Direct observation of Fermi surface in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We have performed a high-precision measurement (5×${10}^8$ coincidence counts) of the basal-plane electron-positron momentum density in well oxygenated, twin-free, single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The raw, processed, and K-space reduced spectra unambiguously show a clear image of a major Fermi surface sheet. The form and profile of that image are in substantial quantitative agreement with theoretical predictions of a Γ-X electron ridge section associated with states in the CuO chains.