Measurement of vortex diffusivity and lifetime below the Kosterlitz-Thouless transition in helium films

Abstract

Using vortex pair motion, we have measured the vortex diffusivity below the Kosterlitz-Thouless transition temperature (T KT) in atomically thin superfluid helium films on both argon and neon substrates. The diffusivities are on the order ofħ/m nearT KT and then fall rapidly to10 −3 ħ/m near0.1 K for argon and to0.03 ħ/m for neon. However, for the thinnest helium film on neon, the diffusivity is large and independent of temperature. At small flow velocities, dissipation due to unpaired vortices indicates a vortex lifetime on the order of seconds for a neon substrate near0.1 K.