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arXiv:1505.04841 · New Journal of Physics

Undamped nonequilibrium dynamics of a nondegenerate Bose gas in a 3D isotropic trap

V. E. Colussi, Cameron Straatsma, Dana Z. Anderson, Murray Holland

Published Oct 14, 2015• 9 citations
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Abstract

We investigate anomalous damping of the monopole mode of a non-degenerate 3D Bose gas under isotropic harmonic confinement as recently reported by the JILA TOP trap experiment (Lobser et al in preparation). Given a realistic confining potential, we develop a model for studying collective modes that includes the effects of anharmonic corrections to a harmonic potential. By studying the influence of these trap anharmonicities throughout a range of temperatures and collisional regimes, we find that the damping is caused by the joint mechanisms of dephasing and collisional relaxation. Furthermore, the model is complimented by Monte Carlo simulations which are in fair agreement with data from the JILA experiment.

DOI
10.1088/1367-2630/17/10/103029

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of the existence of atoms. In the context of Bose gases in the ultracold regime, we investigate the possibility of studying such a nonequilibrium motion of the cloud persisting indenitely throughout a range of temperatures above and below the transition and over a range of collisional regimes given a realistic trapping scenario. For a nondegenerate Bose gas, we characterize the emergence of anomalous damping in the breathing mode by modeling the inuence of the trap anharmonicities via the joint mechanisms of dephasing and collisional relaxation. The model is tested using recent results from the JILA TOP trap experiment with a non degenerate cloud of Rb87 atoms. This work was supported by the U. S. National Science Foundation and AFSOR.
For a gas trapped in a harmonic potential, the sloshing (or Kohn) mode is undamped and its frequency coincides with the trap frequency, independently of the statistics, interaction, and temperature of the gas. However, experimental trap potentials have usually Gaussian shape and anharmonicity effects appear as the temperature and, in the case of Fermions, the filling of the trap are increased. We study the sloshing mode of a degenerate Fermi gas in an anharmonic trap within the Boltzmann equation, including in-medium effects in both the transport and collision terms. The calculated frequency shifts and damping rates of the sloshing mode due to the trap anharmonicity are in satisfactory agreement with the available experimental data. We also discuss higher-order dipole, octupole, and bending modes and show that the damping of the sloshing mode is caused by its coupling to these modes.
We present a theory for the linear dynamics of a weakly interacting Bose gas confined inside a harmonic trap at finite temperature. The theory treats the motions of the condensate and of the noncondensate on an equal footing within a generalized random-phase approximation, which (i) extends the second-order Beliaev-Popov approach by allowing for the dynamical coupling between fluctuations in the thermal cloud, and (ii) reduces to an earlier random-phase scheme when the anomalous density fluctuations are omitted. Numerical calculations of the low-lying spectra in the case of isotropic confinement show that the present theory obeys with high accuracy the generalized Kohn theorem for the dipolar excitations and demonstrate that combined normal and anomalous density fluctuations play an important role in the monopolar excitations of the condensate. Mean-field theory is instead found to yield accurate results for the quadrupolar modes of the condensate. Although the restriction to spherical confinement prevents quantitative comparisons with measured spectra, it appears that the non-mean-field effects that we examine may be relevant to explain the features exhibited by the breathing mode as a function of temperature in the experiments carried out at JILA on a gas of 87Rb^{87}\mathrm{Rb}87Rb atoms.
We study the collective modes of a one-dimensional harmonically trapped Bose-Einstein condensate in the presence of the anomalous density using the time-dependent Hartree-Fock-Bogoliubov theory. Within the hydrodynamic equations, we derive analytical expressions for the mode frequencies and the density fluctuations of the anomalous density which constitutes the minority component at very low temperature and feels an effective external potential exerted by the majority component, i.e., the condensate. On the other hand, we numerically examine the temperature dependence of the breathing mode oscillations of the condensate at finite temperature in the weak-coupling regime. At zero temperature, we compare our predictions with available experimental data, theoretical treatments, and Monte carlo simulations in all interaction regimes and the remaining hindrances are emphasized. We show that the anomalous correlations have a non-negligible role on the collective modes at both zero and finite temperatures.
We investigate the low-energy excitations of a dilute atomic Bose gas confined in a anharmonic trap interacting with repulsive forces. The dispersion law of both surface and compression modes is derived and analyzed for large numbers of atoms in the trap, which show two branches of excitation and appear two critical values, where one of them indicates collective excitation which would be unstable dynamically, and the other one indicates the existing collective mode with lower frequency under anharmonic influence than that in harmonic trapping case. Our work reveals the key role played by the anharmonicity and interatomic forces which introduce a rich structure in the dynamic behavior of these new many-body systems.