Infrared studies of high Tc superconductors

Two open questions about the IR properties of high-Tc superconductors are the superconducting energy gap and the nature of the strong mid-IR absorption. This paper discusses 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-1 for YBa2Cu3O7-δ and 250-300 cm-1 for bisco). At higher frequencies (420-470 cm-1 for YBa2Cu3O7-δ and 470-520 cm-1 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 the authors conclude that neither should be assigned to the superconducting gap. Both are present in the normal state. Furthermore, the authors demonstrate that the clean limit holds, making the gap difficult to see by IR absorption