First Principles Investigation of the Electronic Structures of Copper Oxide Superconductors by the MCSCF Cluster Method

Electronic structures of La-Sr-Cu-0 and Nd-Ce-Cu-O compounds are investigated from first principles, taking CuO6, CuO4, Cu2On and Cu2O7 clusters, as a model. Many-body states are calculated in the presence of the correlation effect by the MCSCF-CI variational method. In the hole-doped CuO6 cluster we have shown that the ground state changes from 1Alg to 3Blg near the doping concentration of the onset of superconductivity, owing to the effect of the apical oxygens. While in the electron-doped CuO4 cluster the ground state is always 3Blg, in which the dopant electron is accommodated in the Cu 4s orbital. The effects of the doped ions are also discussed. In the undoped Cu2On and Cu2O7 clusters two electrons are localized at Cu sites, reflecting the strong correlation effect, and are coupled antiferromagnetically. The destruction mechanism of the antiferromagnetic ordering is shown to be different between in the hole-doped Cu2O11 and in the electron-doped Cu2O7 clusters.