751 research outputs found
Internal Vortex Structure of a Trapped Spinor Bose-Einstein Condensate
The internal vortex structure of a trapped spin-1 Bose-Einstein condensate is
investigated. It is shown that it has a variety of configurations depending on,
in particular, the ratio of the relevant scattering lengths and the total
magnetization.Comment: replacement; minor grammatical corrections but with additional
figure
Topological Excitations in Spinor Bose-Einstein Condensates
We investigate the properties of skyrmion in the ferromagnetic state of
spin-1 Bose-Einstein condensates by means of the mean-field theory and show
that the size of skyrmion is fixed to the order of the healing length. It is
shown that the interaction between two skyrmions with oppositely rotating spin
textures is attractive when their separation is large, following a unique
power-law behavior with a power of -7/2.Comment: 4 pages, 5 figure
Exotic magnetic orders for high spin ultracold fermions
We study Hubbard models for ultracold bosonic or fermionic atoms loaded into
an optical lattice. The atoms carry a high spin , and interact on site
via strong repulsive Van der Waals forces. Making convenient rearrangements of
the interaction terms, and exploiting their symmetry properties, we derive low
energy effective models with nearest-neighbor interactions, and their
properties. We apply our method to , and 5/2 fermions on two-dimensional
square lattice at quarter, and 1/6 fillings, respectively, and investigate
mean-field equations for repulsive couplings. We find for fermions that
the plaquette state appearing in the highly symmetric SU(4) case does not
require fine tuning, and is stable in an extended region of the phase diagram.
This phase competes with an SU(2) flux state, that is always suppressed for
repulsive interactions in absence of external magnetic field. The SU(2) flux
state has, however, lower energy than the plaquette phase, and stabilizes in
the presence of weak applied magnetic field. For fermions a similar
SU(2) plaquette phase is found to be the ground state without external magnetic
field.Comment: final version, 6 pages, 4 figures, epl forma
Temperature-dependent relaxation times in a trapped Bose-condensed gas
Explicit expressions for all the transport coefficients have recently been
found for a trapped Bose condensed gas at finite temperatures. These transport
coefficients are used to define the characteristic relaxation times, which
determine the crossover between the mean-field collisionless and the two-fluid
hydrodynamic regime. These relaxation times are evaluated as a function of the
position in the trap potential. We show that all the relaxation times are
dominated by the collisions between the condensate and the non-condensate
atoms, and are much smaller than the standard classical collision time used in
most of the current literature. The 1998 MIT study of the collective modes at
finite temperature is shown to have been well within the two-fluid hydrodynamic
regime.Comment: 4 pages, 3 figures, to be published in Phys. Rev.
Tunable Cavity Optomechanics with Ultracold Atoms
We present an atom-chip-based realization of quantum cavity optomechanics
with cold atoms localized within a Fabry-Perot cavity. Effective sub-wavelength
positioning of the atomic ensemble allows for tuning the linear and quadratic
optomechanical coupling parameters, varying the sensitivity to the displacement
and strain of a compressible gaseous cantilever. We observe effects of such
tuning on cavity optical nonlinearity and optomechanical frequency shifts,
providing their first characterization in the quadratic-coupling regime.Comment: 4 pages, 5 figure
Mean field ground state of a spin-1 condensate in a magnetic field
We revisit the topic of the mean field ground state of a spin-1 atomic
condensate inside a uniform magnetic field () under the constraints that
both the total number of atoms () and the magnetization () are
conserved. In the presence of an internal state (spin component) independent
trap, we also investigate the dependence of the so-called single spatial mode
approximation (SMA) on the magnitude of the magnetic field and . Our
result indicate that the quadratic Zeeman effect is an important factor in
balancing the mean field energy from elastic atom-atom collisions that are
known to conserve both and .Comment: 13 pages, 9 figures, to be published in New J. Phys.
(http://www.njp.org/
Skyrmions in a ferromagnetic Bose-Einstein condensate
The recently realized multicomponent Bose-Einstein condensates provide
opportunities to explore the rich physics brought about by the spin degrees of
freedom. For instance, we can study spin waves and phase separation,
macroscopic quantum tunneling, Rabi oscillations, the coupling between spin
gradients and superfluid flow, squeezed spin states, vortices and other
topological excitations. Theoretically, there have been already some studies of
the ground-state properties of these systems and their line-like vortex
excitations. In analogy with nuclear physics or the quantum Hall effect, we
explore here the possibility of observing point-like topological excitations or
skyrmions. These are nontrivial spin textures that in principle can exist in a
spinor Bose-Einstein condensate. In particular, we investigate the stability of
skyrmions in a fictitious spin-1/2 condensate of Rb87 atoms. We find that
skyrmions can exist in this case only as a metastable state, but with a
lifetime of the order of, or even longer than, the typical lifetime of the
condensate itself. In addition to determining the size and the lifetime of the
skyrmion, we also present its spin texture and finally briefly consider its
dynamical properties.Comment: 4 pages (REVtex), 3 PDF figures. See also cond-mat/000237
Bose-Einstein condensation in a circular waveguide
We have produced Bose-Einstein condensates in a ring-shaped magnetic
waveguide. The few-millimeter diameter non-zero bias ring is formed from a
time-averaged quadrupole ring. Condensates which propagate around the ring make
several revolutions within the time it takes for them to expand to fill the
ring. The ring shape is ideally suited for studies of vorticity in a
multiply-connected geometry and is promising as a rotation sensor.Comment: 4 pages, 4 figure
Spinor Bose Condensates in Optical Traps
In an optical trap, the ground state of spin-1 Bosons such as Na,
K, and Rb can be either a ferromagnetic or a "polar" state,
depending on the scattering lengths in different angular momentum channel. The
collective modes of these states have very different spin character and spatial
distributions. While ordinary vortices are stable in the polar state, only
those with unit circulation are stable in the ferromagnetic state. The
ferromagnetic state also has coreless (or Skyrmion) vortices like those of
superfluid He-A. Current estimates of scattering lengths suggest that the
ground states of Na and Rb condensate are a polar state and a
ferromagnetic state respectively.Comment: 11 pages, no figures. email : [email protected]
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