Pair-breaking as the fundamental limit to persistent-current stabilization in fermionic superfluids

Abstract: We study the stability of persistent currents in fermionic superfluids with impurities within the BCS regime by using time-dependent density functional theory. Unlike in Bose-Einstein condensates, we find that current stabilization by impurities is intrinsically limited by the pair-breaking threshold. Below the threshold, impurities enhance winding number stability, but pair-breaking continues to drive dissipation of the flow. Above this critical velocity, superflow destabilizes, emitting vortices. Impurities then govern vortex mobility and pinning, exhibiting regimes of collective pinning and hopping. Moreover, pinned vortices do not guarantee dissipationless flow due to ongoing pair-breaking. Our results identify pair breaking as the fundamental mechanism that sets the ultimate limit of persistent-current stability in fermionic superfluids, providing insights relevant to ultracold Fermi gases and neutron-star matter.