354 research outputs found
The flashing ratchet and unidirectional transport of matter
We study the flashing ratchet model of a Brownian motor, which consists in
cyclical switching between the Fokker-Planck equation with an asymmetric
ratchet-like potential and the pure diffusion equation. We show that the motor
really performs unidirectional transport of mass, for proper parameters of the
model, by analyzing the attractor of the problem and the stationary vector of a
related Markov chain.Comment: 11 page
Brownian Motors driven by Particle Exchange
We extend the Langevin dynamics so that particles can be exchanged with a
particle reservoir. We show that grand canonical ensembles are realized at
equilibrium and derive the relations of thermodynamics for processes between
equilibrium states. As an application of the proposed evolution rule, we devise
a simple model of Brownian motors driven by particle exchange. KEYWORDS:
Langevin Dynamics, Thermodynamics, Open SystemsComment: 5 pages, late
Rocking ratchets in 2D Josephson networks: collective effects and current reversal
A detailed numerical study on the directed motion of ac-driven vortices and
antivortices in 2D Josephson junction arrays (JJA) with an asymmetric periodic
pinning potential is reported. Dc-voltage rectification shows a strong
dependence on vortex density as well as an inversion of the vortex flow
direction with ac amplitude for a wide range of vortex density around =1/2
(=), in good agreement with recent experiments by Shal\'om
and Pastoriza [Phys. Rev. Lett. {\bf 94}, 177001 (2005)]. The study of vortex
structures, spatial and temporal correlations, and vortex-antivortex pairs
formation gives insight into a purely collective mechanism behind the current
reversal effect.Comment: 4 pages, 5 figures. Accepted for publication in Phys. Rev. Let
Symmetry Relations for Trajectories of a Brownian Motor
A Brownian Motor is a nanoscale or molecular device that combines the effects
of thermal noise, spatial or temporal asymmetry, and directionless input energy
to drive directed motion. Because of the input energy, Brownian motors function
away from thermodynamic equilibrium and concepts such as linear response
theory, fluctuation dissipation relations, and detailed balance do not apply.
The {\em generalized} fluctuation-dissipation relation, however, states that
even under strongly thermodynamically non-equilibrium conditions the ratio of
the probability of a transition to the probability of the time-reverse of that
transition is the exponent of the change in the internal energy of the system
due to the transition. Here, we derive an extension of the generalized
fluctuation dissipation theorem for a Brownian motor for the ratio between the
probability for the motor to take a forward step and the probability to take a
backward step
Interaction of molecular motors can enhance their efficiency
Particles moving in oscillating potential with broken mirror symmetry are
considered. We calculate their energetic efficiency, when acting as molecular
motors carrying a load against external force. It is shown that interaction
between particles enhances the efficiency in wide range of parameters. Possible
consequences for artificial molecular motors are discussed.Comment: 6 pages, 8 figure
Generic Quantum Ratchet Accelerator with Full Classical Chaos
A simple model of quantum ratchet transport that can generate unbounded
linear acceleration of the quantum ratchet current is proposed, with the
underlying classical dynamics fully chaotic. The results demonstrate that
generic acceleration of quantum ratchet transport can occur with any type of
classical phase space structure. The quantum ratchet transport with full
classical chaos is also shown to be very robust to noise due to the large
linear acceleration afforded by the quantum dynamics. One possible experiment
allowing observation of these predictions is suggested.Comment: 4 pages, 4 figure
Separation quality of a geometric ratchet
We consider an experimentally relevant model of a geometric ratchet in which
particles undergo drift and diffusive motion in a two-dimensional periodic
array of obstacles, and which is used for the continuous separation of
particles subject to different forces. The macroscopic drift velocity and
diffusion tensor are calculated by a Monte-Carlo simulation and by a
master-equation approach, using the correponding microscopic quantities and the
shape of the obstacles as input. We define a measure of separation quality and
investigate its dependence on the applied force and the shape of the obstacles
Efficiency of Energy Transduction in a Molecular Chemical Engine
A simple model of the two-state ratchet type is proposed for molecular
chemical engines that convert chemical free energy into mechanical work and
vice versa. The engine works by catalyzing a chemical reaction and turning a
rotor. Analytical expressions are obtained for the dependences of rotation and
reaction rates on the concentrations of reactant and product molecules, from
which the performance of the engine is analyzed. In particular, the efficiency
of energy transduction is discussed in some detail.Comment: 4 pages, 4 fugures; title modified, figures 2 and 3 modified, content
changed (pages 1 and 4, mainly), references adde
Rectifying fluctuations in an optical lattice
We have realized a Brownian motor by using cold atoms in a dissipative
optical lattice as a model system. In our experiment the optical potential is
spatially symmetric and the time-symmetry of the system is broken by applying
appropriate zero-mean ac forces. We identify a regime of rectification of
forces and a regime of rectification of fluctuations, the latter corresponding
to the realization of a Brownian motor
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