Charge Order in the Falicov-Kimball Model

TL;DR

This paper provides a non-perturbative analysis of charge order in the one-dimensional Falicov-Kimball model, revealing the roles of different electron scattering processes and describing phase transitions with an effective Ising model.

Contribution

It introduces a comprehensive effective Hamiltonian for charge order in the FKM and clarifies the roles of forward and backscattering electrons in phase formation.

Findings

Backscattering electrons induce crystalline phases.

Forward-scattering electrons lead to phase separation.

Critical line for phase transition matches numerical data.

Abstract

We examine the spinless one-dimensional Falicov-Kimball model (FKM) below half-filling, addressing both the binary alloy and valence transition interpretations of the model. Using a non-perturbative technique, we derive an effective Hamiltonian for the occupation of the localized orbitals, providing a comprehensive description of charge order in the FKM. In particular, we uncover the contradictory ordering roles of the forward-scattering and backscattering itinerant electrons: the latter are responsible for the crystalline phases, while the former produces the phase separation. We find an Ising model describes the transition between the phase separated state and the crystalline phases; for weak-coupling we present the critical line equation, finding excellent agreement with numerical results. We consider several extensions of the FKM that preserve the classical nature of the localized states. We also investigate a parallel between the FKM and the Kondo lattice model, suggesting a close relationship based upon the similar orthogonality catastrophe physics of the associated single-impurity models.