Infrared studies of high Tc superconductors

The infrared properties of the high temperature superconductors are dominated by the strong temperature dependent absorption of the free carriers at low frequency and a temperature independent midinfrared band. Dramatic changes in reflectance do occur as the material becomes superconducting but these changes cannot be used to determine the energy gap, since as a result of the development of the superconducting condensate, changes in the real part of the dielectric function dominate. Also, since the relaxation rate of the free carriers is of the order of 200 cm−1 at 100 K the materials are near the clean limit below this temperature and we do not expect to see structure at the energy of the superconducting gap. The reflectance edge observed in many experiments at 450 cm−1 is caused by the onset of the midinfrared absorption which has the character of a direct particle-hole excitation rather than the Holstein scattering of the free carriers.