Twin-boundaries exciton model for high critical temperature superconductors

We propose a model for superconductivity in the 123 materials which attributes a central role to the twin boundaries. These boundaries, parallel to the {110} planes, are always observed in the superconducting phases. The model assumes that the twin boundaries support interfacial states at the Fermi level. The electrons trapped in these states condense into Cooper pairs due to attraction mediated predominantly by electron-hole transitions (excitons) between the Cu-O planes and the Cu-O chain bands. A representative two band model yields an analytic estimate of the order of the experimental value. Some implications of the model are briefly discussed.