Finite temperature and density depletion effects on persistent current state transitions and critical velocity of a toroidal Bose-Einstein condensate

We study the decay of a persistent, quantized current state in a toroidal geometry. Our experiment involves trapping neutral $^{23}$Na atoms in an all optical ``target trap" shaped potential. This potential consists of a disc surrounded by an annular potential. A current in a superfluid can be sustained only below a critical current. This critical current can be tuned by introducing a density perturbation which depletes the local density. The decay time of a persistent current state can also be controlled by enhancing fluctuations of the system thermally. We study the decay at four different temperatures between 30~nK and 190~nK. For each temperature we record the decay at four different perturbation strengths. We find that increasing the magnitude of the density depletion or the temperature leads to a faster decay, and have seen the decay constant change by over two orders of magnitude. We also studied the size of hysteresis loop between different current states as a function of temperature, allowing us to extract a critical velocity. We find that the discrepancies between the experimentally extracted critical velocity and theoretically calculated critical velocity (using local-density approximation ) decreases as the temperature is decreased.