Cluster calculations of the electronic structure of copper oxide superconductors

A semiempirical INDO model suitable for examination of the transition metal complexes is used to study the electronic structure of various clusters representing the La-Sr-Cu-O and Nd-Ce-Cu-O types of the high-Tc superconductors. The clusters are stabilized by embedding in an appropriate Madelung field. The results show a convergent picture independent of the cluster sizes. In the undoped clusters, all copper sites have a ∼ d9 configuration with one unpaired spin coupled antiferromagnetically to the spin of adjacent Cu sites. Fitting the resulting energies to the Heisenberg spin Hamiltonian, the superexchange J values obtained were in excellent agreement with experiments. The hole carriers are mainly of planar O character, while the electron carriers are of Cu character