LATTICE RELAX, CHARGE-TRANSFER, AND MADELUNG POTENTIAL-ENERGY IN YBCO SYSTEMS

Abstract

We have calculated Madelung potential energies by Ewald's method in YBa2Cu3-xMxO7-delta system (M = Ni, Zn, Co, Fe, etc.). The Madelung potential energy, long-range Coulomb interaction, is expected to give a major contribution in these partially ionic copper-oxide superconductors, Concerning site preference of dopant ions, the results from the calculated Madelung energies can not only interpret the site preference of dopant ions substituting for copper sites, which are in accordance with most experimental results, but also illustrate strongly that the holes transfer from sheets to chains on substitution. The apical oxygen is a balancing bridge of charge-transfer, and leads to lattice variation of Cu-0 bond length which is due to long-range Coulomb force. These suggest that this kind of charge-transfer channel is one of important factors to affect T(C) values.