ELECTRONIC-STRUCTURE OF TWIN BOUNDARIES IN YBA2CU3O7

The electronic structure of a twin boundary (TB) in YBa2Cu3O7 is evaluated from ab initio band-structure calculations. Two models are considered: The first assumes the ideal TB atomic configuration. The oxygen-disordered TB model, on the other hand, assumes that in proximity to the TB the oxygen atoms relocate randomly to vacant lattice sites such that the Cu-O bond length is preserved. The calculations have been carried out using 52-atom and 78-atom supercells. For the ideal TB model, the TB presence gives rise to a quasicontinuum of localized states near E(F). These states do not contribute to the conductivity. For the oxygen-disordered TB model, two representative oxygen dislocations are considered: One in which a chain oxygen [O(1)] is displaced in the basal plane, and the other in which a Cu-plane oxygen [O(2), O(3)] is up-displaced into the yttrium plane. For both configurations we find conductive pathways with energy near E(F). These pathways are comprised of zigzag Cu-O chains, oriented in all directions parallel to the TB plane. This model implies, in particular, percolative conduction in the c-axis direction parallel to the TB planes.