New Spontaneous Symmetry Breaking for the CubicXYModel

A Landau expansion including sixth- and eighth-degree terms is studied for the cubic $\mathrm{XY}$ model. A new ordered phase with "generic symmetry" is found. The angle of rotation $\theta$ of the associated order parameter depends continuously on temperature. The transition from the disordered phase into this new phase must always be first order, irrespective of both the sign of the quartic term and the inclusion of higher-degree terms in the expansion. This prediction can be verified experimentally by studying the temperature dependence of $\theta$. ${\mathrm{Tb}}_2$ ${\left(\mathrm{M}\mathrm{o}{\mathrm{O}}_4\right)}_3$ should be a good candidate.