  We have measured the threshold for creating long-lived excitations when a
toroidal Bose-Einstein condensate is stirred by a rotating (optical) barrier of
variable height. When the barrier height is on the order of or greater than
half of the chemical potential, the critical barrier velocity at which we
observe a change in the circulation state is much less than the speed for sound
to propagate around the ring. In this regime we primarily observe discrete
jumps (phase slips) from the non-circulating initial state to a simple,
well-defined, persistent current state. For lower barrier heights, the critical
barrier velocity at which we observe a change in the circulation state is
higher, and approaches the effective sound speed for vanishing barrier height.
The response of the condensate in this small-barrier regime is more complex,
with vortex cores appearing in the bulk of the condensate. We find that the
variation of the excitation threshold with barrier height is in qualitative
agreement with the predictions of an effective 1D hydrodynamic model.
