Optical signature of subgap absorption in the superconducting state of Ba(Fe1-xCox)2As2

The optical conductivity of Ba(Fe0.92Co0.08)(2)As-2 shows a clear signature of the superconducting gap but a simple s-wave description fails in accounting for the low-frequency response. This task is achieved by introducing an extra Drude peak in the superconducting state representing subgap absorption, other than thermally broken pairs. This extra peak and the coexisting s-wave response respect the total sum rule indicating a common origin for the carriers. We discuss the possible origins for this absorption as (i) quasiparticles due to pair breaking from interband impurity scattering in a two-band s(+/-)-gap symmetry model, which includes (ii) the possible existence of impurity levels within an isotropic gap model; or (iii) an indication that one of the bands is highly anisotropic.