609 research outputs found
Tunable transport with broken space-time symmetries
Transport properties of particles and waves in spatially periodic structures
that are driven by external time-dependent forces manifestly depend on the
space-time symmetries of the corresponding equations of motion. A systematic
analysis of these symmetries uncovers the conditions necessary for obtaining
directed transport. In this work we give a unified introduction into the
symmetry analysis and demonstrate its action on the motion in one-dimensional
periodic, both in time and space, potentials. We further generalize the
analysis to quasi-periodic drivings, higher space dimensions, and quantum
dynamics. Recent experimental results on the transport of cold and ultracold
atomic ensembles in ac-driven optical potentials are reviewed as illustrations
of theoretical considerations.Comment: Phys. Rep., in pres
Spin-Orbit Based Coherent Spin Ratchets
The concept of ratchets, driven asymmetric periodic structures giving rise to
directed particle flow, has recently been generalized to a quantum ratchet
mechanism for spin currents mediated through spin-orbit interaction. Here we
consider such systems in the coherent mesoscopic regime and generalize the
proposal of a minimal spin ratchet model based on a non-interacting clean
quantum wire with two transverse channels by including disorder and by
self-consistently treating the charge redistribution in the nonlinear
(adiabatic) ac-driving regime. Our Keldysh-Green function based quantum
transport simulations show that the spin ratchet mechanism is robust and
prevails for disordered, though non-diffusive, mesoscopic structures. Extending
the two-channel to the multi-channel case does not increase the net ratchet
spin current efficiency but, remarkably, yields a dc spin transmission
increasing linearly with channel number.Comment: 23 pages, 7 figures; to be published in Chemical Physic
Experimental Realization of Quantum-Resonance Ratchets
Quantum-resonance ratchets associated with the periodically kicked particle
are experimentally realized for the first time. This is achieved by using a
Bose-Einstein condensate exposed to a pulsed standing light wave and prepared
in an initial state differing from the usual plane wave. Both the standing-wave
potential and the initial state have a point symmetry around some center and
the ratchet arises from the non-coincidence of the two centers. The dependence
of the directed quantum transport on the quasimomentum is studied. A detailed
theoretical analysis is used to explain the experimental results.Comment: Accepted for publication in Physical Review Letters (November 2007
Quantum Ratchets
The concept of thermal ratchets is extended to the system governed by quantum
mechanics. We study a tight-binding model with an asymmetric periodic potential
contacting with a heat bath under an external oscillating field as a specific
example of quantum ratchet. Dynamics of a density operator of this system is
studied numerically by using the quantum Liouville equation. Finite net current
is found in the non-equilibrium steady state. The direction of the current
varies with parameters, in contrast with the classical thermal ratchets.Comment: 7 pages, Latex, 4 ps figures; No change in the text by this
replacement. only the figures are replaced with higher quality ones (but
smaller size
Multi-band quantum ratchets
We investigate directed motion in non-adiabatically rocked ratchet systems
sustaining few bands below the barrier. Upon restricting the dynamics to the
lowest M bands, the total system-plus-bath Hamiltonian is mapped onto a
discrete tight-binding model containing all the information both on the intra-
and inter-well tunneling motion. A closed form for the current in the
incoherent tunneling regime is obtained. In effective single-band ratchets, no
current rectification occurs. We apply our theory to describe rectification
effects in vortex quantum ratchets devices. Current reversals upon variation of
the ac-field amplitude or frequency are predicted.Comment: Accepted for publication in Physical Review Letter
Quantum ratchet transport with minimal dispersion rate
We analyze the performance of quantum ratchets by considering the dynamics of
an initially localized wave packet loaded into a flashing periodic potential.
The directed center-of-mass motion can be initiated by the uniform modulation
of the potential height, provided that the modulation protocol breaks all
relevant time- and spatial reflection symmetries. A poor performance of quantum
ratchet transport is characterized by a slow net motion and a fast diffusive
spreading of the wave packet, while the desirable optimal performance is the
contrary. By invoking a quantum analog of the classical P\'eclet number, namely
the quotient of the group velocity and the dispersion of the propagating wave
packet, we calibrate the transport properties of flashing quantum ratchets and
discuss the mechanisms that yield low-dispersive directed transport.Comment: 6 pages; 3 figures; 1 tabl
Quantum Ratchets at High Temperatures
Using the continued-fraction method we solve the Caldeira-Leggett master
equation in the phase-space (Wigner) representation to study Quantum ratchets.
Broken spatial symmetry, irreversibility and periodic forcing allows for a net
current in these systems. We calculate this current as a function of the force
under adiabatic conditions. Starting from the classical limit we make the
system quantal. In the quantum regime tunnel events and over-barrier wave
reflection phenomena modify the classical result. Finally, using the
phase-space formalism we give some insights about the decoherence in these
systems.Comment: submitted to Physia E (proceedings of conference "Frontiers of
Quantum and Mesoscopic Thermodynamics", Prague 26-29 July 2004
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