What is the true charge transfer gap in parent insulating cuprates?

TL;DR

This paper proposes that the true charge transfer gap in parent insulating cuprates is significantly smaller than traditionally believed, affecting their electronic properties and understanding of high-temperature superconductivity.

Contribution

It introduces a revised estimate of the charge transfer gap and a unified framework for interpreting various experimental data on cuprates.

Findings

Charge transfer gap is 0.4-0.5 eV, not 1.5-2.0 eV.

Competition between 3d9 ground state and charge transfer state.

Evolution of CuO2 planes under doping explained.

Abstract

A large body of experimental data point towards a charge transfer instability of parent insulating cuprates to be their unique property. We argue that the true charge transfer gap in these compounds is as small as 0.4-0.5\,eV rather than 1.5-2.0\,eV as usually derived from the optical gap measurements. In fact we deal with a competition of the conventional (3d$^9$) ground state and a charge transfer (CT) state with formation of electron-hole dimers which evolves under doping to an unconventional bosonic system. Our conjecture does provide an unified standpoint on the main experimental findings for parent cuprates including linear and nonlinear optical, Raman, photoemission, photoabsorption, and transport properties anyhow related with the CT excitations. In addition we suggest a scenario for the evolution of the CuO$_2$ planes in the CT unstable cuprates under a nonisovalent doping.