminal velocity on the system parameters is analyzed numerically and compared with that obtained using the adiabatic approximation. Among other results, it is found that, for appropriate parameter values, the direction of the average terminal velocity can be reversed by varying the forcing strength. Furthermore, certain aspects of the observed phenomenology are explained by means of symmetry arguments
Very large stochastic resonance gains in finite sets of interacting identical subsystems driven by subthreshold rectangular pulses
We study the phenomenon of nonlinear stochastic resonance (SR) in a complex
noisy system formed by a finite number of interacting subunits driven by
rectangular pulsed time periodic forces. We find that very large SR gains are
obtained for subthreshold driving forces with frequencies much larger than the
values observed in simpler one-dimensional systems. These effects are explained
using simple considerations.Comment: 4 pages, 5 figures. to appear in Phys. Rev.
Overdamped Deterministic Ratchets Driven By Multifrequency Forces
We investigate a dissipative, deterministic ratchet model in the overdamped
regime driven by a {\it rectangular force}. Extensive numerical calculations
are presented in a diagram depicting the drift velocity as a function of a wide
range of the driving parameter values. We also present some theoretical
considerations which explain some features of the mentioned diagram. In
particular, we proof the existence of regions in the driving parameter space
with bounded particle motion possessing zero current. Moreover, we present an
explicit analytical expression for the drift velocity in the adiabatic limit.Comment: 12 pages, 6 figure
Effects of additive noise on vibrational resonance in a bistable system
We study the overdamped motion of a particle in a bistable potential subject to the action of a bichromatic force and additive noise, within the context of the vibrational resonance phenomenon. Under appropriate conditions, we obtain analytical expressions for the relevant observables which quantifies this phenomenon. The theoretical results are compared with those obtained by the numerical solution of the stochastic differential equation which describes the dynamics of the system. The limits of validity of the theoretical approach are also discussed.España Dirección General de Enseñanza Superior Projects Nos. BFM2002- 03822 and BFM2000-096
Quantum ratchet with Lindblad rate equations
A quantum random walk model is established on a one-dimensional periodic
lattice that fluctuates between two possible states. This model is defined by
Lindblad rate equations that incorporate the transition rates between the two
lattice states. Leveraging the system's symmetries, the particle velocity can
be described using a finite set of equations, even though the state space is of
infinite dimension. These equations yield an analytical expression for the
velocity in the long-time limit, which is employed to analyze the
characteristics of directed motion. Notably, the velocity can exhibit multiple
inversions, and to achieve directed motion, distinct, nonzero transition rates
between lattice states are required.Comment: 8 pages, 3 figure
Comment on "Soliton ratchets induced by excitation of internal modes"
Very recently Willis et al. [Phys. Rev. E {\bf 69}, 056612 (2004)] have used
a collective variable theory to explain the appearance of a nonzero energy
current in an ac driven, damped sine-Gordon equation. In this comment, we prove
rigorously that the time-averaged energy current in an ac driven nonlinear
Klein-Gordon system is strictly zero.Comment: 3 pages, 1 figure, Submitted to Phys. Rev.
Resonant activation in a simple kinetic model
We present a very simple Markovian kinetic model displaying a stochastic resonant behavior which is similar to the one found in the escape of a particle over a fluctuating potential barrier. The basic mechanism that is responsible for the existence of resonance is identified. This allows the generalization of the model in different ways, leading to a variety of models where a similar phenomenon is to be expected. It is also shown that the initial conditions play an important role in determining whether the resonant activation actually shows up.España Dirección General de Investigación Cientifíca y Técnica Grant No. PB92-068
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