Ising-Kondo lattice with transverse field: A possiblef-moment Hamiltonian forURu2Si2

We study the phase diagram of the Ising-Kondo lattice with transverse magnetic field as a possible model for the weak-moment heavy-fermion compound ${\mathrm{URu}}_2$ ${\mathrm{Si}}_2$, in terms of two low-lying f singlets in which the uranium moment is coupled by on-site exchange to the conduction-electron spins. In the mean-field approximation for an extended range of parameters at half-filling, we show that the conduction-electron magnetization responds logarithmically to f-moment formation due to nesting; the ordered moment in the antiferromagnetic state is anomalously small, and the Néel temperature is of the order observed, though agreement for both of these quantities is not attained simultaneously. The model gives a qualitatively correct temperature dependence, but not magnitude, of the specific heat. The majority of the specific-heat jump at the Néel temperature arises from the formation of a spin gap in the conduction-electron spectrum. We also discuss the single-impurity version of the model and speculate on ways to increase the specific-heat coefficient. In the limits of small bandwidth and of small Ising-Kondo coupling, we find that the model corresponds to anisotropic Heisenberg and Hubbard models, respectively. © 1996 The American Physical Society.