65 research outputs found
Three-dimensional vortex dynamics in Bose-Einstein condensates
We simulate in the mean-field limit the effects of rotationally stirring a
three-dimensional trapped Bose-Einstein condensate with a Gaussian laser beam.
A single vortex cycling regime is found for a range of trap geometries, and is
well described as coherent cycling between the ground and the first excited
vortex states. The critical angular speed of stirring for vortex formation is
quantitatively predicted by a simple model. We report preliminary results for
the collisions of vortex lines, in which sections may be exchanged.Comment: 4 pages, 4 figures, REVTeX 3.1; Submitted to Physical Review A (6
March 2000
Fourier transforming a trapped Bose-Einstein condensate by waiting a quarter of the trap period: simulation and applications
We investigate the property of isotropic harmonic traps to Fourier transform a weakly interacting BoseâEinstein condensate (BEC) every quarter of a trap period. We solve the GrossâPitaevskii equation numerically to investigate the time evolution of interacting BECs in the context of the Fourier transform, and we suggest potential applications
Coherent Dynamics of Vortex Formation in Trapped Bose-Einstein Condensates
Simulations of a rotationally stirred condensate show that a regime of simple
behaviour occurs in which a single vortex cycles in and out of the condensate.
We present a simple quantitative model of this behaviour, which accurately
describes the full vortex dynamics, including a critical angular speed of
stirring for vortex formation. A method for experimentally preparing a
condensate in a central vortex state is suggested.Comment: 4 pages, 4 figures, REVTeX 3.1; Submitted to Physical Review Letters
(5 February 1999); See http://www.physics.otago.ac.nz/research/bec/vortex for
MPEG movies and further information; Accepted for Physical Review Letters (24
June 1999); Changes: updated Figs 1 and 2 (new style), minor typos fixed,
more discussion at en
Pade approximations of solitary wave solutions of the Gross-Pitaevskii equation
Pade approximants are used to find approximate vortex solutions of any
winding number in the context of Gross-Pitaevskii equation for a uniform
condensate and condensates with axisymmetric trapping potentials. Rational
function and generalised rational function approximations of axisymmetric
solitary waves of the Gross-Pitaevskii equation are obtained in two and three
dimensions. These approximations are used to establish a new mechanism of
vortex nucleation as a result of solitary wave interactions.Comment: In press by Journal of Physics: Mathematics and Genera
Observation of Superfluid Flow in a Bose-Einstein Condensed Gas
We have studied the hydrodynamic flow in a Bose-Einstein condensate stirred
by a macroscopic object, a blue detuned laser beam, using nondestructive {\em
in situ} phase contrast imaging. A critical velocity for the onset of a
pressure gradient has been observed, and shown to be density dependent. The
technique has been compared to a calorimetric method used previously to measure
the heating induced by the motion of the laser beam.Comment: 4 pages, 5 figure
Dynamic instability of a rotating Bose-Einstein condensate
We consider a Bose-Einstein condensate subject to a rotating harmonic
potential, in connection with recent experiments leading to the formation of
vortices. We use the classical hydrodynamic approximation to the non-linear
Schr\"odinger equation to determine almost analytically the evolution of the
condensate. We predict that this evolution can exhibit dynamical instabilities,
for the stirring procedure previously demonstrated at ENS and for a new
stirring procedure that we put forward. These instabilities take place within
the range of stirring frequency and amplitude for which vortices are produced
experimentally. They provide therefore an initiating mechanism for vortex
nucleation.Comment: 4 pages, 3 figures, last version including comparison with
experiment
Vortices in a Bose-Einstein Condensate
We have created vortices in two-component Bose-Einstein condensates. The
vortex state was created through a coherent process involving the spatial and
temporal control of interconversion between the two components. Using an
interference technique, we map the phase of the vortex state to confirm that it
possesses angular momentum. We can create vortices in either of the two
components and have observed differences in the dynamics and stability.Comment: 4 pages with 3 figure
Scattering of light and atoms in a Fermi-Dirac gas with BCS pairing
We theoretically study the optical properties of a Fermi-Dirac gas in the
presence of a superfluid state. We calculate the leading quantum-statistical
corrections to the standard column density result of the electric
susceptibility. We also consider the Bragg diffraction of atoms by means of
light-stimulated transitions of photons between two intersecting laser beams.
Bardeen-Cooper-Schrieffer pairing between atoms in different internal levels
magnifies incoherent scattering processes. The absorption linewidth of a
Fermi-Dirac gas is broadened and shifted. Bardeen-Cooper-Schrieffer pairing
introduces a collisional local-field shift that may dramatically dominate the
Lorentz-Lorenz shift. For the case of the Bragg spectroscopy the static
structure function may be significantly increased due to superfluidity in the
nearforward scattering.Comment: 13 pages, 6 figures; to appear in PR
Vortex Nucleation in a Stirred Bose-Einstein Condensate
We studied the nucleation of vortices in a Bose-Einstein condensate stirred
by a laser beam. We observed the vortex cores using time-of-flight absorption
imaging. By varying the size of the stirrer, we observed either discrete
resonances or a broad response as a function of the frequency of the stirrer's
motion. Stirring beams small compared to the condensate size generated vortices
below the critical rotation frequency for the nucleation of surface modes,
suggesting a local mechanism of generation. In addition, we observed the
centrifugal distortion of the condensate due to the rotating vortex lattice and
found evidence for bent vortices
Stable and unstable vortices in multicomponent Bose-Einstein condensates
We study the stability and dynamics of vortices in two-species condensates as
prepared in the recent JILA experiment (M. R. Andrews {\em et al.}, Phys. Rev.
Lett. 83 (1999) 2498). We find that of the two available configurations, in
which one specie has vorticity and the other one has , only one is
linearly stable, which agrees with the experimental results. However, it is
found that in the unstable case the vortex is not destroyed by the instability,
but may be transfered from one specie to the other or display complex
spatiotemporal dynamics.Comment: 4 EPS figures, now features a three-dimensional stud
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