Optical conductivity in ferropnictides with and without gap nodes

The real part of the optical conductivity of a superconductor with an extended s-wave gap as well as the corresponding reflectance and optical scattering rate are calculated within BCS theory. We consider the case when gap nodes exist in certain directions on the Fermi surface and when there is instead a finite spectral gap everywhere. When nodes exist they can be lifted by increasing disorder. Static residual impurities wash out anisotropy and produce a finite gap in all momentum directions before eventually becoming isotropic. This provides a mechanism for the emergence of an additional gap energy scale, the magnitude of which can be tuned through the control of the elastic scattering. Application to the ferropnictides is discussed within a two-band model with the aim of identifying signatures of nodes and of node lifting when several bands are involved.