POSITRON TRAPPING AT DEFECTS IN COPPER-OXIDE SUPERCONDUCTORS

Positron states and lifetimes at defects in the copper oxide superconductors La2-xSrxCuO4, YBa2Cu3O7-x, and Bi2Sr2CaCu2O8+x are calculated with use of the superposed-atom model. In the Bi2Sr2CaCu2O8+x compound, we find that the smaller metal-ion vacancies appear to only bind positrons weakly, while missing oxygens do not trap positrons. In contrast, metal-ion vacancies in La2-xSrxCuO4 and YBa2Cu3O7-x bind positrons by approximately 1 eV, and oxygen-related defects appear to be the weak-binding sites in these materials. The sites that bind positrons only weakly, by energies approximately k(B)T, are of particular interest in view of the complex temperature dependences of the annihilation characteristics that are observed in these materials.