Chemical aspects and the correlated electronic structure of superconductor oxocuprates

Abstract

An analysis of structure and chemical binding of the superconducting oxocuprates is made resulting in approximate groundstate configurations of their anionic complexes. A simple analytic tight-binding model is introduced which allows to consider self-consistently the interplay of charge transfer and strong on-site correlation (self-interaction correction to the mean-field theory). Besides yielding the charge transfer gap, it naturally explains the collectivization of strongly localized electrons at integer occupation numbers. This is due to occupation number frustration by bonding charge transfer and also explains heavy-fermion behaviour near integer valences. An analytic LCAO analysis of the electronic structure of the oxocuprates yields a spin-polarized groundstate with one copper 3d-hole and about one half of an oxygen 2p-hole per unit cell in the undoped [CuO 2] 2− -layer Whereas the level of the missing 3d-electron is high above the Fermi level, the conduction gap is from oxygen holes to conduction states with oxygen 2p-admixture. Doping this layer by extracting electrons (e.g. by transferring them into an overlapping chain band in the case of the 1 2 3 structure) leads to metallic oxygen hole conductivity and at the same time to collectivization of the copper d-electrons. All features of this picture are confirmed by the known results of polarized core-level spectroscopy. As a further common peculiarity of all superconducting oxocuprates a strong anharmonicity of one special oxygen atom which lies outside of the CuO 2-layers but which is responsible for doping them, is discussed.