HEAT-CAPACITY AND THE COMMENSURATE-INCOMMENSURATE TRANSITION OF HE-4 ADSORBED ON GRAPHITE

The heat capacity of He-4 adsorbed on graphite has been precisely measured over a fine grid of coverages for temperatures extending down to 100 mK and for coverages up to five atomic layers. This paper focuses mainly on the transitional region between the square-root 3 X square-root 3 commensurate structure and the incommensurate phase of the first adsorbed layer. At the commensurate coverage, rho1/3, the low-temperature data have been used to extract a phonon energy gap of 10.5 K, in agreement with the neutron-scattering value obtained for adsorbed He-3. At coverages slightly greater than rho1/3 the data are consistent with an incommensurate domain-wall solid. However, an abrupt change in behavior occurs at rho=1.1 rho1/3, which is interpreted as signalling the entrance into a coexistence region involving a commensurate structure in which 2/5 of the graphite adsorption sites are occupied by He-4 atoms. At rho2/5 the system undergoes a sharp first-order transition near 1 K where the entropy per atom changes by 0.2k(B). At somewhat higher coverages there is evidence for a 3/7 commensurate structure that transforms at 0.6 K into the floating incommensurate solid. Data for the second adsorbed layer indicate a solid registered relative to the compressed incommensurate first layer. Here the ratio of second- to first-layer densities is 4/7, in agreement with recent results for He-3 on graphite.