Dipole-mode spectrum in a resonantly interacting Fermi-Fermi mixture of ¹⁶¹Dy and ⁴⁰K

We report on studies of hydrodynamic behavior of an ultracold strongly interacting fermionic mixture of ¹⁶¹Dy and ⁴⁰K. Employing an interspecies Feshbach resonance we investigated the dipole-mode spectrum of the mixture for different strengths of repulsive and attractive interactions. We measured frequencies and damping rates of the oscillation modes, observing a striking frequency locking in the unitarity regime. For intermediate interaction strengths we observed mean field contributions to the frequencies and damping of the locked mode. We extensively studied the damping rates of the oscillations across the resonance identifying three contributions, one related to the locked mode itself, one related to the spatial mixing of the two clouds, called "slow damping mode", and one related to the drag between the two species, called "fast damping mode". The fast damping is of particular interest as it allows for a characterization of the collisional properties on a time scale faster than all other relevant time scales, making it more robust against typical experimental limitations on resonance, such as heating. From this fast mode we derive a microscopic friction coefficient, which is directly related to the number of collisions in the unitarity limited regime.