  We study how temperature affects the lifetime of a quantized, persistent
current state in a toroidal Bose-Einstein condensate (BEC). When the
temperature is increased, we find a decrease in the persistent current
lifetime. Comparing our measured decay rates to simple models of thermal
activation and quantum tunneling, we do not find agreement. We also measured
the size of hysteresis loops size in our superfluid ring as a function of
temperature, enabling us to extract the critical velocity. The measured
critical velocity is found to depend strongly on temperature, approaching the
zero temperature mean-field solution as the temperature is decreased. This
indicates that an appropriate definition of critical velocity must incorporate
the role of thermal fluctuations, something not explicitly contained in
traditional theories.
