A positron study of the electronic structure of YBa2Cu3O7- delta

The authors have measured the two-dimensional electron-positron momentum density (resolved momentum components along (100) and (010)) for single crystals of YBa2Cu3O7- delta at a significantly higher level of statistical precision for this orientation than that embraced in earlier published works. They have found in their data structures suggesting that the positron annihilates predominantly on oxygen sites in the copper-oxygen chains. These structures are also present in the results of a companion ligand field calculation and arise from spatial variations in the electron and positron wavefunction overlaps. Even at the present level of statistical precision, the authors are unable to argue for or against the existence of a Fermi surface.