1,089 research outputs found
Dynamical instability and dispersion management of an attractive condensate in an optical lattice
We investigate the stability of an attractive Bose-Einstein condensate in a
moving 1D optical lattice in the presence of transverse confinement. By means
of a Bogoliubov linear stability analysis we find that the system is
dynamically unstable for low quasimomenta and becomes stable near the band
edge, in a specular fashion with respect to the repulsive case. For low
interactions the instability occurs via long wavelength excitations that are
not sufficient for spoiling the condensate coherence, producing instead an
oscillating density pattern both in real and momentum space. This behaviour is
illustrated by simulations for the expansion of the condensate in a moving
lattice.Comment: 5 pages, 4 figure
Correlated bosons in a one-dimensional optical lattice: Effects of the trapping potential and of quasiperiodic disorder
We investigate the effect of the trapping potential on the quantum phases of
strongly correlated ultracold bosons in one-dimensional periodic and
quasiperiodic optical lattices. By means of a decoupling meanfield approach, we
characterize the ground state of the system and its behavior under variation of
the harmonic trapping, as a function of the total number of atoms. For a small
atom number the system shows an incompressible Mott-insulating phase, as the
size of the cloud remains unaffected when the trapping potential is varied.
When the quasiperiodic potential is added the system develops a
metastable-disordered phase which is neither compressible nor Mott insulating.
This state is characteristic of quasidisorder in the presence of a strong
trapping potential.Comment: Accepted for publication in PR
Enhancement of the scissors mode of an expanding Bose-Einstein condensate
We study the time-evolution of the scissors mode of a Bose-Einstein
condensate during the ballistic expansion after release from the magnetic trap.
We show that despite the nontrivial character of the superfluid expansion, the
sinusoidal behavior of the scissor oscillations is recovered after an
asymptotic expansion, with an enhancement of the final amplitude. We
investigate this phenomenon with a condensate held in an elongated
magnetostatic potential, whose particular shape allows for the excitation of
the scissors mode.Comment: RevTeX, 5 figure
Observation of subdiffusion of a disordered interacting system
We study the transport dynamics of matter-waves in the presence of disorder
and nonlinearity. An atomic Bose-Einstein condensate that is localized in a
quasiperiodic lattice in the absence of atom-atom interaction shows instead a
slow expansion with a subdiffusive behavior when a controlled repulsive
interaction is added. The measured features of the subdiffusion are compared to
numerical simulations and a heuristic model. The observations confirm the
nature of subdiffusion as interaction-assisted hopping between localized states
and highlight a role of the spatial correlation of the disorder.Comment: 8 pages, to be published on Physical Review Letter
Subdiffusion of nonlinear waves in quasiperiodic potentials
We study the spatio-temporal evolution of wave packets in one-dimensional
quasiperiodic lattices which localize linear waves. Nonlinearity (related to
two-body interactions) has destructive effect on localization, as recently
observed for interacting atomic condensates [Phys. Rev. Lett. 106, 230403
(2011)]. We extend the analysis of the characteristics of the subdiffusive
dynamics to large temporal and spatial scales. Our results for the second
moment consistently reveal an asymptotic and
intermediate laws. At variance to purely random systems
[Europhys. Lett. 91, 30001 (2010)] the fractal gap structure of the linear wave
spectrum strongly favors intermediate self-trapping events. Our findings give a
new dimension to the theory of wave packet spreading in localizing
environments
Effects of interaction on the diffusion of atomic matter waves in one-dimensional quasi-periodic potentials
We study the behaviour of an ultracold atomic gas of bosons in a bichromatic
lattice, where the weaker lattice is used as a source of disorder. We
numerically solve a discretized mean-field equation, which generalizes the
one-dimensional Aubry-Andr\`e model for particles in a quasi-periodic potential
by including the interaction between atoms. We compare the results for
commensurate and incommensurate lattices. We investigate the role of the
initial shape of the wavepacket as well as the interplay between two competing
effects of the interaction, namely self-trapping and delocalization. Our
calculations show that, if the condensate initially occupies a single lattice
site, the dynamics of the interacting gas is dominated by self-trapping in a
wide range of parameters, even for weak interaction. Conversely, if the
diffusion starts from a Gaussian wavepacket, self-trapping is significantly
suppressed and the destruction of localization by interaction is more easily
observable
Single vortex states in a confined Bose-Einstein condensate
It has been demonstrated experimentally that non-axially symmetric vortices
precess around the centre of a Bose-Einstein condensate. Two types of single
vortex states have been observed, usually referred to as the S-vortex and the
U-vortex. We study theoretically the single vortex excitations in spherical and
elongated condensates as a function of the interaction strength. We solve
numerically the Gross-Pitaevskii equation and calculate the angular momentum as
a function of precession frequency. The existence of two types of vortices
means that we have two different precession frequencies for each angular
momentum value. As the interaction strength increases the vortex lines bend and
the precession frequencies shift to lower values. We establish that for given
angular momentum the S-vortex has higher energy than the U-vortex in a rotating
elongated condensate. We show that the S-vortex is related to the solitonic
vortex which is a nonlinear excitation in the nonrotating system. For small
interaction strengths the S-vortex is related to the dark soliton. In the
dilute limit a lowest Landau level calculation provides an analytic description
of these vortex modes in terms of the harmonic oscillator states
Localized Asymmetric Atomic Matter Waves in Two-Component Bose-Einstein Condensates Coupled with Two Photon Microwave Field
We investigate localized atomic matter waves in two-component Bose-Einstein
condensates coupled by the two photon microwave field. Interestingly, the
oscillations of localized atomic matter waves will gradually decay and finally
become non-oscillating behavior even if existing coupling field. In particular,
atom numbers occupied in two different hyperfine spin states will appear
asymmetric occupations after some time evolution.Comment: 4 pages, 4 figure
Engineering fast and stable splitting of matter waves
When attempting to split coherent cold atom clouds or a Bose-Einstein
condensate (BEC) by bifurcation of the trap into a double well, slow adiabatic
following is unstable with respect to any slight asymmetry, and the wave
"collapses" to the lower well, whereas a generic fast chopping splits the wave
but it also excites it. Shortcuts to adiabaticity engineered to speed up the
adiabatic process through non-adiabatic transients, provide instead quiet and
robust fast splitting. The non-linearity of the BEC makes the proposed shortcut
even more stable
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