Thin films of superconducting Y B ~& u ~O , -~ (T, = 90 K), deposited on A1203, have been used to study reflection and absorption from 0.15 to 6 eV. Values for charge transfer gaps are found near 4 and 5eV. In the weak-absorption region below 4eV there appear broad maxima near 0.6, 1.5 and 3 eV-very similar to the well-known d-d transitions in the magnetic oxides of the 3d transition metals.Recent reflection and transmission measurements on the high-T, superconductor Y B ~& U ~O ~-~ exhibit a Drude-like absorption down to low energy and the lack of other absorption features in the energy range between 0.1 and 3 eV [l-51. These results were in contrast to some earlier work on sintered samples of Laz-,Sr,Cu04 and YBa2Cu307-8, where the reflectance data appeared to exhibit some structure in the range between 0.2 and 3eV. In particular, an absorption maximum near 0.5eV[6,71 has been derived from a Kramers-Kronig analysis of the reflectance spectra. Yet from the more recent data, in particular from the measurements on highly oriented thin films, it has been concluded that no sharp excitations exist which would qualify for the intermediate boson in the coupling mechanism for the superconductivity in the high-Tc cuprates [ll. In this report we present new spectroscopical data, in the energy range from 0.15 eV up to 6 eV both in reflection and transmission. The data were taken at 300 K on thin films of superconducting Y B ~& U ~O ~-~ with 6% 0, comparable in optical quality to those of ref. [l]. We find weak absorption in an energy range up to = 4 eV, followed by strong absorption at higher energies. In addition, we observe three pronounced absorption maxima in the range between 0.5 and 3 eV which we interpret as valence conserving d-d transitions of CU++.
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