Mediated Interactions and Sound Propagation in a Degenerate ¹³³Cs-⁶Li Bose-Fermi Mixture

We present our work with Bose-Einstein condensates of ¹³³Cs immersed in degenerate Fermi gases of ⁶Li. Excitations near the Fermi surface can mediate interactions between bosonic atoms, altering both the ground state and the dynamics of the condensate. Using a high-resolution microscope objective, we image and project optical potentials on the mixtures. The interspecies interactions can be tuned with a Feshbach resonance.

We investigate the propagation of sound in the mixtures from strongly attractive to strongly repulsive interspecies interactions. We observe strikingly different behavior across the Feshbach resonance: the sound velocity is heavily reduced with negative interactions, but we do not measure this effect with positive interactions. Our measurements further clarify the two- and three-body contributions to the fermion-mediated interactions between bosons