Optical signature of sub-gap absorption in the superconducting state of Ba(Fe,Co)2As2

Abstract

The optical conductivity of Ba(Fe${}_{0.92}$Co${}_{0.08}$)${}_2$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 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 $s_{\pm }$ 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.

5 pages, 4 figures

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• Optical observation of spin-density-wave fluctuations in Ba122 iron-based superconductors
• Optical properties of the optimally doped Ca${}_{8.5}$La${}_{1.5}$(Pt${}_3$As${}_8$)(Fe${}_2$As${}_2$)${}_5$ single crystal
• Infrared probe of the gap evolution across the phase diagram of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$
• THz electrodynamics of BaFe${}_{1.91}$Ni${}_{0.09}$As${}_2$ film analyzed in the framework of multiband Eliashberg theory
• Two-band conductivity of a FeSe${}_{0.5}$Te${}_{0.5}$ film by reflectance measurements in the terahertz and infrared range
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