Pinning of a nanometric size vortex in superfluid He-4

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