Infrared studies of high Tc superconductors

The authors discuss the IR reflectance of epitaxially grown laser-deposited films of YBa2Cu3O7-δ and of single crystals of Bi2Sr2CaCu2O8. Below Tc, both show zero absorption at low frequencies (120-140 cm for YBa2Cu3O7-δ and 250-300/cm for bisco). At higher frequencies (420-470/cm for YBa2Cu3O7-δ and 470-520/cm for bisco), there is a characteristic knee in the reflectance and a corresponding minimum in σ1(ω). Although these features are revealed clearly below Tc, it is concluded that neither should be assigned to the superconducting gap. Both are present in the normal state. Furthermore, the clean limit holds, making the gap difficult to see by IR absorption. There are two other reasons why the authors believe that neither of the obvious IR features is the superconducting gap. First, they occur at nearly the same energy in reduced Tc crystals and Tc ≈ 91 K films and thus do not scale with Tc. Second, they occur at different frequencies in YBa2Cu3O7 and Bi2Sr2CaCu2O8, although Tc is nearly identical for these materials. Alternate interpretations are that the mid-IR band is a direct electronic absorption, whose energy is determined by the material, and that the minimum is a phonon feature.