Friday, 15 November – 17:00 Room 138, SISSA (Santorio building)
Silvia Chiacchiera (University of Coimbra): Collective modes of Fermi gases in realistic 3D and 2D traps
Abstract: Usually theoretical calculations for the collective modes of trapped cold gases are performed in harmonic traps. However, experimental trap potentials have typically a Gaussian shape and anharmonicity effects appear as the temperature and, in the case of Fermions, the filling of the trap are increased. These effects are not only quantitative, but also qualitative. Consider the sloshing (Kohn) mode, a center-of-mass oscillation of the cloud: in a harmonic trap it is undamped and takes place at the trap frequency, whatever the statistics, temperature, number of particles and interaction strength are (Kohn theorem). However, in an anharmonic trap its frequency is shifted and it acquires a damping, both effects depending on the system parameters. Since the sloshing mode is often used as a tool to determine with high precision the trap frequency, and as a reference for the other modes, it is fundamental to understand its properties. For the other modes, anharmonicity implies also a frequency shift and a different damping, and its effect cannot be reproduced in an accurate way by a simple rescaling.
In this seminar I will present two studies performed in the framework of the Boltzmann equation including in-medium effects and solved approximately via the phase-space moments method extended beyond lowest order: the sloshing mode in a 3D trap and the quadrupole mode in a 2D trap, for which experimental data are available. The calculated frequency shifts and damping rates of the sloshing mode in 3D are in very good agreement with the experimental data. On the other side, the computed damping of the quadrupole mode in 2D is smaller than the observed one, even in the weakly interacting and non-interacting regime, where our picture is more reliable. We try to model the experimental procedure as close as possible, but none of the many effects considered solves the puzzle.
 P.-A. Pantel, D. Davesne, S. Chiacchiera, and M. Urban, Trap anharmonicity and sloshing mode of a Fermi gas, Phys. Rev. A 86, 023635 (2012).
 S. Chiacchiera, D. Davesne, T.Enss, and M. Urban, Damping of the quadrupole mode in a two-dimensional Fermi gas, Phys. Rev. A, Phys. Rev. A 88, 053616 (2013).