Pinning of a nanometric size vortex in superfluid 4He

The incidental observation of a second, lower critical velocity in the flow of superfluid He-4 through a micro-aperture contaminated with micro-particles is interpreted as due to a vortex becoming pinned in the immediate vicinity of the nucleation site for phase-slips. The micro-particles are atomic clusters of solid hydrogen or air of nanometric size, located within nanometres of the nucleation site. The pinned vortex influence is also seen on the scatter of the critical transition which is found to be twice its usual value. (C) 2000 Elsevier Science B.V. All rights reserved.