2,536 research outputs found
Accurate control of a Bose-Einstein condensate by managing the atomic interaction
We exploit the variation of the atomic interaction in order to move
ultra-cold atoms across an AC-driven periodic lattice. By breaking relevant
symmetries, a gathering of atoms is achieved. Accurate control of the gathered
atoms positions can be demonstrated via the control of the atomic localization
process. The localization process is analyzed with the help of the nonlinear
Floquet states where the Landau-Zener tunneling between states is observed and
controlled. Transport effects in the presence of disorder are discussed.Comment: 14 pages, 5 Figures, PACS numbers: 03.75.Lm, 05.60.-k, 63.20.P
Inhomogeneous soliton ratchets under two ac forces
We extend our previous work on soliton ratchet devices [L. Morales-Molina et
al., Eur. Phys. J. B 37, 79 (2004)] to consider the joint effect of two ac
forces including non-harmonic drivings, as proposed for particle ratchets by
Savele'v et al. [Europhys. Lett. 67}, 179 (2004); Phys. Rev. E {\bf 70} 066109
(2004)]. Current reversals due to the interplay between the phases, frequencies
and amplitudes of the harmonics are obtained. An analysis of the effect of the
damping coefficient on the dynamics is presented. We show that solitons give
rise to non-trivial differences in the phenomenology reported for particle
systems that arise from their extended character. A comparison with soliton
ratchets in homogeneous systems with biharmonic forces is also presented. This
ratchet device may be an ideal candidate for Josephson junction ratchets with
intrinsic large damping
Ratchet behavior in nonlinear Klein-Gordon systems with point-like inhomogeneities
We investigate the ratchet dynamics of nonlinear Klein-Gordon kinks in a
periodic, asymmetric lattice of point-like inhomogeneities. We explain the
underlying rectification mechanism within a collective coordinate framework,
which shows that such system behaves as a rocking ratchet for point particles.
Careful attention is given to the kink width dynamics and its role in the
transport. We also analyze the robustness of our kink rocking ratchet in the
presence of noise. We show that the noise activates unidirectional motion in a
parameter range where such motion is not observed in the noiseless case. This
is subsequently corroborated by the collective variable theory. An explanation
for this new phenomenom is given
Current and entanglement in a Bose-Hubbard lattice
We study the generation of entanglement for interacting cold atoms in an
optical lattice. The entanglement is generated by managing the interaction
between two distinct atomic species. It is found that the current of one of the
species can be used as a good indicator of entanglement generation. The
thermalization process between the species is also shown to be closely related
to the evolution of the current.Comment: 10 pages, 5 figure
Optimization of soliton ratchets in inhomogeneous sine-Gordon systems
Unidirectional motion of solitons can take place, although the applied force
has zero average in time, when the spatial symmetry is broken by introducing a
potential , which consists of periodically repeated cells with each cell
containing an asymmetric array of strongly localized inhomogeneities at
positions . A collective coordinate approach shows that the positions,
heights and widths of the inhomogeneities (in that order) are the crucial
parameters so as to obtain an optimal effective potential that yields
a maximal average soliton velocity. essentially exhibits two
features: double peaks consisting of a positive and a negative peak, and long
flat regions between the double peaks. Such a potential can be obtained by
choosing inhomogeneities with opposite signs (e.g., microresistors and
microshorts in the case of long Josephson junctions) that are positioned close
to each other, while the distance between each peak pair is rather large. These
results of the collective variables theory are confirmed by full simulations
for the inhomogeneous sine-Gordon system
Resonant ratcheting of a Bose-Einstein condensate
We study the rectification process of interacting quantum particles in a
periodic potential exposed to the action of an external ac driving. The
breaking of spatio-temporal symmetries leads to directed motion already in the
absence of interactions. A hallmark of quantum ratcheting is the appearance of
resonant enhancement of the current (Europhys. Lett. 79 (2007) 10007 and Phys.
Rev. A 75 (2007) 063424). Here we study the fate of these resonances within a
Gross-Pitaevskii equation which describes a mean field interaction between many
particles. We find, that the resonance is i) not destroyed by interactions, ii)
shifting its location with increasing interaction strength. We trace the
Floquet states of the linear equations into the nonlinear domain, and show that
the resonance gives rise to an instability and thus to the appearance of new
nonlinear Floquet states, whose transport properties differ strongly as
compared to the case of noninteracting particles
Periodically driven Quantum Ratchets: Symmetries and Resonances
We study the quantum version of a tilting and flashing Hamiltonian ratchets, consisting of a periodic potential and a time-periodic driving field. The system dynamics is governed by a Floquet evolution matrix bearing the symmetry of the corresponding Hamiltonian. The dc-current appears due to the desymmetrization of Floquet eigenstates, which become transporting when all the relevant symmetries are violated. Those eigenstates which mostly contribute to a directed transport reside in phase space regions corresponding to classical resonances. Quantum dynamics leads to the dependence of the average velocity on the initial phase of the ac-field. A resonant enhancement (or suppression) of the dc-current, due to avoided crossings between different Floquet states takes place upon tuning some control parameters. Our studies are predominantly aimed at experimental realizations of ac-driven quantum ratchets with cold atoms
Analytical approach to soliton ratchets in asymmetric potentials
We use soliton perturbation theory and collective coordinate ansatz to
investigate the mechanism of soliton ratchets in a driven and damped asymmetric
double sine-Gordon equation. We show that, at the second order of the
perturbation scheme, the soliton internal vibrations can couple {\it
effectively}, in presence of damping, to the motion of the center of mass,
giving rise to transport. An analytical expression for the mean velocity of the
soliton is derived. The results of our analysis confirm the internal mode
mechanism of soliton ratchets proposed in [Phys. Rev. E {\bf 65} 025602(R)
(2002)].Comment: 9 figures. Submitted to Phys. Rev.
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