Nuclear Spin Capacity of sup 3 He Adsorbed on Graphite

Precise heat capacity results are presented for sup 3 He adsorbed an 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 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 from around zero point vacancies. As the coverage is increased the peak remains located at 2.5 mK but suddenly grows in amplitude while the temperature dependence changes towards T sup -2. The anomaly reaches its greatest magnitude at 0.24 atoms/A sup 2 where promotion of atoms into the 4th layer also occurs. At this same coverage magnetization measurement exhibit a large ferromagnetic peak. It is not clear if the ferromagnetic anomaly should be attributed to the 2nd solid layer or to the 3rd liquid layer.