ELECTRON-POSITRON P-SPACE AND K-SPACE DENSITIES IN YBA2CU3O7-DELTA

We have performed an ultra-high precision measurement (5 x 10(8) coincidence counts) of the basal-plane electron-positron momentum density (MD) in well oxygenated, twin-free, single crystals of YBa2Cu3O7-delta. The anisotropies of the raw and processed MD spectra and LCW-transformed spectra (k-space densities) not only show the D2 symmetry appropriate to the untwinned crystals but, more importantly, unambiguously show a clear image of a major Fermi surface sheet. The form of the FS image is in substantial agreement with theoretical predictions of a GAMMA-X electron ridge section associated with states in the Cu-O chains.