111 research outputs found
Three-dimensional vortex structure of a fast rotating Bose-Einstein condensate with harmonic-plus-quartic confinement
We address the challenging proposition of using real experimental parameters
in a three-dimensional numerical simulation of fast rotating Bose-Einstein
condensates. We simulate recent experiments [V. Bretin, S. Stock, Y. Seurin and
J. Dalibard, Phys. Rev. Lett. 92, 050403 (2004); S. Stock, V. Bretin, S. Stock,
F. Chevy and J. Dalibard, Europhys. Lett. 65, 594 (2004)] using an anharmonic
(quadratic-plus-quartic) confining potential to reach rotation frequencies
() above the trap frequency (). Our numerical results are
obtained by propagating the 3D Gross-Pitaevskii equation in imaginary time. For
, we obtain an equilibrium vortex lattice similar (as
size and number of vortices) to experimental observations. For
we observe the evolution of the vortex lattice into an
array of vortices with a central hole. Since this evolution was not visible in
experiments, we investigate the 3D structure of vortex configurations and
3D-effects on vortex contrast. Numerical data are also compared to recent
theory [D. E. Sheehy and L. Radzihovsky, Phys. Rev. A 70, 063620 (2004)]
describing vortex lattice inhomogeneities and a remarkably good agreement is
found.Comment: to appear in Phys Rev A 71 (2005
Spontaneous Dissociation of 85Rb Feshbach Molecules
The spontaneous dissociation of 85Rb dimers in the highest lying vibrational
level has been observed in the vicinity of the Feshbach resonance which was
used to produce them. The molecular lifetime shows a strong dependence on
magnetic field, varying by three orders of magnitude between 155.5 G and 162.2
G. Our measurements are in good agreement with theoretical predictions in which
molecular dissociation is driven by inelastic spin relaxation. Molecule
lifetimes of tens of milliseconds can be achieved close to resonance.Comment: 4 pages, 3 figure
Spin textures in rotating two-component Bose-Einstein condensates
We investigate two kinds of coreless vortices with axisymmetric and
nonaxisymmetric configurations in rotating two-component Bose-Einstein
condensates. Starting from the Gross-Pitaevskii energy functional in a rotating
frame, we derive a nonlinear sigma model generalized to the two-component
condensates. In terms of a pseudospin representation, an axisymmetric vortex
and a nonaxisymmetric one correspond to spin textures referred to as a
"skyrmion" and a "meron-pair", respectively. A variational method is used to
investigate the dependence of the sizes of the stable spin textures on system
parameters, and the optimized variational function is found to reproduce well
the numerical solution. In the SU(2) symmetric case, the optimal skyrmion and
meron-pair are degenerate and transform to each other by a rotation of the
pseudospin. An external rf-field that couples coherently the hyperfine states
of two components breaks the degeneracy in favor of the meron-pair texture due
to an effective transverse pseudomagnetic field. The difference between the
intracomponent and intercomponent interactions yields a longitudinal
pseudomagnetic field and a ferromagnetic or an antiferromagnetic pseudospin
interaction, leading to a meron-pair texture with an anisotropic distribution
of vorticity.Comment: 14 pages, 15 figure
Ultracold Molecule Production Via a Resonant Oscillating Magnetic Field
A novel atom-molecule conversion technique has been investigated. Ultracold 85Rb atoms sitting in a dc magnetic field near the 155 G Feshbach resonance are associated by applying a small sinusoidal oscillation to the magnetic field. There is resonant atom to molecule conversion when the modulation frequency closely matches the molecular binding energy. We observe that the atom to molecule conversion efficiency depends strongly on the frequency, amplitude, and duration of the applied modulation and on the phase space density of the sample. This technique offers high conversion efficiencies without the necessity of crossing or closely approaching the Feshbach resonance and allows precise spectroscopic measurements. Efficiencies of 55% have been observed for pure Bose-Einstein condensates
Theory of vortex-lattice melting in a one-dimensional optical lattice
We investigate quantum and temperature fluctuations of a vortex lattice in a
one-dimensional optical lattice. We discuss in particular the Bloch bands of
the Tkachenko modes and calculate the correlation function of the vortex
positions along the direction of the optical lattice. Because of the small
number of particles in the pancake Bose-Einstein condensates at every site of
the optical lattice, finite-size effects become very important. Moreover, the
fluctuations in the vortex positions are inhomogeneous due to the inhomogeneous
density. As a result, the melting of the lattice occurs from the outside
inwards. However, tunneling between neighboring pancakes substantially reduces
the inhomogeneity as well as the size of the fluctuations. On the other hand,
nonzero temperatures increase the size of the fluctuations dramatically. We
calculate the crossover temperature from quantum melting to classical melting.
We also investigate melting in the presence of a quartic radial potential,
where a liquid can form in the center instead of at the outer edge of the
pancake Bose-Einstein condensates.Comment: 17 pages, 17 figures, submitted to Phys. Rev. A, references update
Spontaneous squeezing of a vortex in an optical lattice
We study the equilibrium states of a vortex in a Bose-Einstein condensate in
a one-dimensional optical lattice. We find that quantum effects can be
important and that it is even possible for the vortex to be strongly squeezed,
which reflects itself in a different quantum mechanical uncertainty of the
vortex position in two orthogonal directions. The latter is observable by
measuring the atomic density after an expansion of the Bose-Einstein condensate
in the lattice.Comment: 8 pages, 3 figures, more details added, some new citation
Hysteresis effects in rotating Bose-Einstein condensates
We study the formation of vortices in a dilute Bose-Einstein condensate
confined in a rotating anisotropic trap. We find that the number of vortices
and angular momentum attained by the condensate depends upon the rotation
history of the trap and on the number of vortices present in the condensate
initially. A simplified model based on hydrodynamic equations is developed, and
used to explain this effect in terms of a shift in the resonance frequency of
the quadrupole mode of the condensate in the presence of a vortex lattice.
Differences between the spin-up and spin-down response of the condensate are
found, demonstrating hysteresis phenomena in this system.Comment: 16 pages, 7 figures; revised after referees' report
Production Efficiency of Ultracold Feshbach Molecules in Bosonic and Fermionic Systems
We investigate the production efficiency of ultracold molecules in bosonic
Rb and fermionic K when the magnetic field is swept across a
Feshbach resonance. For adiabatic sweeps of the magnetic field, the conversion
efficiency of each species is solely determined by the phase space density of
the atomic cloud, in contrast to a number of theoretical predictions. Our novel
model for the adiabatic pairing process, developed from general physical
principles, accurately predicts the conversion efficiency for {\it both}
ultracold gases of bosons and of fermions. In the non-adiabatic regime our
measurements of the Rb molecule conversion efficiency follow a Landau
Zener model, with a conversion efficiency that is characterized by the density
divided by the time derivative of the magnetic field.Comment: 5 pages, 3 figure
Bragg Spectroscopy of Vortex Lattices in Bose-Einstein condensates
We have measured the velocity field of a vortex lattice within a sodium
Bose-Einstein condensate using Bragg scattering. The phase gradient of the
macroscopic wavefunction was mapped into the spatial structure of the
diffracted atom cloud, allowing for single shot measurement of the rotation
parameters. A combination of spectral and spatial information yields a complete
description of the superfluid flow, coarse-grained over the lattice structure,
including direct and independent measurements of the rate and sense of
rotation. Signatures of the microscopic quantum rotation have also been
observed.Comment: 5 pages, 5 Figures, A movie built from the CM data is available in
our Webpage: http://www.physics.gatech.edu/chandra/index.htm; added Fig.5
presents new data, showing signatures of the microscopic vortex structure in
the diffracted clou
The Experimental Observation of a Superfluid Gyroscope in a dilute Bose Condensed Gas
We have observed a superfluid gyroscope effect in a dilute gas Bose-Einstein
condensate. A condensate with a vortex possesses a single quantum of angular
momentum and this causes the plane of oscillation of the scissors mode to
precess around the vortex line. We have measured the precession rate of the
scissors oscillation. From this we deduced the angular momentum associated with
the vortex line and found a value close to per particle, as predicted
for a superfluid.Comment: 4 pages 5 fig
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