Dynamical and scaling effects in two-dimensional superfluid helium

The results of a series of measurements of Kosterlitz-Thouless behavior in helium films are presented. Use is made of high-Q piezoelectric crystals so as to probe the phenomena in a single flat substrate geometry at high frequencies. Variations of the superfluid density and dissipation in the vicinity of the transition were measured. Dynamical effects were probed by comparing transitions at two different frequencies. Results are compared with a comprehensive numerical calculation in which finite-size scaling effects are explicitly introduced. A consistent picture for the dependence of critical thickness on temperature, of the scaling of the transition shape with temperature, and of dynamical effects is obtained.