Heat Capacity of sup 3 He Adsorbed on Graphite at Millikelvin Temperatures and Near Third Layer Promotion.

The heat capacity of sup 3 He adsorbed on a graphite substrate has been measured for temperatures between 2 and 200 mK and for coverages corresponding to the region near third layer promotion. The second layer is observed to undergo a first- order transition between the fluid phase and presumably a registered solid. As a function of coverage, the transition is complete before atoms are promoted into the next layer. At 2.5 mK, the nuclear spin contribution to the heat capacity of the higher coverage state exhibits a peak which appears to be inconsistent with the usual exchange model. One possible explanation attributes the peak to magnetic polarons which form around zero-point vacancies.