Abstract
Hubbard model may be used for cuprate high-T c superconductors (HTSC). For these HTSC, the Cu3d and 02p energy levels are close together. The topmost valence band is the oxygen band. Since 0 −2 ions cannot exist in free state, the oxygen Hubbard parameter U may depend on hole concentration z of oxygen ion. For sufficiently strong z-dependence of U, the Fermi sea of oxygen band may become unstable toward electron density fluctuations, leading to HTSC. Theoretical T c values of La 2−x(Sr, Ba)CuO 4 and YBa 2Cu 3O 7−x (maximum T c∼38 and 92°K) are consistent with experimental data for 02p-Cu3d separations of −0.4 and 1 eV. For T c∼250°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.
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