Saint-Jalm, Raphaël

Date:   Friday, August 24, 2018
Time:   10:00
Place:   ETH Zurich, Hönggerberg, HPF G 6
Host:    Laura Corman / Tilman Esslinger

Second sound in a 2D Bose gas

Raphaël Saint-Jalm
Laboratoire Kastler Brossel, Collège de France, Paris

The propagation of sound is an important feature for the understanding of superfluid systems, in particular for dilute quantum fluids. In the case of strongly interacting Fermi gases, two sound modes have been observed. The first mode corresponds to the in-phase oscillation of the density of the superfluid phase and of the normal phase, whereas the second mode corresponds to the out-of-phase oscillation of the entropies of these two phases. Because of their high compressibility, weakly interacting Bose gases behave differently, and the nature of the two modes is modified. In particular, the second sound is mainly due to superfluid part of the gas and its study is helpful to gain information about the superfluid properties of the gas.

The dimensionality of the gas is an important parameter for the propagation of sound, as it influences the thermodynamics of the gas. I will focus on the case of two-dimensional Bose gases, where the superfluid fraction exhibits a jump at the critical temperature.

I will present the experiments that we have been performing with two-dimensional uniform Bose gases: We observe the propagation of sound and measure its velocity and its damping rate from a highly degenerate regime to above the critical temperature Tc. Below Tc, our observations are in good agreement with the theoretical values of the two-fluids model. Above Tc, we still observe propagation of sound, which we attribute to collisionless sound.