Chemical Geminal Charge Transfer, Vibronic Degeneracy Effects, and Doping Excitons in Superconductivity

A quantum chemical view of charge transfer related to the Jahn–Teller effect involving two-electron functions (geminals) is compared with BCS theory, the Cooper pair, and Bipolaron theory. For different isotope effects, we have proposed a Renner–Teller mechanism for cyclic crystals (instead of linear molecules) and a Jahn–Teller effect for degenerate vibronic states (instead of degenerate electronic states alone). To illustrate examples of special doping structure in superconductors, we consider (YBa2Cu3O7–0.25)4and (Nd2–0.125Ce0.125CuO4–0.625)2×4and employ exciton-like Covalon states to render an improved description of the superconductive mechanism.