Vortex Diffusivity in Two-Dimensional Helium Films

The first measurements of vortex diffusivity in two-dimensional superfluid helium films are reported. The diffusivity appears to be a universal function of $\frac{{\sigma }_s}{T}$, where ${\sigma }_s$ is the areal superfluid density, and is $\frac{0.4\hslash }{m}$ at the Kosterlitz-Thouless transition. In contrast with theoretical expectations, the diffusivity is a rapidly increasing function of $\frac{T}{{\sigma }_s}$ near the transition temperature.