Theoretical considerations on high Tc superconductivity

Abstract

In the Introduction of the paper the different theoretical models for the prediction of high T, superconductivity, which were done before its dimvery in ceramic materials, are reviewed. Atter this a number of models to interpret the experimentally found high superconductive transition temperature are mentioned. In the second part of the paper the results of Hartree-Fock and DODS band structure calculations on (Cu-O), chains and on the 2-dimensional Cu-0 plane are presented. We have found that in the 2D case the Fermi level is in a broad band due to 0 2p orbitals, while there are narrow Cu 3d bands in the valence band region. This gives the possibility that the polarization of the localized electrons in the narrow bands provide an effective attraction between the mobile electrons in the broad band which can lead to formation of Cooper pairs and therefore also to a superconductive state. The DODS calculation of the (Cu-O), chain leads to a spin alternation on the 0 atom in agreement with the experimentally found antiferromagnetic behavior of the ceramic materials. This gives rise to a spin-polaron (through a spin-flip) mechanism of superconductivity if positive holes are formed. Finally it is discussed how the band structures of the ceramic materials could be improved by taking into account charge transfer between the planes (via different cluster calculations), the effect of the Madelung potential of the crystal and last but not least correcting the bands for correlation effects using a generalized electronic polaron model.