Calculations of the far-infrared conductivity, which include inelastic as well as arbitrary amounts of elastic impurity scattering, are presented for a superconductor with nodes in the gap. The results are compared with the normal-state conductivity and with previous results for isotropic s-wave superconductors in the clean limit. The effect of adding impurity scattering is also considered, and differences and similarities between the response for s- and d-wave superconductors are noted. The results are compared with recent experimental data on a high-quality single crystal before and after radiation damage. The data are found to be inconsistent with isotropic s waves but display all the expected qualitative features for d waves.

• Received 28 November 1994

DOI:https://doi.org/10.1103/PhysRevB.51.11798