Superconductivity on a YBa2Cu3O7 lattice

The interaction between two electrons near a conducting and polarizable plane is found to be attractive at large separations if the polarizability is large enough. Assuming such an interaction, a variational Bardeen-Cooper-Schrieffer superconducting state is constructed based upon the YBa2Cu3O7 tight-binding electronic structure. There is no charge-density-wave instability, nor s-wave superconducting instability. However, allowing the phase of the order parameter to vary along the Fermi lines separates the paired electrons sufficiently to sample only the attractive electron-electron interaction. As the polarizability is increased from zero over a reasonable range, 900 superconductivity is first obtained in CuO2 planes. The corresponding Cooper-pair electrons are related by a reflection symmetry of the Brillouin zone rather than the usual k to -k inversion. The paired electrons move along third-neighbor copper rows. This particular l = 3 state would have triplet pairing but neighboring l = 2 and l = 4 superconducting states have singlet pairing. Superconductivity, with a smaller gap, simultaneously arises on the CuO3 chains with Cooper-pair electrons on second-neighbor chains. The ratio 2Δ0/k/sub B/T/sub c/ is found to be 5.08 for the planes. Predicted values for a range of superconducting properties of the 900 state are in reasonable accord with experiment