Competing Correlated Insulators in multi-orbital systems coupled to phonons

TL;DR

This paper investigates how electron-electron interactions and phonon couplings influence different types of Mott insulators in a multi-orbital system, revealing a rich phase diagram with competing phases.

Contribution

It demonstrates the coexistence and competition between electron-electron and electron-phonon interactions in multi-orbital Mott insulators, identifying distinct phases and a transitional boundary.

Findings

Identification of a high-spin Mott insulator stabilized by Hund's coupling.

Discovery of a low-spin Mott-bipolaronic insulator favored by phonons.

Existence of an intermediate phase at the boundary between the two insulators.

Abstract

We study the interplay between electron-electron interaction and a Jahn-Teller phonon coupling in a two-orbital Hubbard model. We demonstrate that the e-ph interaction coexists with the Mott localization driven by the Hubbard repulsion U, but it competes with the Hund's coupling J. This interplay leads to two spectacularly different Mott insulators, a standard high-spin Mott insulator with frozen phonons which is stable when the Hund's coupling prevails, and a low-spin Mott-bipolaronic insulator favoured by phonons, where the characteristic features of Mott insulators and bipolarons coexist. The two phases are separated by a sharp boundary along which an intriguing intermediate solution emerges as a kind of compromise between the two solutions.