Optical signature of sub-gap absorption in the superconducting state of Ba(Fe,Co)2As2
Abstract
The optical conductivity of Ba(Fe0.92Co0.08)2As2 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 sub-gap 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.