Mechanism for Depinning a Macroscopic Vortex in Superfluid Helium

We measure a superfluid vortex, pinned to a thin wire along part or all of its length. After ceasing rotation, this vortex is metastable and heating the fluid can lead to depinning. Several observations show that the key to the depinning is the counterflow velocity through the inlet hole for the helium fill line: the importance of the rate of change of temperature rather than temperature itself, the behavior for different size inlet holes, the location where the vortex depins, and the relative stability of fully or partially attached vortices. All of these observations point to the role of the thermally induced velocity. Although we cannot prove how this velocity leads to depinning, one possibility is that the velocity field enlarges vortices trapped nearby on the cell wall until vortex loops break free and subsequently interact with the trapped central vortex. We comment on the level of reproducibility of our measurements and what it implies for such wall vortices.