Optical properties of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ thin films

Temperature-dependent reflectance and transmittance measurements (1250-8000 ${\mathrm{cm}}^{-1\mathrm{}}$) of thin Y-Ba-Cu-O films are reported and the complex dielectric response $\varepsilon \left(\omega \right)$ is directly determined from the measured data. We find that $\varepsilon \left(\omega \right)$ is more complex than predicted by any of the existing models, in particular the marginal and the nested Fermi liquids, as well as the two-component approach (Drude and midinfrared terms). A phenomenological analysis reveals either a two-fluid or a complicated one-fluid model, which is yet to be explained theoretically. The near-infrared (NIR) transmittance is weakly temperature dependent in the normal state, but almost temperature independent in the superconducting state. Both the normal and the superconducting states are anomalous and non-Fermi-liquid-like with renormalization of energies much higher than any superconducting energy gap. Such observations have previously been reported only in powder absorbance measurements but not in single-crystal and thick-film reflectance data. We present a quantitative analysis in which the powder absorbance results are recovered from thin-film data. The NIR response is argued to be of crucial importance for the understanding of the high-temperature superconductors in both the normal and the superconducting states.