Dynamics of the decay of dark solitons in superfluid Fermi gases*

Dark solitons are solitary matter waves which retain their shape while propagating at a constant velocity. They emerge in a wide variety of physical systems, including ultracold atomic gases. In superfluid Bose gases, dark solitons have been observed to decay into quantized vortices through the so-called snake instability mechanism. Recent experiments in superfluid Fermi gases have also interpreted soliton decay via this mechanism. However, using both numerical simulations and a perturbative analysis based on a low-energy effective field theory, we show that there is a qualitative difference between soliton decay in the BEC- and BCS-regimes of superfluid Fermi gases.
In the BEC-regime, the characteristic snaking deformations of the soliton plane are induced by fluctuations of the amplitude of the order parameter, while in the BCS-limit, fluctuations of the phase destroy the soliton core through the formation of local Josephson currents, without the occurrence of a snaking pattern. The difference between both mechanisms should be experimentally observable, providing an incentive to consider both past and future experiments from a new perspective.