Analysis of high- Tc superconductivity in modified copper oxides in terms of superexchange cooper pair formation via the oxygen anions

Abstract

We propose, within the BCS framework, a quantitative model for superconductivity in modified copper oxides, adopting a superexchange mechanism formulated and applied in earlier work on magnetic ordering and exchange coupling constants in insulating magnetic solids containing transition-metal cations. In all previous systems of magnetic insulators, superexchange is a consequence of the enhancement of permutation symmetry for a system of two cations and one (or more) diamagnetic anion(s) relative to the sum of the two separate subsystems of cations and anions. In the modified copper oxides, excitonic transitions Cu 3+O 2−↔Cu 2+O 1− occur upon doping or valency imbalance, with delocalized electron-hole states. Two delocalized electrons can give rise to Cooper pair formation through indirect exchange via the O 2− ions of the lattice. In evaluating the coupling, a many-electron formalism is employed throughout, with strong electron-electron correlations. Following in part an analysis given earlier [Phys. Rev. B 26 (1982) 3656] for superconductivity in simple (i.e. non-transition) metals, the scattering-matrix elements in the BCS formalism are evaluated numerically. The model is capable of explaining high critical temperatures, the occurrence of high- T c superconductivity close to a metal-to-insulator transition, as well as very small coherence lengths.