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

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)], the critical velocity v(c) was measured for atomic Bose-Einstein condensates (BECs) using a moving repulsive Gaussian potential and v(c) was minimized when the potential height V-0 of the obstacle was close to the condensate chemical potential mu. Here we numerically investigate the evolution of the critical vortex shedding in a two-dimensional BEC with increasing V-0 and show that the minimum v(c) at the critical strength V-0c approximate to mu 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 v(c) is examined. The particle density at the tip of the obstacle decreases as V-0 increases to V-c0 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 sigma as v(c) similar to s(-gamma) with gamma approximate to 1/2.

Automated summary

When a superfluid flows past an obstacle at a certain critical velocity, quantized vortices can be formed in the wake. This study investigates the critical vortex shedding in a two-dimensional BEC and finds that the minimum critical velocity occurs when the obstacle's height is close to the condensate chemical potential. Numerical simulations reveal that the minimum critical velocity is a result of local density reduction and vortex pinning effect caused by the repulsive obstacle. The spatial distribution of superflow and the formation of vortex dipole at the critical strength are also examined.