Orientational Phases of Hydrogen Molecules on a Triangular Lattice

Abstract

A mean-field theory for the ordering of hydrogen molecules on a triangular net is derived in terms of the quadrupole coupling constant $\Gamma$, the crystal field $V_c$, and the temperature $T$. The phase diagram consists of six regions, separated by first- and second-order transitions. For almost all values of $\frac{V_c}{\Gamma }$ for which ordering occurs, the disordered to ordered phase transition is continuous. At $T=0\mathrm{}$ K, all phases have energy gaps except for a ferrorotational phase which behaves like an $\mathrm{XY}$ model.