Observation of single-vortex pinning in superfluid He-4

Flow-induced pinning and unpinning of vortices have been investigated exper imentally in superfluid He-4 at temperatures varying from 14 to 44 mK. The lifetime and unpinning velocities of pinned vortices could be measured by f ollowing the apparent changes of the critical velocity for phase slips. The pinned vortex size and location can be deduced from the magnitude of these changes and are found, in these experiments, to be of nanometer size. The experimental observations indicate that unpinning takes place by quantum tu nnelling and provide information on the phenomenon of superflow collapse in pinholes. (C) 1998 Elsevier Science B.V. All rights reserved.