ORDERING IN HIGH-TC SUPERCONDUCTING SYSTEMS - EFFECTIVE PAIR INTERACTIONS IN Y-BA-CU-O

Effective pair interactions are examined in the nonstoichiometric as well as the Zn-doped Y-Ba-Cu-O systems in terms of the real-space-scattering-cluster coherent-potential approximation. Large negative pair energies are found for the first and second in-chain neighbors if vacancies occupy the O(1) sublattice, while concomitantly rather small positive pair energies characterize the interchain neighbors. Negative pair energies are also found for a disordered O(2) sublattice if a copper atom bridges the two sites. This result is in good agreement with experiment and with other calculations. For the Zn doped copper sublattices no such clear-cut behavior is observed with regard to the possible ordering of sites occupied by Cu or Zn atoms.