X-ray absorption measurements on high-Tcsuperconductors: Cu-valence and cation-bond-length effects

We present the results of Cu and Y $K$-edge and rare-earth and Ba $L$-edge x-ray absorption measurements on a series of new quaternary high-$T_c$ superconducting compounds ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$ and $R{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ ($R=\mathrm{Y},\mathrm{E}\mathrm{r},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{H}\mathrm{o}$). The Cu $K$-edge results are shown to reflect the Cu valence state in these materials. The other measurements are discussed in the context of binary rare-earth oxides and the quantitative influence of bond lengths on continuum resonances in such spectra.