High-frequency Raman scattering in the superconducting and semiconducting phases of ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_x$-type single crystals and ceramics

We report the high-frequency Raman spectra ($\Delta \omega >1000\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$) of ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_x$ and various analogs, measured at room temperature. In semiconducting single crystals and ceramics of ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_x$ ($x\approx 6$) and semiconducting single crystals of ${\mathrm{Sr}}_2\mathrm{Y}{\mathrm{Cu}}_{2.4}{\mathrm{Al}}_{0.6}{\mathrm{O}}_x$ we find a peak at 1220 ${\mathrm{cm}}^{-1\mathrm{}}$ ($A_{1g}$ symmetry) and a broadband at 2600 ${\mathrm{cm}}^{-1\mathrm{}}$ ($B_{1g}$). The 1220-${\mathrm{cm}}^{-1\mathrm{}}$ peak can be interpreted as scattering from excitations of single spins near defects or two-phonon scattering. The 2600-${\mathrm{cm}}^{-1\mathrm{}}$ band is attributed to scattering from spin-pair excitations. In the spectra of superconducting ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_x$ crystals and ceramics ($x\approx 7$) these bands are not present. In all samples, semiconductors and superconductors, we observe flat and featureless spectral intensity ($A_{1g}$ symmetry) extending up to $\Delta \omega =5000\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, the origin of which is not understood at present. The similarity between the high-frequency spectra of ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_x$ ($x\sim 6$) and ${\mathrm{Sr}}_2\mathrm{Y}{\mathrm{Cu}}_{2.4}{\mathrm{Al}}_{0.6}{\mathrm{O}}_x$ [Al on Cu(1)-chain sites] shows that the magnetic scattering is determined by the structure of the Cu(2)-O planes and is not affected by the distance between the planes or by the Cu(1)-chain structure.