Electronic structure of YBa2Cu3O7 ceramics at the MP2 electron correlation level

The electronic structure of the superconducting ground state of YBa2Cu3O7 ceramics is calculated at the MP2 electron correlation level using the self-consistent embedded-cluster method. The [Cu3O12]14- quantum cluster is embedded in a finite array of point charges which produce the correct value of the Madelung potential at each cluster site. The results obtained reveal the strong influence of electron correlation effects on the charge distribution. The coupling between chains and planes is found to be essentially ionic while covalent bonding is predominant within both chains and planes. The symmetry of the holes on oxygens in the CuO2 planes is 2pσ in accordance with nuclear magnetic resonance and x-ray absorption experiments. A partially unoccupied state associated with the orbitals 3dz2-y2 and 3dx2 is found to be present on the copper ions. The unpaired spin is located on the Cu1 atom in the chains; in the CuO2 planes, the hypothetical Zhang-Rice singlet is indicated. (author)