True Charge-Transfer Gap in Parent Insulating Cuprates

Abstract

A large body of experimental data points toward a charge-transfer (CT) instability of parent insulating cuprates to be their unique property. It is argued that the true CT 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 (3d9) ground state and a CT state with the formation of electron-hole dimers which evolves under doping to an unconventional bosonic system. My 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 related with the CT excitations. In addition I suggest a scenario for the evolution of the CuO2 planes in the CT unstable cuprates under nonisovalent doping. © 2011 American Physical Society.