SUPERFLUIDITY OF HE-4 FILMS ADSORBED ON HYDROGEN PLATED GRAPHITE

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