Singlet Magnetism in Heavy Fermions

We consider singlet magnetism for the uranium ions in UP${\mathrm{t}}_3$ and UR${\mathrm{u}}_2$S${\mathrm{i}}_2$ assuming that time-reversal symmetry is broken for the coherent state of intermediate valence. The relative weight of the two involved configurations should be different for UP${\mathrm{t}}_3$ and UR${\mathrm{u}}_2$S${\mathrm{i}}_2$. If in UP${\mathrm{t}}_3$ the configuration ${5f}^1$ on the U ion prevails in the coherent state below the magnetic transition, the magnetic moment will vanish for the particular choice of the ionic wave function. In the case of UR${\mathrm{u}}_2$S${\mathrm{i}}_2$, the phase transition is nonmagnetic in the first approximation—the magnetic moment arises from a small admixture of a half-integer spin configuration.