523 research outputs found
Vortex energy and vortex bending for a rotating Bose-Einstein condensate
For a Bose-Einstein condensate placed in a rotating trap, we give a
simplified expression of the Gross-Pitaevskii energy in the Thomas Fermi
regime, which only depends on the number and shape of the vortex lines.
Then we check numerically that when there is one vortex line, our simplified
expression leads to solutions with a bent vortex for a range of rotationnal
velocities and trap parameters which are consistent with the experiments.Comment: 7 pages, 2 figures. submitte
Low-Lying Excitations from the Yrast Line of Weakly Interacting Trapped Bosons
Through an extensive numerical study, we find that the low-lying,
quasi-degenerate eigenenergies of weakly-interacting trapped N bosons with
total angular momentum L are given in case of small L/N and sufficiently small
L by E = L hbar omega + g[N(N-L/2-1)+1.59 n(n-1)/2], where omega is the
frequency of the trapping potential and g is the strength of the repulsive
contact interaction; the last term arises from the pairwise repulsive
interaction among n octupole excitations and describes the lowest-lying
excitation spectra from the Yrast line. In this case, the quadrupole modes do
not interact with themselves and, together with the octupole modes, exhaust the
low-lying spectra which are separated from others by N-linear energy gaps.Comment: 5 pages, RevTeX, 2 figures, revised version, submitted to PR
Pathogenic Potential to Humans of Bovine Escherichia coli O26, Scotland
Escherichia coli O26 and O157 have similar overall prevalences in cattle in Scotland, but in humans, Shiga toxinâproducing E. coli O26 infections are fewer and clinically less severe than E. coli O157 infections. To investigate this discrepancy, we genotyped E. coli O26 isolates from cattle and humans in Scotland and continental Europe. The genetic background of some strains from Scotland was closely related to that of strains causing severe infections in Europe. Nonmetric multidimensional scaling found an association between hemolytic uremic syndrome (HUS) and multilocus sequence type 21 strains and confirmed the role of stx<sub>2</sub> in severe human disease. Although the prevalences of E. coli O26 and O157 on cattle farms in Scotland are equivalent, prevalence of more virulent strains is low, reducing human infection risk. However, new data on E. coli O26âassociated HUS in humans highlight the need for surveillance of non-O157 enterohemorrhagic E. coli and for understanding stx<sub>2</sub> phage acquisition
Anomalous rotational properties of Bose-Einstein condensates in asymmetric traps
We study the rotational properties of a Bose-Einstein condensate confined in
a rotating harmonic trap for different trap anisotropies. Using simple
arguments, we derive expressions for the velocity field of the quantum fluid
for condensates with or without vortices. While the condensed gas describes
open spiraling trajectories, on the frame of reference of the rotating trap the
motion of the fluid is against the trap rotation. We also find explicit
formulae for the angular momentum and a linear and Thomas-Fermi solutions for
the state without vortices. In these two limits we also find an analytic
relation between the shape of the cloud and the rotation speed. The predictions
are supported by numerical simulations of the mean field Gross-Pitaevskii
model.Comment: 4 RevTeX pages, 2 EPS figures; typos fixed, reference adde
Split vortices in optically coupled Bose-Einstein condensates
We study a rotating two-component Bose-Einstein condensate in which an
optically induced Josephson coupling allows for population transfer between the
two species. In a regime where separation of species is favored, the ground
state of the rotating system displays domain walls with velocity fields normal
to them. Such a configuration looks like a vortex split into two halves, with
atoms circulating around the vortex and changing their internal state in a
continuous way.Comment: 4 EPS pictures, 4 pages; Some errata have been corrected and thep
resentation has been slightly revise
Superfluid toroidal currents in atomic condensates
The dynamics of toroidal condensates in the presence of condensate flow and
dipole perturbation have been investigated. The Bogoliubov spectrum of
condensate is calculated for an oblate torus using a discrete-variable
representation and a spectral method to high accuracy. The transition from
spheroidal to toroidal geometry of the trap displaces the energy levels into
narrow bands. The lowest-order acoustic modes are quantized with the dispersion
relation with . A condensate
with toroidal current splits the co-rotating and
counter-rotating pair by the amount: . Radial dipole excitations are the lowest energy dissipation modes.
For highly occupied condensates the nonlinearity creates an asymmetric mix of
dipole circulation and nonlinear shifts in the spectrum of excitations so that
the center of mass circulates around the axis of symmetry of the trap. We
outline an experimental method to study these excitations.Comment: 8 pages, 8 figure
Generation and evolution of vortex-antivortex pairs in Bose-Einstein condensates
We propose a method for generating and controlling a spatially separated
vortex--antivortex pair in a Bose-Einstein condensate trapped in a toroidal
potential. Our simulations of the time dependent Gross-Pitaevskii equation show
that in toroidal condensates vortex dynamics are different from the dynamics in
the homogeneous case. Our numerical results agree well with analytical
calculations using the image method. Our proposal offers an effective example
of coherent generation and control of vortex dynamics in atomic condensates.Comment: 4 pages, 2 figure
Vortex nucleation in Bose-Einstein condensates in time-dependent traps
Vortex nucleation in a Bose-Einstein condensate subject to a stirring
potential is studied numerically using the zero-temperature, two-dimensional
Gross-Pitaevskii equation. It is found that this theory is able to describe the
creation of vortices, but not the crystallization of a vortex lattice. In the
case of a rotating, slightly anisotropic harmonic potential, the numerical
results reproduce experimental findings, thereby showing that finite
temperatures are not necessary for vortex excitation below the quadrupole
frequency. In the case of a condensate subject to stirring by a narrow rotating
potential, the process of vortex excitation is described by a classical model
that treats the multitude of vortices created by the stirrer as a continuously
distributed vorticity at the center of the cloud, but retains a potential flow
pattern at large distances from the center.Comment: 22 pages, 7 figures. Changes after referee report: one new figure,
new refs. No conclusions altere
Generating vortex rings in Bose-Einstein condensates in the line-source approximation
We present a numerical method for generating vortex rings in Bose-Einstein
condensates confined in axially symmetric traps. The vortex ring is generated
using the line-source approximation for the vorticity, i.e., the rotational of
the superfluid velocity field is different from zero only on a circumference of
given radius located on a plane perpendicular to the symmetry axis and coaxial
with it. The particle density is obtained by solving a modified
Gross-Pitaevskii equation that incorporates the effect of the velocity field.
We discuss the appearance of density profiles, the vortex core structure and
the vortex nucleation energy, i.e., the energy difference between vortical and
ground-state configurations. This is used to present a qualitative description
of the vortex dynamics.Comment: Accepted for publication in Phys. Rev.
Vortex phase diagram in trapped Bose-Einstein condensation
The vortex phase diagram in the external rotation frequency versus
temperature is calculated for dilute Bose-Einstein condensed gases. It is
determined within the Bogoliubov-Popov theory for a finite temperature where
the condensate and non-condensate fractions are treated in an equal footing.
The temperature dependences of various thermodynamic instability lines for the
vortex nucleation are computed to construct the phase diagram. Experiments are
proposed to resolve a recent controversy on the vortex creation problem
associated with the quantized vortex observation in Rb atom gases.Comment: 11 pages, 8 figure
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