SUPERFLUIDITY OF HE-4 FILMS ADSORBED ON HYDROGEN PLATED GRAPHITE

The superfluidity of He-4 on graphite with various preplatings of HD is under investigation using a torsional oscillator. Results for a bilayer of HD follow a similar pattern to those obtained earlier on pure 4He films and are consistent with third sound measurements on graphite plated with a bilayer of hydrogen. In our case the first He-4 layer solidifies, consistent with behaviour observed in NMR studies of He-3 on a similarly prepared substrate. At a coverage of 11 nm(-2) there is the onset of a region in which the transition temperature increases rapidly with coverage and the transition itself becomes sharper. At certain coverages pronounced signatures have been observed in the period shift and dissipation, which may indicate a further phase transition in the film at lower temperatures.