1,758 research outputs found
Weakly Interacting Bose-Einstein Condensates Under Rotation: Mean-field versus Exact Solutions
We consider a weakly-interacting, harmonically-trapped Bose-Einstein
condensed gas under rotation and investigate the connection between the
energies obtained from mean-field calculations and from exact diagonalizations
in a subspace of degenerate states. From the latter we derive an approximation
scheme valid in the thermodynamic limit of many particles. Mean-field results
are shown to emerge as the correct leading-order approximation to exact
calculations in the same subspace.Comment: 4 pages, RevTex, submitted to PR
Bose-Einstein condensates in strong electric fields -- effective gauge potentials and rotating states
Magnetically-trapped atoms in Bose-Einstein condensates are spin polarized.
Since the magnetic field is inhomogeneous, the atoms aquire Berry phases of the
Aharonov-Bohm type during adiabatic motion. In the presence of an eletric field
there is an additional Aharonov-Casher effect. Taking into account the
limitations on the strength of the electric fields due to the polarizability of
the atoms, we investigate the extent to which these effects can be used to
induce rotation in a Bose-Einstein condensate.Comment: 5 pages, 2 ps figures, RevTe
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
Rotational Dynamics of Vortices in Confined Bose-Einstein Condensates
We derive the frequency of precession and conditions for stability for a
quantized vortex in a single-component and a two-component Bose-Einstein
condensate. The frequency of precession is proportional to the gradient of the
free energy with respect to displacement of the vortex core. In a two-component
system, it is possible to achieve a local minimum in the free energy at the
center of the trap. The presence of such a minimum implies the existence of a
region of energetic stability where the vortex cannot escape and where one may
be able to generate a persistent current.Comment: 6 Pages, 6 Figure
Rotating Bose gas with hard-core repulsion in a quasi-2D harmonic trap: vortices in BEC
We consider a gas of N(=6, 10, 15) Bose particles with hard-core repulsion,
contained in a quasi-2D harmonic trap and subjected to an overall angular
velocity about the z-axis. Exact diagonalization of the
many-body Hamiltonian matrix in given subspaces of the total (quantized)
angular momentum L, with (e.g. for L=N=15, n =240782)
was carried out using Davidson's algorithm. The many-body variational ground
state wavefunction, as also the corresponding energy and the reduced
one-particle density-matrix were calculated. With the usual identification of
as the Lagrange multiplier associated with L for a rotating
system, the phase diagram (or the stability line) was determined
that gave a number of critical angular velocities at which the ground state angular momentum and the associated
condensate fraction undergo abrupt jumps.
A number of (total) angular momentum states were found to be stable at
successively higher critical angular velocities $\Omega_{{\bf c}i}, \
i=1,2,3,...L_{z}>N\Omega_{{\bf c}i}_{z}(\sim 4)$ orders of magnitude in the moderately to the weakly
interacting regime.Comment: Revtex, 11 pages, 1 table as ps file, 4 figures as ps file
Vortex stabilization in a small rotating asymmetric Bose-Einstein condensate
We use a variational method to investigate the ground-state phase diagram of
a small, asymmetric Bose-Einstein condensate with respect to the dimensionless
interparticle interaction strength and the applied external rotation
speed . For a given , the transition lines between no-vortex
and vortex states are shifted toward higher relative to those for the
symmetric case. We also find a re-entrant behavior, where the number of vortex
cores can decrease for large . In addition, stabilizing a vortex in a
rotating asymmetric trap requires a minimum interaction strength. For a given
asymmetry, the evolution of the variational parameters with increasing
shows two different types of transitions (sharp or continuous), depending on
the strength of the interaction. We also investigate transitions to states with
higher vorticity; the corresponding angular momentum increases continuously as
a function of
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
A single transcription factor is sufficient to induce and maintain secretory cell architecture
We hypothesized that basic helix–loop–helix (bHLH) MIST1 (BHLHA15) is a “scaling factor” that universally establishes secretory morphology in cells that perform regulated secretion. Here, we show that targeted deletion of MIST1 caused dismantling of the secretory apparatus of diverse exocrine cells. Parietal cells (PCs), whose function is to pump acid into the stomach, normally lack MIST1 and do not perform regulated secretion. Forced expression of MIST1 in PCs caused them to expand their apical cytoplasm, rearrange mitochondrial/lysosome trafficking, and generate large secretory granules. Mist1 induced a cohort of genes regulated by MIST1 in multiple organs but did not affect PC function. MIST1 bound CATATG/CAGCTG E boxes in the first intron of genes that regulate autophagosome/lysosomal degradation, mitochondrial trafficking, and amino acid metabolism. Similar alterations in cell architecture and gene expression were also caused by ectopically inducing MIST1 in vivo in hepatocytes. Thus, MIST1 is a scaling factor necessary and sufficient by itself to induce and maintain secretory cell architecture. Our results indicate that, whereas mature cell types in each organ may have unique developmental origins, cells performing similar physiological functions throughout the body share similar transcription factor-mediated architectural “blueprints.
Stability of rotating states in a weakly-interacting Bose-Einstein condensate
We investigate the lowest state of a rotating, weakly-interacting
Bose-Einstein condensate trapped in a harmonic confining potential that is
driven by an infinitesimally asymmetric perturbation. Although in an
axially-symmetric confining potential the gas has an axially-symmetric
single-particle density distribution, we show that in the presence of the small
asymmetric perturbation its lowest state is the one given by the mean-field
approximation, which is a broken-symmetric state. We also estimate the rate of
relaxation of angular momentum when the gas is no longer driven by the
asymmetric perturbation and identify two regimes of "slow" and "fast"
relaxation. States of certain symmetry are found to be more robust.Comment: 6 pages, RevTe
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