Superfluidity and film structure inHe4adsorbed on graphite

We have completed a torsional-oscillator study of superfluidity in thin films of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ adsorbed on the basal plane of graphite for coverages from 11/2 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 film. 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. © 1996 The American Physical Society.