Low Temperature Heat-Capacity Anomalies in Two-Dimensional SolidH3e

The heat capacity of second-layer solid ${}^3\mathrm{He}$ adsorbed on graphite has been measured down to extremely low temperatures below $100\mu \mathrm{K}$. We observed a double-peak structure for a low-density registered solid, which strongly suggests that the system is a highly frustrated spin- $1/2$ two-dimensional (2D) antiferromagnet with a disordered ground state. As the density increases it approaches a 2D nearest-neighbor Heisenberg ferromagnet with a single rounded peak, which can be explained semiquantitatively by considering higher-order exchange processes up to six-spin exchange.