304 research outputs found
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.
Frequency and damping of hydrodynamic modes in a trapped Bose-condensed gas
Recently it was shown that the Landau-Khalatnikov two-fluid hydrodynamics
describes the collision-dominated region of a trapped Bose condensate
interacting with a thermal cloud. We use these equations to discuss the low
frequency hydrodynamic collective modes in a trapped Bose gas at finite
temperatures. We derive a variational expressions based on these equations for
both the frequency and damping of collective modes. A new feature is our use of
frequency-dependent transport coefficients, which produce a natural cutoff by
eliminating the collisionless low-density tail of the thermal cloud. Above the
superfluid transition, our expression for the damping in trapped inhomogeneous
gases is analogous to the result first obtained by Landau and Lifshitz for
uniform classical fluids. We also use the moment method to discuss the
crossover from the collisionless to the hydrodynamic region. Recent data for
the monopole-quadrupole mode in the hydrodynamic region of a trapped gas of
metastable He is discussed. We also present calculations for the damping of
the analogous monopole-quadrupole condensate mode in the superfluid
phase.Comment: 22 pages, 10 figures, submitted to Physical Review
Particle-Localized Ground State of Atom-Molecule Bose-Einstein Condensates in a Double-Well Potential
We study the effect of atom-molecule internal tunneling on the ground state
of atom-molecule Bose-Einstein condensates in a double-well potential. In the
absence of internal tunneling between atomic and molecular states, the ground
state is symmetric, which has equal-particle populations in two wells. From the
linear stability analysis, we show that the symmetric stationary state becomes
dynamically unstable at a certain value of the atom-molecule internal tunneling
strength. Above the critical value of the internal tunneling strength, the
ground state bifurcates to the particle-localized ground states. The origin of
this transition can be attributed to the effective attractive inter-atomic
interaction induced by the atom-molecule internal tunneling. This effective
interaction is similar to that familiar in the context of BCS-BEC crossover in
a Fermi gas with Feshbach resonance. Furthermore, we point out the possibility
of reentrant transition in the case of the large detuning between the atomic
and molecular states.Comment: 34 pages,10 figure
- …