Heat capacity ofHe3adsorbed on graphite at millikelvin temperatures and near third-layer promotion

The heat capacity of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ 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 in consistent with the usual exchange model. One possible explanation attributes the peak to magnetic polarons which form around zero-point vacancies.