title: Status of trivalent copper and charge-transfer excitons in high-TC cuprates.
abstract: 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".
authors.affiliation: ICMCB, CNRS, Bordeaux I University, 87 Av. Dr. A. Schweitzer, 33608 Pessac Cedex, France. pouchard@icmcb-bordeaux.cnrs.fr
authors.forename: Michel
authors.initials: M
authors.lastname: Pouchard
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Doumerc
authors.forename: Antoine
authors.initials: A
authors.lastname: Villesuzanne