Damping of Collective Oscillations in a Box Trap

We model numerically the lowest-lying collective mode of a Bose gas in a box trap excited by a kick in the potential, as in a recent experiment. Our analysis is performed at finite temperatures (below the critical region), based on the so-called “ZNG” model, in which the condensate is described by a dissipative Gross-Pitaevskii equation which is itself self-consistently coupled to a dynamical thermal cloud described by a quantum Boltzmann equation – a model which has proven most successful in describing damping observed in harmonic traps. For typical parameters probed far from the hydrodynamic region, we find a single oscillation -- whose frequency agrees well with experiments -- with the thermal cloud rapidly damping out higher frequency modes primarily through self-consistent dynamical mean-field coupling. Our results are confirmed by an independent analysis with the stochastic projected Gross-Pitaevskii equation. Intuitively, we find damping in a box trap to depend much more weakly on temperature than in harmonic traps, in broad agreement with experimental data.