Singlet Magnetism in Heavy Fermions

We consider singlet magnetism for the uranium ions in UPt₃ and URu₂Si₂ 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 UPt₃ and URu₂Si₂. If in UPt₃ the configuration 5f¹ 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 URu₂Si₂, the phase transition is nonmagnetic in the first approximation---the magnetic moment arises from a small admixture of a half-integer spin configuration.