First Principles Investigation of the ElectronicStructures of Copper Oxide Superconductors bythe MCSCF Cluster Method

Electronic structures of La-Sr-Cu-O and Nd-Ce-Cu-O compounds are investigated from first principles, taking CuO 6 , CuO 4 , Cu 2 O 11 and Cu 2 O 7 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 CuO 6 cluster we have shown that the ground state changes from 1 A 1g to 3 B 1g near the doping concentration of the onset of superconductivity, owing to the effect of the apical oxygens. While in the electron-doped CuO 4 cluster the ground state is always 3 B 1g , in which the dopant electron is accommodated in the Cu 4s orbital. The effects of the doped ions are also discussed. In the undoped Cu 2 O 11 and Cu 2 O 7 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 Cu 2 O 11 and in the electron-doped Cu 2 O 7 clusters.