Infrared Reflectivity of High‐Tc YBa₂Cu₃O7–x and Related Ceramics

Infrared reflectivity of several Y–Ba–Cu–O‐type ceramic samples with T c ≈ 90 K is measured from 10 to 4000 cm −1 at room temperature and from 10 to 650 cm −1 as a function of temperature. The room temperature spectrum of the best‐surface Y–Ba–Cu–O sample is fitted using the model for the dielectric function consisting of the sum of the Drude term and five classical phonon oscillators. Due to the strong anisotropy the Drude term belongs predominantly to the E ‖ c electric field orientation whereas the phonon modes solely to the E ⊥ c orientation (B lu modes). The low temperature spectra in all samples show a systematic reflectivity increase below ≈ 180 to 220 cm −1 in agreement with recent results of other laboratories. The interpretation of this feature is discussed in terms of the gap opening below T c . The smaller gap values than those obtained from tunneling experiments give an evidence for the strong gap anisotropy.