31,972 research outputs found
Irreversible and reversible modes of operation of deterministic ratchets
We discuss a problem of optimization of the energetic efficiency of a simple
rocked ratchet. We concentrate on a low-temperature case in which the
particle's motion in a ratchet potential is deterministic. We show that the
energetic efficiency of a ratchet working adiabatically is bounded from above
by a value depending on the form of ratchet potential. The ratchets with
strongly asymmetric potentials can achieve ideal efficiency of unity without
approaching reversibility. On the other hand we show that for any form of the
ratchet potential a set of time-protocols of the outer force exist under which
the operation is reversible and the ideal value of efficiency is also achieved.
The mode of operation of the ratchet is still quasistatic but not adiabatic.
The high values of efficiency can be preserved even under elevated
temperatures
Irreversible and reversible modes of operation of deterministic ratchets
We discuss a problem of optimization of the energetic efficiency of a simple
rocked ratchet. We concentrate on a low-temperature case in which the
particle's motion in a ratchet potential is deterministic. We show that the
energetic efficiency of a ratchet working adiabatically is bounded from above
by a value depending on the form of ratchet potential. The ratchets with
strongly asymmetric potentials can achieve ideal efficiency of unity without
approaching reversibility. On the other hand we show that for any form of the
ratchet potential a set of time-protocols of the outer force exist under which
the operation is reversible and the ideal value of efficiency is also achieved.
The mode of operation of the ratchet is still quasistatic but not adiabatic.
The high values of efficiency can be preserved even under elevated
temperatures
Quantum Brownian motion in ratchet potentials: duality relation and its consequences
Quantum Brownian motion in ratchet potentials is investigated by means of an
approach based on a duality relation. This relation links the long-time
dynamics in a tilted ratchet potential in the presence of dissipation with the
one in a driven dissipative tight-binding model. The application to quantum
ratchet yields a simple expression for the ratchet current in terms of the
transition rates in the tight-binding system.Comment: Chemical Physics (in press
Chaos and Correspondence in Classical and Quantum Hamiltonian Ratchets: A Heisenberg Approach
Previous work [Gong and Brumer, Phys. Rev. Lett., 97, 240602 (2006)]
motivates this study as to how asymmetry-driven quantum ratchet effects can
persist despite a corresponding fully chaotic classical phase space. A simple
perspective of ratchet dynamics, based on the Heisenberg picture, is
introduced. We show that ratchet effects are in principle of common origin in
classical and quantum mechanics, though full chaos suppresses these effects in
the former but not necessarily the latter. The relationship between ratchet
effects and coherent dynamical control is noted.Comment: 21 pages, 7 figures, to appear in Phys. Rev.
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
Ratchet effect on a relativistic particle driven by external forces
We study the ratchet effect of a damped relativistic particle driven by both
asymmetric temporal bi-harmonic and time-periodic piecewise constant forces.
This system can be formally solved for any external force, providing the
ratchet velocity as a non-linear functional of the driving force. This allows
us to explicitly illustrate the functional Taylor expansion formalism recently
proposed for this kind of systems. The Taylor expansion reveals particularly
useful to obtain the shape of the current when the force is periodic, piecewise
constant. We also illustrate the somewhat counterintuitive effect that
introducing damping may induce a ratchet effect. When the force is symmetric
under time-reversal and the system is undamped, under symmetry principles no
ratchet effect is possible. In this situation increasing damping generates a
ratchet current which, upon increasing the damping coefficient eventually
reaches a maximum and decreases toward zero. We argue that this effect is not
specific of this example and should appear in any ratchet system with tunable
damping driven by a time-reversible external force.Comment: 1 figur
Quantum Ratchet Accelerator without a Bichromatic Lattice Potential
In a quantum ratchet accelerator system, a linearly increasing directed
current can be dynamically generated without using a biased field. Generic
quantum ratchet acceleration with full classical chaos [Gong and Brumer, Phys.
Rev. Lett. 97, 240602 (2006)] constitutes a new element of quantum chaos and an
interesting violation of a sum rule of classical ratchet transport. Here we
propose a simple quantum ratchet accelerator model that can also generate
linearly increasing quantum current with full classical chaos. This new model
does not require a bichromatic lattice potential. It is based on a variant of
an on-resonance kicked-rotor system, periodically kicked by two optical lattice
potentials of the same lattice constant, but with unequal amplitudes and a
fixed phase shift between them. The dependence of the ratchet current
acceleration rate on the system parameters is studied in detail. The cold-atom
version of our new quantum ratchet accelerator model should be realizable by
introducing slight modifications to current cold-atom experiments.Comment: 9 pages, 6 figures, submitted to Phys. Rev.
Depinning of kinks in a Josephson-junction ratchet array
We have measured the depinning of trapped kinks in a ratchet potential using
a fabricated circular array of Josephson junctions. Our ratchet system consists
of a parallel array of junctions with alternating cell inductances and
junctions areas. We have compared this ratchet array with other circular
arrays. We find experimentally and numerically that the depinning current
depends on the direction of the applied current in our ratchet ring. We also
find other properties of the depinning current versus applied field, such as a
long period and a lack of reflection symmetry, which we can explain
analytically.Comment: to be published in PR
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