261 research outputs found
Unidirectional flow of flat-top solitons
We numerically demonstrate the unidirectional flow of flat-top solitons when
interacting with two reflectionless potential wells with slightly different
depths. The system is described by a nonlinear Schr\"{o}dinger equation with
dual nonlinearity. The results show that for shallow potential wells, the
velocity window for unidirectional flow is larger than for deeper potential
wells. A wider flat-top solitons also have a narrow velocity window for
unidirectional flow than those for thinner flat-top solitons.Comment: 5 pages, 7 figure
Soliton response to transient trap variations
The response of bright and dark solitons to rapid variations in an expulsive
longitudinal trap is investigated. We concentrate on the effect of transient
changes in the trap frequency in the form of temporal delta kicks and the
hyperbolic cotangent functions. Exact expressions are obtained for the soliton
profiles. This is accomplished using the fact that a suitable linear
Schrodinger stationary state solution in time can be effectively combined with
the solutions of non-linear Schrodinger equation, for obtaining solutions of
the Gross-Pitaevskii equation with time dependent scattering length in a
harmonic trap. Interestingly, there is rapid pulse amplification in certain
scenarios
Mermin-Ho vortex in ferromagnetic spinor Bose-Einstein condensates
The Mermin-Ho and Anderson-Toulouse coreless non-singular vortices are
demonstrated to be thermodynamically stable in ferromagnetic spinor
Bose-Einstein condensates with the hyperfine state F=1. The phase diagram is
established in a plane of the rotation drive vs the total magnetization by
comparing the energies for other competing non-axis-symmetric or singular
vortices. Their stability is also checked by evaluating collective modes.Comment: 4 pages, 4 figure
Two-component Bose gas in an optical lattice at single-particle filling
The Bose-Hubbard model of a two-fold degenerate Bose gas is studied in an
optical lattice with one particle per site and virtual tunneling to empty and
doubly-occupied sites. An effective Hamiltonian for this system is derived
within a continued-fraction approach. The ground state of the effective model
is studied in mean-field approximation for a modulated optical lattice. A
dimerized mean-field state gives a Mott insulator whereas the lattice without
modulations develops long-range correlated phase fluctuations due to a
Goldstone mode. This result is discussed in comparison with the superfluid and
the Mott-insulating state of a single-component hard-core Bose.Comment: 11 page
Mott insulators in an optical lattice with high filling factors
We discuss the superfluid to Mott insulator transition of an atomic Bose gas
in an optical lattice with high filling factors. We show that also in this
multi-band situation, the long-wavelength physics is described by a single-band
Bose-Hubbard model. We determine the many-body renormalization of the tunneling
and interaction parameters in the effective Bose-Hubbard Hamiltonian, and
consider the resulting model at nonzero temperatures. We show that in
particular for a one or two-dimensional optical lattice, the Mott insulator
phase is more difficult to realize than anticipated previously.Comment: 5 pages, 3 figures, title changed, major restructuring, resubmitted
to PR
Kinetic Theory of Collective Excitations and Damping in Bose-Einstein Condensed Gases
We calculate the frequencies and damping rates of the low-lying collective
modes of a Bose-Einstein condensed gas at nonzero temperature. We use a complex
nonlinear Schr\"odinger equation to determine the dynamics of the condensate
atoms, and couple it to a Boltzmann equation for the noncondensate atoms. In
this manner we take into account both collisions between
noncondensate-noncondensate and condensate-noncondensate atoms. We solve the
linear response of these equations, using a time-dependent gaussian trial
function for the condensate wave function and a truncated power expansion for
the deviation function of the thermal cloud. As a result, our calculation turns
out to be characterized by two dimensionless parameters proportional to the
noncondensate-noncondensate and condensate-noncondensate mean collision times.
We find in general quite good agreement with experiment, both for the
frequencies and damping of the collective modes.Comment: 10 pages, 8 figure
't Hooft-Polyakov Monopoles in an Antiferromagnetic Bose-Einstein Condensate
We show that an antiferromagnetic spin-1 Bose-Einstein condensate, which can
for instance be created with Na-23 atoms in an optical trap, has not only
singular line-like vortex excitations, but also allows for singular point-like
topological excitations, i.e., 't Hooft-Polyakov monopoles. We discuss the
static and dynamic properties ofthese monopoles.Comment: Four pages of ReVTeX and 1 postscript figur
Extension of Bogoliubov theory to quasi-condensates
We present an extension of the well-known Bogoliubov theory to treat low
dimensional degenerate Bose gases in the limit of weak interactions and low
density fluctuations. We use a density-phase representation and show that a
precise definition of the phase operator requires a space discretisation in
cells of size . We perform a systematic expansion of the Hamiltonian in
terms of two small parameters, the relative density fluctuations inside a cell
and the phase change over a cell. The resulting macroscopic observables can be
computed in one, two and three dimensions with no ultraviolet or infrared
divergence. Furthermore this approach exactly matches Bogoliubov's approach
when there is a true condensate. We give the resulting expressions for the
equation of state of the gas, the ground state energy, the first order and
second order correlations functions of the field. Explicit calculations are
done for homogeneous systems.Comment: 32 pages, 2 figures; typos corrected in revised versio
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