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Mixtures of superfluids
Marion Delehaye (LKB)

Ultracold atoms are unique tools to probe the physics of quantum matter. Indeed, the high degree of tunability of ultracold Bose and Fermi gases makes them ideal systems for quantum simulation and for exploring macroscopic manifestations of quantum effects, such as superfluidity. In this work, we have realized the first Bose-Fermi superfluid mixture, with ultracold gases of 6Li and 7Li. The properties of the mixture are investigated by initiating a Bose-Fermi counterflow through their dipole modes. The superfluid critical velocity is measured in the BEC-BCS crossover, and is found close to the sound velocity of the Fermi gas near unitarity. We compare our findings to recent theoretical predictions. Raising the temperature of the mixture, we observe an unexpected synchronization of the motion of the two clouds, interpreted with a Zeno-like model induced by
dissipation. Finally, this Bose-Fermi mixture offers the unique possibility to create a homogeneous trap for the Fermi gas. By a fine tuning of the interactions, we propose to use the Bose-Fermi repulsion to compensate the curvature of the harmonic trap for fermions. For a spin-polarized Fermi gas in such a trap, we theoretically predict the existence of a superfluid with a shell structure and we provide first experimental evidence for this topologically new superfluid.