Quench-induced spontaneous currents in rings of ultracold fermionic atoms

Abstract

We have measured the rate of spontaneous current formation in ring-shaped ensembles of fermionic $^6$Li atoms, following a thermal quench through the BCS superfluid phase transition. For the fastest quenches, the mean square winding number follows a scaling law with exponent $\sigma$ = 0.24(2), in line with predictions of the Kibble-Zurek (KZ) model for mean-field BCS theory. We use a hybrid quench protocol involving simultaneous evaporation and interaction ramps, with a long system lifetime allowing characterization of a different rate of spontaneous current formation in the slow-quench regime, where finite-size effects are important. Comparing our observations to a quasi-1D stochastic Ginzburg-Landau model, we find quantitative agreement for fast quenches, but only qualitative agreement for slow quenches.Comment: 6 pages, 4 figure