Competition between crystalline electric field singlet and itinerant states of f electrons

TL;DR

This paper proposes a new phase transition in lattice fermion systems with f^2 configurations, showing a first-order transition between itinerant and localized states, contrasting with traditional Mott transitions.

Contribution

It introduces a novel phase transition mechanism between itinerant and localized f-electron states in lattice systems with simplified f^2 configurations, analyzed via mean-field approximation.

Findings

First-order phase transition between itinerant and localized states.

High-temperature insulator-to-metal transition in half-filled systems.

Comparison with two-impurity Kondo systems with f^1 configuration.

Abstract

A new kind of phase transition is proposed for lattice fermion systems with simplified f^2 configurations at each site. The free energy of the model is computed in the mean-field approximation for both the itinerant state with the Kondo screening, and a localized state with the crystalline electric field (CEF) singlet at each site. The presence of a first-order phase transition is demonstrated in which the itinerant state changes into the localized state toward lower temperatures. In the half-filled case, the insulating state at high temperatures changes into a metallic state, in marked contrast with the Mott transition in the Hubbard model. For comparison, corresponding states are discussed for the two-impurity Kondo system with f^1 configuration at each site.