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

TL;DR

This paper investigates the optical conductivity of Ba(Fe,Co)2As2 in its superconducting state, revealing sub-gap absorption features that challenge simple models and suggest complex underlying mechanisms.

Contribution

It introduces an extra Drude peak to account for sub-gap absorption in the optical response, highlighting the need for more nuanced models of the superconducting state.

Findings

Identification of a sub-gap absorption peak in optical conductivity

The extra peak obeys the sum rule, indicating a common carrier origin

Possible origins include impurity effects, pair-breaking, or band anisotropy

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.