Study of the superfluid transition in two-dimensional /sup 4/He films

We have studied the superfluid transition of thin two-dimensional /sup 4/He films adsorbed on an oscillating substrate. The superfluid mass and dissipation support the Kosterlitz-Thouless picture of the phase transition in a two-dimensional superfluid. In addition we observe finite-velocity effects which are not explained by current dynamic theories. We also report data on 2D /sup 3/He-/sup 4/He mixtures.