Heat capacity of multilayers ofHe3adsorbed on graphite at low millikelvin temperatures

Precise heat-capacity results are presented for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ adsorbed on graphite. The temperature range of the data is from 2 to 200 mK, while the coverages span from somewhat below monolayer completion up through five atomic layers. Promotion of atoms into the second, third, and fourth layers is clearly observed. Nuclear-spin exchange energies of the order of a few tenths of a mK are found for the submonolayer incommensurate solid phase. These values differ significantly from those recently inferred from NMR experiments. Data for the second-layer fluid yield ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ quasiparticle effective masses that agree well with the corresponding first-layer values and range from one to five times the bare ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ mass. Prior to third-layer promotion, the second layer undergoes a first-order phase transition. By comparison with the phase diagram for the first layer, the new phase in the second layer is assumed to be a registered solid. Registry is now with respect to the first ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ layer, which continues to exist as a triangular-lattice solid incommensurate with the graphite substrate. The registered phase exhibits a large, sharp heat-capacity anomaly at 2.5 mK.