Critical Vortex Shedding in a Strongly Interacting Fermionic Superfluid

Abstract

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 $v_{\mathrm{c}}$ for vortex shedding is measured as a function of the obstacle travel distance $L$. The observed $v_{\mathrm{c}}$ 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 $v_{\mathrm{c}}$ on $L$, characterized by a constant value of $\eta =-\frac{\mathrm{d}(1/v_{\mathrm{c}})}{\mathrm{d}(1/L)}$. However, as the system is tuned across the resonance, a step increase of $\eta$ develops about a characteristic distance $L_{\mathrm{c}}$ as $L$ is increased, where $L_{\mathrm{c}}$ 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 $v_{\mathrm{c}}$ to the participation of pair breaking in the vortex shedding dynamics of a fermionic superfluid.

arXiv comment8 pages, 8 figures

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