"Ridge" Fermi Surface Study in Metallic (R)Ba$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7-δ}$ (R = Y, Dy) and in Oxygen Deficient YBa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7-δ}$ by Positron Annihilation

We present a detailed positron 2D-ACAR measurements study of the "ridge" Fermi surface of (R)Ba$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7-δ}$ (R = Y, Dy) compared to full-potential linearized augmented plane wave and linear muffin-tin orbital calculations. From different 2D-ACAR projections measured in DyBa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7-δ}$, a 3D model of the momentum density of the ridge was established and is in good agreement with LMTO calculations of YΒa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7}$, confirming the description of the ridge Fermi surface by local density approximation calculations. The ridge is also studied in oxygen deficient YΒa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7-δ}$. With growing δ, the ridge disappears without changing in width. This is consistent with full-potential linearized augmented plane wave calculations of YΒa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{6.5}$, and supports the phase separation model of Mesot et al.