Critical Vortex Shedding in a Strongly Interacting Fermionic Superfluid

We study the critical vortex shedding in a strongly interacting fermionic superfluid of $^{6}$Li across the BEC-BCS crossover. By moving an optical obstacle in the sample and directly imaging the vortices after time of flight, the critical velocity $v_{{\rm c}}$ for vortex shedding is measured as a function of the obstacle travel distance $L$. The observed $v_{\rm c}$ increases with decreasing $L$, where the rate of increase is the highest in the unitary regime. In the deep BEC regime, an empirical dissipation model well captures the dependence of $v_{{\rm c}}$ on $L$, characterized by a constant value of $\eta = -\frac{{\rm d}(1/v_{{\rm c}})}{{\rm d}(1/L)}$. However, as the system is tuned across the resonance, a step increase of $\eta$ develops about a characteristic distance $L_{\rm c}$ as $L$ is increased, where $L_{\rm c}$ 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 $v_{{\rm c}}$ to the participation of pair breaking in the vortex shedding dynamics of a fermionic superfluid.