Abstract

The optical properties of high T_c films, 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 T_c the reflectance of high quality (T_c~ 88 K and ΔT_c, ≈ 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 T_c (2), do not shift when T_c decreases due to oxygen deficiency, and (3) occur at a substantially higher energy in T_c ≈ 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 T_c.

© 1989 Optical Society of America