Dual Bose-Fermi Superfluids

We study the dynamics of superfluid counterflow in a Bose-Fermi mixture of lithium atoms. First, by tuning the interaction strength we measure the critical velocity $v_c$ of the system in the BEC-BCS crossover in the low temperature regime and we compare it to the recent prediction of Castin {\em et al.}, Comptes Rendus Physique,{\bf 16}, 241 (2015). Second, raising the temperature of the mixture slightly above the superfluid transitions reveals an unexpected phase-locking of the oscillations of the clouds induced by dissipation. Finally, we investigate the lifetime of the Bose-Fermi mixture. We show theoretically and experimentally that, for weak inter-species coupling, the loss rate is proportional to Tan's contact parameter. At unitarity where the fermion-fermion scattering length diverges, we show that the loss rate is proportional the $4/3$ power of the fermionic density. This study demonstrates that impurity-induced losses can be used as a quantitative probe of many-body correlations.