Interacting hole-spin model for oxide superconductors

It is shown that high-temperature superconductivity in cuprous oxide can be understood in terms of an interacting hole-spin model with superexchange and Kondo-type exchange interactions. Many important properties will be discussed, with special emphasis on the s-wave nature of Cooper pairs, reentrant behavior of Tc, finite isotope effect, and for YBa2Cu3O7-x, strong correlation between superconducting transition temperature and Cu(1)-O(4) bond length. The isotope effect has an origin quite different from the Bardeen-Cooper-Schrieffer theory, and the calculated value of its coefficient a is consistent with the experimental value 0.02. The main results of NMR studies, i.e. the s-wave nature of Cooper pairs as observed in O17 nuclei, and some puzzling aspects of the Cu NMR relaxation rates are discussed on the basis of the proposed model. A possible explanation to the anomaly in Cu NMR is given. It is predicted that oxygen ions should show a small displacement below the superconducting transition temperature Tc.