INTERFACIAL FRICTION OF SUBMONOLAYER HELIUM FILMS ADSORBED ON THE SURFACE OF GRAPHITE

The frictional coupling between an adsorbed helium film and the surface of graphite has been studied using torsional oscillator methods and exfoliated graphite as a substrate, in the temperature range 50mK to 2K. The observed period shift is consistent with the film decoupling from the substrate as the temperature is reduced, and is accompanied by a maximum in dissipation. Ar coverages below registry and in the incommensurate solid region the data are well described by Debye relations with a thermally activated relaxation time. This would correspond to a vanishing of the interfacial friction for the solid monolayer as T --> 0. At coverages between 0.064 Angstrom(-2) and 0.08 Angstrom(-2) a double peaked structure in the dissipation is observed. The results relate to the work of Krim et al.(l) who have used a quartz microbalance to study the ''nanotribology'' of adsorbed monolayers. In the present work the use of the graphite substrate offers the possibility of studying these effects on a system with a rich and well characterised submonolayer phase diagram.