Heat Capacity of Multilayers of sup 3 He Adsorbed on Graphite at Millikelvin Temperatures

Precise heat capacity results are presented for sup 3 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 2nd, 3rd, and 4th layers is clearly observed. Prior to third layer promotion, the 2nd layer undergoes a first-order phase transition. By comparison with the known phase diagram for the 1st layer, the new phase in the 2nd layer is assumed to be a registered lattice gas. The registry is now presumably with respect to the 1st sup 3 He layer which exists as a triangular-lattice solid incommensorate with the graphite substrate. The registered phase exhibits a large, sharp heat capacity anomaly at 2.5 mK. When approached from higher temperatures, the temperature dependence tends towards T sup (-0.5). This suggests that the anomaly may be due to antiferromagnetic polarons which form around zero point vacancies.