Characteristic features of the copper-oxygen interaction, electron pairs, and the asymmetric π bond in high-Tc superconductors

The experimental data on the structural features and bond character in layers of high-T-c superconductors and related materials are analyzed. Correlations are found between T-c and the orthorhombic distortion parameter and between T-c and the incommensurability of the layers in the structure. It is pointed out that the electric field around the CuO2 plane is asymmetric. On the basis of this analysis it is proposed that the unusual properties of high-T-c superconductors are due to the mixed covalent-ionic bond character in the CuO2 layers, a consequence of which is the presence of electron pairs in the layer. It is shown that in the presence of asymmetry of the electric field around the CuO2 planes these pre-existing electron pairs can give rise to superconductivity via the asymmetric delocalized pi bond. (C) 1997 American Institute of Physics.