Direct measurement of vortex diffusivity in thin films of He4

We report direct measurements of vortex diffusivity, D, as a function of temperature through the Kosterlitz-Thouless transition. We find that D is a rapidly varying function of temperature near the transition. It is very small well below the transition, increases to 1/4Latin small letter h with stroke/m at the static transition temperature, and apparently diverges at the point where the superfluid density vanishes. The diffusivity appears to be insensitive to the presence of He3 impurities and to the nature of the substrate.