Jahn-teller (reverse sign) mechanism for superconductive pairing

Band structure calculations and some recent experiments indicate the involvement of both d_θ(z²) and d_ε{lunate}(x²-y²) type states in the carrier states of cuprate superconductors. Electron-phonon coupling, or an excitonic process having similar symmetry properties, leads to a d_θ-d_ε{lunate} pairing mechanism that has the same form as, but a reverse sign to that in the BCS theory so that states that lie close in energy are repelled and those that lie farther apart are attracted. Cooper pairs and gap parameters are derived and are found to be relatively stable against lattice distortion, to be of short range and to exhibit various kinds of isotope effects. The reverse sign model shows stability in situations where d_θ and d_ε{lunate} states are degenerate or are split, the latter provided that the electron-phonon coupling is sufficiently strong so that the superconducting gap energies exceed the splitting.