Small moments in heavy fermions: a model Hamiltonian

Abstract

We examine a model Hamiltonian for the heavy-fermion URu 2Si 2 based on two crystal-field-split singlets for the uranium spin coupled by on-site exchange to the conduction electrons. It is equivalent to an Ising-Kondo lattice with transverse magnetic field. Within the mean-field approximation, we find a logarithmic response of the conduction electrons to moment formation. Over an appreciable range of parameters, the moments in the antiferromagnetic state are anomalously small as observed, and a reasonable Néel temperature is obtained. The model also gives a qualitatively correct temperature-dependence, but not magnitude, of the specific heat, and we find that the majority of the specific heat jump at the Néel temperature arises from the formation of a spin gap in the conduction electron spectrum.