Status of trivalent copper and charge-transfer excitons in high-TC cuprates

A chemical bonding approach based on tight-binding cluster and band calculations, taking into account on-site Coulomb repulsion (Hubbard U parameter) to differentiate doubly and singly occupied states, was applied to high- T C superconducting cuprates and related compounds. This work provides rational insight and explanations for issues such as (i) the actual oxidation number Cu (I+) for formally trivalent copper in oxides such as La 2Li 1/2Cu 1/2O 4, (ii) the dominant oxygen character of the doping holes in (CuO 2) ( n- ) planes, (iii) the Mott-Hubbard character of the insulator-to-metal transition triggered by hole doping, leading to an oxygen-to-copper charge transfer of avalanche type, (iv) the occurrence of an excitonic phase with anisotropic Frenkel-type excitons, (v) the role of Coulomb interactions between excitons and between doping holes and their exciton surroundings, and (vi) the on-time pairing of doping holes by means of an "excitonic glue".