Infrared look at the high Tc superconductors

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 Δ T c , ≈ 0.5 K) YBa 2 Cu 3 O 7 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 Bi 2 Sr 2 CaCu 2 O 8 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 .