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 $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ 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 $x=0.77\mathrm{}$.