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 (YBa{sub 2}Cu{sub 3}O{sub 7-0.25}){sub 4} and Nd{sub 2-0.125}Ce{sub 0.125}CuO{sub 4-0.625}){sub 2x4} and employ exciton-like Covalon states to render an improved description of the superconductive mechanism.