Observation of single-vortex pinning in superfluid ⁴He

Flow-induced pinning and unpinning of vortices have been investigated experimentally in superfluid ⁴He at temperatures varying from 14 to 44 mK. The lifetime and unpinning velocities of pinned vortices could be measured by following 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 tunnelling and provide information on the phenomenon of superflow collapse in pinholes.