Superfluidity and film structure in He-4 adsorbed on graphite

We have completed a torsional-oscillator study of superfluidity in thin films of He-4 adsorbed on the basal plane of graphite for coverages from 1 1/2 to 7 atomic layers. In contrast to superfluidity in films adsorbed on amorphous substrates, we find several different regimes of superfluid behavior. Superfluidity in the second layer shows an anomalous temperature dependence and is destroyed by solidification of the him. The superfluid film appears to coexist with a two-dimensional surface gas for low coverages in the third layer. Evidence of a phase transition, which may be the reconstruction of an underlying layer, appears just above the completion of the third layer. Modulation of the superfluid signal with coverage is seen through the completion of the sixth layer.