Orientation-dependent x-ray-absorption near-edge studies of high-Tcsuperconductors

Orientation-dependent x-ray-absorption near-edge measurements have been made on the Cu and Y $K$ edges in oriented ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_{4}$ and $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ superconductors. The orientation dependence of the Cu edges is quite similar to that for other ${\mathrm{Cu}}^{2+}$ planar compounds, and allows a clear identification of a number of near-edge features. The results also demonstrate the strong similarity of the Cu-O planes in the two materials, and clearly establish that ${\mathrm{Cu}}^{1+}$ is being formed on the linear-chain sites for $x=0.77$.