ISING-KONDO LATTICE WITH TRANSVERSE FIELD - A POSSIBLE F-MOMENT HAMILTONIAN FOR URU2SI2

We study the phase diagram of the Ising-Kondo lattice with transverse magnetic field as a possible model for the weak-moment heavy-fermion c ompound URu2Si2, in terms of two low-lying f singlets in which the ura nium moment is coupled by on-site exchange to the conduction-electron spins. Ln the mean-field approximation for an extended range of parame ters at half-filling, we show that the conduction-electron magnetizati on responds logarithmically to f-moment formation due to nesting; the ordered moment in the antiferromagnetic state is anomalously small, an d the Neel temperature is of the order observed, though agreement for both of these quantities is not attained simultaneously. The model giv es a qualitatively correct temperature dependence, but not magnitude, of the specific heat. The majority of the specific-heat jump at the Ne el temperature arises from the formation of a spin gap in the conducti on-electron spectrum. We also discuss the single impurity version of t he model and speculate on ways to increase the specific-heat coefficie nt. In the limits of small bandwidth and of small Ising-Kondo coupling , we find that the model corresponds to anisotropic Heisenberg and Hub bard models, respectively.