Dissipation in adsorbed submonolayer helium films and two dimensional melting

Abstract

The mechanical dissipation of a submonolayer helium film adsorbed on the surface of graphite has been studied as a function of coverage and temperature through the two dimensional melting transition. The measurements were made with a high Q torsional oscillator over the temperature range 1.5 to 5K on both⁴He and³He films. The largest dissipation was observed in the incommensurate solid phase and increased with decreasing temperature and increasing surface density. Plots of dissipation vs. temperature break away from a common curve at the “melting temperature” defined by the observed maxima in the film heat capacity. This may provide evidence for continuous melting of the two dimensional solid. It is believed that the dissipation arises not from interfacial friction between the adsorbate and substrate but rather from internal friction in the solid crystallites. The effect of surface morphology has been investigated by using Grafoil and UCAR ZYX as substrates.