COMPUTATIONAL STUDY OF HOLE SPECIES IN MODELS OF THE CUPRATE SUPERCONDUCTORS

We have performed Hartree-Fock calculations for copper-oxygen cluster fragments self-consistently embedded within a polarizable set of surrounding ions representing the rest of the crystal. Models for pure La2CuO4, La2CuO4 containing Sr2+ impurity substituted for La3+, and pure YBa2Cu3O7 were considered. We find that when a hole is added to the normal charge state of the cluster, it occupies Cu-O hybrid orbitals in the Cu-O plane regions of the crystal. Localized holes on a single oxygen ion can be formed, but this requires 0.2 eV or 1.4 eV more energy in the case of La2CuO4 or YBa2Cu3O7 models, respectively. In the case of Sr impurities substituted for La3+ in La2CuO4 clusters, the hole becomes localized on an axial oxygen ion. These results are presented and discussed in the light of experimental data.