Critical Vortex Shedding in a Strongly Interacting Fermionic Superfluid

We study the critical vortex shedding in a strongly interacting fermionic superfluid of Li-6 across the BEC-BCS crossover. By moving an optical obstacle in the sample and directly imaging the vortices after the time of flight, the critical velocity u(vor) for vortex shedding is measured as a function of the obstacle travel distance L. The observed uvor increases with decreasing L, where the rate of increase is the highest in the unitary regime. In the deep Bose-Einstein condensation regime, an empirical dissipation model well captures the dependence of u(vor) on L, characterized by a constant value of n = -[d(1/u(vor))/d(1/L)]. However, as the system is tuned across the resonance, a step increase of. develops about a characteristic distance L-c as L is increased, where L-c is comparable to the obstacle size. This bimodal behavior is strengthened as the system is tuned towards the BCS regime. We attribute this evolution of u(vor) to the emergence of the underlying fermionic degree of freedom in the vortex-shedding dynamics of a Fermi condensate.