RIDGE FERMI-SURFACE STUDY IN METALLIC (R)BA2CU3O7-DELTA (R=Y, DY) AND IN OXYGEN-DEFICIENT YBA2CU3O7-DELTA BY POSITRON-ANNIHILATION

We present a detailed positron 2D-ACAR measurements study of the ''ridge'' Fermi surface of (R)Ba2Cu3O7-delta (R = Y, Dy) compared to full-potential linearized augmented plane wave and Linear muffin-tin orbital calculations. From different 2D-ACAR projections measured in DYBa2CU3O7-delta, it 3D model of the momentum density of the ridge was established and is in good agreement with LMTO calculations of YBa2Cu3O7, confirming the description of the ridge Fermi surface by local density approximation calculations. The ridge is also studied in oxygen deficient YBa2Cu3O7-delta. With growing delta, the ridge disappears without changing in width. This is consistent with full-potential linearized augmented plane wave calculations of YBa2Cu3O6.5, and supports the phase separation model of Mesot et al.