Heat capacity of multilayerHe3adsorbed on graphite

Abstract

Heat-capacity measurements in the multilayer region of adsorbed ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ on Grafoil display a new set of transitions. These effects first appear as small peaks in the film heat capacity at low temperature, for coverages greater than second-layer completion, $n=0.186\mathrm{}$ ${\mathrm{\AA }}^{-2\mathrm{}}$. With increasing film coverage, the peaks grow in height until they dominate the film heat capacity for the highest coverages measured, up to $n=0.275\mathrm{}$ ${\mathrm{\AA }}^{-2\mathrm{}}$. Eight films in this range of coverage have been studied for temperatures between 0.04 and 1.5 K. Two important observations have been made with regard to the data: the incremental low temperature, $T<\mathrm{}0.5\mathrm{}$ K, heat-capacity asymptotes to a value similar to bulk liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, and the signal near the peak is exponential in form with an activation energy close to the latent heat of bulk liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at the peak temperature. The peaks are assumed to signal evaporation of bulk liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. These results suggest a distinct transition from two- to three-dimensional behavior in helium films. Stability of the proposed three-dimensional phase versus that of the flat film is discussed in terms of current nucleation theory. Reevaluation of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ data for the same coverage range is encouraged by these findings.