Single bipolaronic Transition in Jahn-Teller Model

TL;DR

This paper studies a two-orbital Jahn-Teller model revealing a single bipolaronic transition that differs from Hubbard models, with implications for understanding electron-phonon interactions and metal-insulator transitions.

Contribution

It demonstrates a unified bipolaronic transition in a Jahn-Teller model, contrasting with orbital-selective transitions in Hubbard models, due to inter-orbital coupling effects.

Findings

Polaron crossover occurs at weaker coupling than bipolaron binding.

A single bipolaronic transition occurs even with different bandwidths.

Transition behavior is driven by inter-orbital Jahn-Teller coupling.

Abstract

We investigate the bipolaronic crossover and the pairing transition for a two-orbital model with Jahn-Teller coupling to a two-fold degenerate phonon mode. The evolution from weak to strong coupling is reminiscent of the behavior of the single-band Holstein model: The polaron crossover in which the electrons and phonons become strongly entangled occurs for a weaker coupling than the binding of bipolarons, which gives rise to a metal-insulator transition. Interestingly, a single bipolaronic transition takes place also when the two bands have significantly different bandwidths, as opposed to the case of repulsive Hubbard-like interactions for which an orbital-selective Mott transition has been reported. This behavior is related to the inter-orbital nature of the Jahn-Teller coupling.