The electron-positron momentum density and Fermi surface of YBa2Cu3O7-x

The search for evidence of Fermi surface in the cuprate high temperature superconductors has provided the greatest challenge for the 2D ACAR technique thus far in its relatively short history. Much of the effort in both theory and experiment has focussed on the Yttrium compound where the small size of samples, twinning and structural oxygen defects have made the experiments difficult, complicated the interpretation of results and led to some confusion. With the advent of good twin-free crystals that confusion has now been resolved. In this paper we shall briefly review the earlier experiments on twinned crystals and then describe in more detail, and in the context of the contemporary bandstructure theory, our recent clear resolution of a major Fermi surface section in untwinned crystals of YBa2Cu3O7-X