The optical properties of high
Tcfilms, crystals, and bulk samples have been studied between 15 and 300 K, over the 50-40,000-cm−1 (6-meV-5-eV) range. The normal state spectra have a low frequency Drude contribution and a strong midinfrared absorption; Drude parameters correspond to the clean limit (1/τ < 2Δ or l >ξ ).Below Tcthe reflectance of high quality (Tc~ 88 K and Δ Tc, ≈ 0.5 K) YBa2Cu3O7 films prepared by laser deposition is 100% ± 1% below 150 cm−1. There is an onset of absorption at 150 cm-1 and structure in the 450-500-cm−1 region. These, however, are not the superconducting energy gap because they (1) are present above Tc (2), do not shift when Tcdecreases due to oxygen deficiency, and (3) occur at a substantially higher energy in Tc≈ 85 K Bi2Sr2CaCu2O8 crystals. Instead, they are temperature independent midinfrared excitations. The superconducting gap may (perhaps) be inferred (with rather wide bounds) from the temperature dependence of certain infrared vibrational features, which narrow below Tc.