Charge Transfer Excitons and the Metal-Insulator Transition in the High Temperature Superconductors

TL;DR

This paper investigates how nearest neighbour Coulomb repulsion influences charge transfer excitons and the metal-insulator transition in high-temperature superconductors using a one-dimensional copper-oxide chain model.

Contribution

It demonstrates the emergence of charge transfer excitons as lowest energy excitations and analyzes the role of Coulomb repulsion near the transition, validating the particle-hole symmetry.

Findings

Charge transfer excitons become the lowest excitations above a critical Coulomb repulsion.

Nearest neighbour repulsion creates in-gap states and pins the Fermi level upon doping.

Good particle-hole symmetry is observed near the transition at V=0.

Abstract

The effects of the nearest neighbour Coulomb repulsion, V, are considered in the one dimensional copper-oxide chain using the modified Lanczos method. Above a critical value of V we find that charge transfer excitons are the lowest lying energy exitations in the insulating phase. Close to the metal-insulator transition and $V=0$ there is good `particle-hole' symmetry, showing that the mapping from a two band model to a one band model is appropriate. The effect of the nearest neighbour repulsion is to create states in the gap and to pin the Fermi level upon hole doping.