One key property of a superfluid is the existence of a threshold (critical) velocity for dissipationless flow. Measuring this critical velocity not only unambiguously illustrates the superfluidity of the system, but furthermore provides important insight about the excitation mechanisms that eventually cause the breakdown of superfluidity.
In this paper we report on the measurement of critical velocities in an ultracold superfluid Fermi gas. The experimental signature was very clear, an abrupt onset of dissipation, when the velocity of the moving lattice was increased past a critical value.
We determined critical velocities for variable atom-atom interactions magnetically tuned through the BEC-BCS crossover. Most quantities describing the system vary monotonically across the BEC-BCS crossover, whereas the critical velocity shows a pronounced peak on resonance, which was clearly confirmed by our experiment. The maximum in the value of the critical velocity is caused by the change of the dissipation mechanism (pair breaking vs. phonon excitation) On the BCS side, the pairs become fragile and break more easily, whereas on the BEC side, the pairs are stable, but now the lower velocity of sound determines the critical velocity for the onset of dissipation.