POSITRON-ANNIHILATION IN YBA2CU3O7-DELTA - A CLEARER PICTURE

We have measured the 2D-ACAR Spectrum in untwinned YBa2Cu3O7-delta cry stals both conducting and insulating. These new two-particle momentum distributions are consistent with the prior work of West et al. (1) an d Smedskjaer et al. (2). They are also in remarkable agreement with FL APW band structure calculations. Doping of the samples with Ni, Zn, Al produces characteristic changes. Lifetimes results agree with those o f Hentrich et al. (3); they are extended to untwinned samples.