Nuclear antiferromagnetism in a registered ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ solid

Recent heat-capacity measurements of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ adsorbed on graphite by Greywall and Busch are interpreted in terms of a triangular second-layer solid having √7 × √7 registry with respect to the first adsorbed layer. The observed entropy change in the millikelvin region is well accounted for by an antiferromagnetic Heisenberg spin Hamiltonian in which only a subset of second-layer spins forming a Kagomé net are strongly coupled.