Critical velocity for vortex shedding in a Bose-Einstein condensate

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 V0, the critical velocity vc shows a dip structure having a minimum at V0≈μ, where μ is the chemical potential of the condensate. At fixed V0≈7μ, we observe that the ratio of vc to the speed of sound cs monotonically increases for decreasing σ/ξ, where σ is the beam width and ξ 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 vc/cs 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.