He-4 adsorption and superfluid transition on C-60

We present adsorption isotherm data of He-4 on C-60 for 1.50 K < T < 1.68 K determined by measurements of the frequency of quartz crystal microbalances (QCM's) coated with C-60 films. We find a Kosterlitz-Thouless transition with a jump in the areal superfluid density close to predictions. By comparing the adsorbed He-4 on two QCM's we derive an upper limit for the amount of He-4 intercalated into C-60 at low temperature of 0.05 He-4 atoms per C-60 molecule. The low coverage portion of the adsorption data shows an apparent excess adsorption of He-4 onto the C-60.