Sharp optical lines in rare-earth barium copper oxides

We report the results of a sensitive technique of optical spectroscopy applied to bulk and thin-film samples of the high-${\mathit{T}}_{\mathit{c}}$ superconductor R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ with R=Er, Ho, and Yb. Sharp crystal electric field lines were observed in these samples at 77 and 4.2 K due to impurities of less than 1% of the so-called ‘‘green phase,’’ ${\mathit{R}}_2$ ${\mathrm{BaCuO}}_5$. Expected strong magnetic dipole transitions in the superconductor were not observed, probably because of strain broadening of the energy levels involved. The method can reveal absorption lines corresponding to oscillator strengths of ${10}^{\mathrm{-}10}$, for expected linewidths, in opaque or thin-film samples.