Minimum critical velocity of a Gaussian obstacle in a Bose-Einstein condensate

Abstract

When a superfluid flows past an obstacle, quantized vortices can be created in the wake above a certain critical velocity. In the experiment by Kwon et al. [Phys. Rev. A 91, 053615 (2015)1050-294710.1103/PhysRevA.91.053615], the critical velocity vc was measured for atomic Bose-Einstein condensates (BECs) using a moving repulsive Gaussian potential, and vc was minimized when the potential height V0 of the obstacle was close to the condensate chemical potential μ. Here we numerically investigate the evolution of the critical vortex shedding in a two-dimensional BEC with increasing V0 and show that the minimum vc at the critical strength V0c≈μ results from the local density reduction and vortex-pinning effect of the repulsive obstacle. The spatial distribution of the superflow around the moving obstacle just below vc is examined. The particle density at the tip of the obstacle decreases as V0 increases to Vc0, and at the critical strength, a vortex dipole is suddenly formed and dragged by the moving obstacle, indicating the onset of vortex pinning. The minimum vc exhibits power-law scaling with the obstacle size σ as vc∼σ-γ with γ≈1/2.