Negative-U centres model of high-Tc superconductivity in metal oxides

Negative-U centres model of superconductivity, proposed by the present authors for low-coordination materials describes well the high-T-c superconducting state in metal oxides. The model explains both the high transition temperature T-c and specific (with a maximum) dependence of T-c on chemical composition of samples, observed in many metal oxides. In the framework of the model the superconducting current arises through Bose condensation of bipolarons in a band constructed from negative-ii centre states. A quantitative description is provided for the example of Y1Ba2Cu3Ox metal oxides. Account is taken of the low-coordination Cu-O chains and the disproportionation reaction 2Cu(2+) <-> Cu++Cu3+, which occurs with increasing x and hence increasing 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.