Critical Velocity for Vortex Shedding in a Bose-Einstein Condensate

We present the measurements of the critical velocity for vortex shedding in a highly oblate Bose-Einstein condensate with a moving repulsive Gaussian potential. As a function of the potential barrier height $V_0$, the critical velocity shows a dip structure having a minimum at $V_0=\mu$, where mu is the chemical potential of the condensate. In a condition of $V_0/\mu\approx7$, where the radius of the density-depleted hole by the potential is close to the potential beam waist $\sigma$, we find that the critical velocity monotonically increases and approaches $0.4c$ for vanishing $\sigma/\xi$, where $c$ is the speed of sound and $\xi$ is the healing length of the condensate. The upper bound for the critical velocity is in good quantitative agreement with the theoretical predictions of the critical velocity of a two-dimensional superflow past a circular cylinder. We will also discuss the effects of the beam profile imperfection on the critical velocity.