Phase diagram forUPd3subject to symmetry-breaking stress and magnetic field

We show on the basis of a Landau theory that ${\mathrm{UPd}}_3$ will exhibit a number of distinct quadrupolar ordered phases and a tetracritical point, when subject to symmetry-breaking stress and magnetic field. Without the symmetry-breaking fields, a transition to a quadrupolar ordered triple-Q phase occurs. The two fields split this transition and at the tetracritical point, the unmodulated, single-, double-, and triple-Q phases coexist. In agreement with the requirement of thermodynamic stability, our theory predicts that no phase may occupy more than 180° of the angle at the tetracritical point. The splitting of the transition to the quadrupolar ordered phase, which has been observed in a number of experiments, could be explained by our results.