DISSIPATION IN SUBMONOLAYER HELIUM FILMS ADSORBED ON GRAPHITE

We have investigated the influence of an adsorbed helium film on the d amping of a torsional oscillator containing exfoliated graphite. The d amping depends crucially on the phase of the adsorbed film; in the inc ommensurate solid a significant dissipation is measured which increase s with decreasing temperature. The origin is believed to be internal f riction in the two dimensional solid rather than surface friction. The results provide a novel probe of the 2D melting transition. They are being extended to lower temperatures, including an investigation of th e domain wall solid phase in order to elucidate the dissipation mechan ism.