Optical properties of YBa2Cu3O7-δ thin films

Temperature-dependent reflectance and transmittance measurements (1250-8000 cm-1) of thin Y-Ba-Cu-O films are reported and the complex dielectric response (ω) is directly determined from the measured data. We find that (ω) 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.