Vortex dynamics in superfluid helium films

A high-Q torsional oscillator was used to measure the superfluid density and excess dissipation in thin films of He4 as a function of temperature and film thickness. The effects of rotation on the Kosterlitz-Thouless transition were studied and the rotation-induced dissipation was measured. The first direct measurements of vortex diffusivity D as a function of the temperature through the superfluid transition were obtained. We find that D is a rapidly varying function of temperature in the vicinity of the transition. It is very small well below the transition, increases to order Latin small letter h with stroke/m at the static transition temperature, and appears to diverge at the point where the superfluid density vanishes. The diffusivity also appears to be insensitive to the presence of He3 impurities and to the nature of the substrate.