Optical conductivity of Ba_0.6K_0.4Fe₂As₂: The effect of in-plane and out-of-plane doping in the superconducting gap

¹ LPEM, ESPCI- ParisTech, CNRS, UPMC - 10 rue Vauquelin, F-75231 Paris Cedex 5, France
² Beijing National Laboratory for Condensed Matter Physics, National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences - P.O. Box 603, Beijing 100190, China
³ National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University Nanjing 210093, China

Received: 20 September 2013
Accepted: 17 November 2013

Abstract

We measured the in-plane optical conductivity of a nearly optimally doped single crystal of Ba_0.6K_0.4Fe₂As₂. Upon entering the superconducting state the optical conductivity vanishes below , indicating a fully gapped system. A quantitative modeling of the optical response of this material requires two different isotropic gaps, the larger of which dominates the London penetration depth. The temperature dependence of these gaps indicate a strong interband interaction, but no impurity-scattering–induced pair breaking is present. This contrasts to the large residual conductivity observed in optimally doped BaFe_1.84Co_0.08As₂ and strongly supports an gap symmetry for both compounds.