Slips of the superfluid wave-function phase occur when the flow velocity through an aperture exceeds a critical threshold ${\mathit{v}}_{\mathit{c}}$. We propose a phase slip mechanism in which a microscopic vortex half ring is nucleated at the wall of the aperture. Its subsequent trajectory is described. The properties of the vortex close to the wall are inferred from the Gross-Pitaevskii equation and yield, at T∼0.1 K, a vortex radius at nucleation of 14 Å and a critical velocity of 21 m/s, in agreement with experiment.

• Received 20 July 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.380