Interplay of Orbital Degeneracy and Superconductivity in a Molecular Conductor

TL;DR

This paper investigates how orbital degeneracy influences superconductivity in a molecular lattice model with Jahn-Teller interactions, revealing an intersite pairing mechanism absent in non-degenerate models, and maps the phase diagram across coupling regimes.

Contribution

It introduces a novel intersite pairing mechanism driven by orbital degeneracy in a Jahn-Teller molecular model, analyzed through analytical and mean-field methods.

Findings

Orbital degeneracy induces intersite pairing absent in non-degenerate models.

Superconductivity stability region mapped using BCS theory.

Phase diagram computed for weak and strong electron-phonon coupling in one dimension.

Abstract

We study electron propagation in a molecular lattice model. Each molecular site involves doubly degenerate electronic states coupled to doubly degenerate molecular vibration, leading to a so--called E-e type of Jahn-Teller Hamiltonian. For weak electron-phonon coupling and in the anti-adiabatic limit we find that the orbital degeneracy induces an intersite pairing mechanism which is absent in the standard non-degenerate polaronic model. In this limit we analyse the model in the presence of an additional on-site repulsion and we determine, within BCS mean field theory, the region of stability of superconductivity. In one dimension, where powerful analytical techniques are available, we are able to calculate the phase diagram of the model both for weak and for strong electron-phonon coupling.