Effect of Crystal-Field Splitting and Inter-Band Hybridization on the Metal-Insulator Transitions of Strongly Correlated Systems

TL;DR

This paper explores how crystal-field splitting and inter-band hybridization influence the metal-insulator transition in a two-band Hubbard model, revealing different transition pathways depending on the splitting magnitude.

Contribution

It provides a detailed analysis of the Mott transition in a two-band system considering realistic effects like crystal-field splitting and hybridization, extending understanding beyond single-band models.

Findings

Large splitting leads to a two-step transition: from two-band to one-band metal, then to Mott insulator.

Small splitting causes a direct transition from two-band metal to Mott insulator with orbital polarization.

Finite-temperature effects can induce bad metallic behavior near the orbital polarization transition.

Abstract

We investigate a quarter-filled two-band Hubbard model involving a crystal-field splitting, which lifts the orbital degeneracy as well as an inter-orbital hopping (inter-band hybridization). Both terms are relevant to the realistic description of correlated materials such as transition-metal oxides. The nature of the Mott metal-insulator transition is clarified and is found to depend on the magnitude of the crystal-field splitting. At large values of the splitting, a transition from a two-band to a one-band metal is first found as the on-site repulsion is increased and is followed by a Mott transition for the remaining band, which follows the single-band (Brinkman-Rice) scenario well documented previously within dynamical mean-field theory. At small values of the crystal-field splitting, a direct transition from a two-band metal to a Mott insulator with partial orbital polarization is found, which takes place simultaneously for both orbitals. This transition is characterized by a vanishing of the quasiparticle weight for the majority orbital but has a first-order character for the minority orbital. It is pointed out that finite-temperature effects may easily turn the metallic regime into a bad metal close to the orbital polarization transition in the metallic phase.