Critical velocity for vortex shedding in a Bose-Einstein condensate

Abstract

We present measurements of the critical velocity for vortex shedding in a highly oblate Bose-Einstein condensate with a moving repulsive Gaussian laser beam. As a function of the barrier height V-0, the critical velocity v(c) shows a dip structure having a minimum at V-0 approximate to mu, where mu is the chemical potential of the condensate. At fixed V-0 approximate to 7 mu, we observe that the ratio of v(c) to the speed of sound c(s) monotonically increases for decreasing sigma/xi, where sigma is the beam width and xi is the condensate healing length. We explain our results with the density reduction effect of the soft boundary of the Gaussian obstacle, based on the local Landau criterion for superfluidity. The measured value of v(c)/c(s) with our stiffest obstacle is about 0.4, which is in good agreement with theoretical predictions for a two-dimensional superflow past a circular cylinder.