Stability of persistent currents in superfluid fermionic rings

We investigate the stability of persistent currents in superfluid fermionic gases confined to a ring geometry. The studies cover interaction regimes from strong (unitary Fermi gas) to weak (Bardeen-Cooper-Schrieffer regime) couplings. Stability is tested with respect to the presence of an external defect within the ring. The dissipation mechanism related to vortex generation is present in all interaction regimes. Interestingly, while the corresponding critical winding number is found to be independent of the regime, the flow energy dissipation and its origin strongly depend on it. Vortex emission is accompanied by Cooper pair breaking, which occurs even beyond the vortex core in the weakly interacting regime. The pair-breaking mechanism prevents the imprinting of a persistent current with a winding number above a threshold, which decreases as we approach the BCS regime. Our study reveals the existence of two types of critical winding numbers above which the currents cease to be persistent in Fermi superfluids: one related to the proliferation of quantum vortices and the other with the onset of the pair-breaking mechanism.