Negative-U centres model of high-T-c superconductivity in metal oxides

Negative-U centres model of superconductivity, proposed by the present auth ors for low-coordination materials describes well the high-T-c superconduct ing state in metal oxides. The model explains both the high transition temp erature T-c and specific (with a maximum) dependence of T-c on chemical com position of samples, observed in many metal oxides. In the framework of the model the superconducting current arises through Bose condensation of bipo larons in a band constructed from negative-ii centre states. A quantitative description is provided for the example of Y1Ba2Cu3Ox metal oxides. Accoun t is taken of the low-coordination Cu-O chains and the disproportionation r eaction 2Cu(2+) <-> Cu++Cu3+, which occurs with increasing x and hence incr easing degree of Cu-O bond covalence. The differently charged states D-+,D- 0,D-- of the negative-U centre correspond to CU3+,2+,+ copper ion states.