Optical conductivity of Ba0.6 K0.4 Fe2 As2 : The effect of in-plane and out-of-plane doping in the superconducting gap

TL;DR

This study investigates the optical conductivity of Ba0.6K0.4Fe2As2, revealing a fully gapped superconducting state with two isotropic gaps, supporting s(+/-) gap symmetry and indicating strong interband coupling without impurity pair breaking.

Contribution

First detailed optical conductivity analysis of Ba0.6K0.4Fe2As2 showing two isotropic gaps and supporting s(+/-) symmetry in this compound.

Findings

Optical conductivity vanishes below ~20 meV in the superconducting state.

Two isotropic gaps are needed to fit the optical response.

Strong interband coupling without impurity pair breaking evidence.

Abstract

We measured the in-plane optical conductivity of a nearly optimally doped (Ba,K)Fe2As2 single crystal with Tc = 39.1 K. Upon entering the superconducting state the optical conductivity below ~20 meV vanishes, strongly suggesting a fully gapped system. A BCS-like fit requires two different isotropic gaps to describe the optical response of this material. The temperature dependence of the gaps and the penetration depth suggest a strong interband coupling, but no impurity scattering induced pair breaking is present. This contrasts to the large residual conductivity observed in optimally doped Ba(Fe,Co)2As2 and strongly supports an s(+/-) gap symmetry for these compounds.