Breathing mode of a Bose-Einstein condensate repulsively interacting with a fermionic reservoir

We investigate the fundamental breathing mode of a small-sized elongated Bose-Einstein condensate coupled to a large Fermi sea, which consists of fully spin-polarized atoms in the collisionless regime. Our observations show a dramatic shift of the breathing frequency when the mixture undergoes phase separation at strong interspecies repulsion. We find that the maximum frequency shift in the full phase-separation limit depends essentially on the atom number ratio of the components. We interpret the experimental observations by modeling the complex dynamics of the collisionless fermions within two complementary approaches. One model assumes an adiabatic response of the Fermi sea, while the other one considers single fermion trajectories for a fully phase-separated mixture. Our models capture the observed features over the full range of interest.