THEORY OF HIGH-T-C CUPRATE SUPERCONDUCTIVITY

Hubbard model may be used for cuprate high-T-c superconductors (HTSC). For these HTSC, the Cu3d and O2p energy levels are close together. Th e topmost valence band is the oxygen band. Since O-2 ions cannot exist in free state, the oxygen Hubbard parameter U may depend on hole conc entration z of oxygen ion. For sufficiently strong z-dependence of U, the Fermi sea of oxygen band may become unstable toward electron densi ty fluctuations, leading to HTSC. Theoretical T-c values of La-2-x(Sr, Ba)CuO4 and YBa2Cu3O7-x (maximum T-c similar to 38 and 92 degrees K) a re consistent with experimental data for O2p-Cu3d separations of -0.4 and 1 eV. For T-c similar to 250 degrees K of the epitaxial films, the separation may be 4 eV. Theory explains the universality of oxygen in cuprate HTSC and the absence of of isotope effects.