The Ising-Kondo lattice with transverse field: an f-moment Hamiltonian for URu2Si2?

TL;DR

This paper investigates a theoretical model combining Ising-Kondo interactions and transverse fields to explain the magnetic and thermodynamic properties of URu2Si2, capturing its small ordered moments and specific heat behavior.

Contribution

It introduces a mean-field model for URu2Si2 that accounts for weak moments, small antiferromagnetic moments, and specific heat features, linking it to known models in certain limits.

Findings

Conduction electron magnetization responds logarithmically to f-moment formation.

Ordered moments in the antiferromagnetic state are anomalously small.

Most of the specific heat jump is due to a spin gap in the conduction electron spectrum.

Abstract

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 URu2Si2, 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, we show that the conduction electron magnetization responds logarithmically to f-moment formation, that the ordered moment in the antiferromagnetic state is anomalously small, and that the Neel temperature is of the order observed. The model gives a qualitatively correct temperature-dependence, but not magnitude, of the specific heat. The majority of the specific heat jump at the Neel 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.