Charge Transfer Excitons and Possible Exitonic Pairing in the Extended Three Band Hubbard Model

TL;DR

This paper uses exact diagonalization to study the extended Hubbard model, revealing how nearest neighbor interactions lead to excitons and potential pairing mechanisms relevant for superconductivity.

Contribution

It demonstrates the role of copper-oxygen repulsion in forming excitons and suggests a link between excitonic excitations and superconducting correlations near phase separation.

Findings

Nearest neighbor repulsion induces Frenkel excitons in the insulating regime.

Charge-transfer and density-density correlations diverge near phase separation.

Softening of dynamic modes indicates excitonic pairing potential.

Abstract

Exact diagonalisations of the extended Hubbard model are performed. In the insulating regime it is shown that the nearest neighbour copper-oxygen repulsion, $V$, leads to Frenkel excitons in the charge transfer gap at values of $V$ of the order of copper-oxygen hybridisation, $t$. In the metallic regime it is shown that the static charge-transfer and density-density correlation functions diverge as a function of $V$, indicating a charge-tansfer instability and phase separation. This is accompanied by a softening of the $q \to 0$ mode of the dynamic correlation functions which is associated with the excitonic excitations responsible for the superconducting correlations observed in the proximity of the phase separation boundary of ref. \cite{cv95b}.