Suppression of phonon propagation in a Bose superfluid immersed in a degenerate Fermi gas

We investigate the behavior of sound propagation in a Bose-Einstein condensate (BEC) immersed in a degenerate Fermi gas (DFG). The interaction between the bosons and the surrounding fermions modifies the phonon dispersion with density perturbations of the DFG. Sound waves in the interacting mixture propagate with modified velocity as coupled excitations of the two species.

Our experiments utilize mixtures of bosonic ¹³³Cs and fermionic ⁶Li atoms cooled to quantum degeneracy and prepared in a linear optical trap. Using a microscope objective, we optically excite sound wave-packets and observe the resulting BEC dynamics in situ. We control the interspecies interaction and study its effect on the speed and decay rate of the phonons. We observe significant reduction of the sound speed in the presence of strong interspecies attraction. Our observations help develop more accurate theoretical models and lay groundwork for future applications of Bose-Fermi mixtures for quantum simulation.