Abstract

We review the effect of elastic and inelastic scattering on the normal state infrared conductivity, and describe modifications to the real and imaginary parts which result from a transition to an s-wave superconducting state. The zero frequency limit of the imaginary part is related to the temperature-dependent penetration depth and, at finite frequency, provides information about the superconducting gap. In the high T_c cuprates the gap appears to have d-wave symmetry. This profoundly modifies both the real and the imaginary parts of the optical conductivity. After describing these modifications we introduce the conductivity-derived scattering rate, which directly probes the inelastic scattering processes, and is much larger in the oxides than in conventional superconductors. This quantity is also significantly modified by the d-wave symmetry of the gap.