Penetration Depth and Quasiparticle Conductivity of Co- and K-Doped BaFe2As2 Crystals, Investigated by a Microwave Coplanar Resonator Technique

In this work, an experimental approach toward reaching new insights into the mechanism of superconductivity in the iron-based compounds is presented. The temperature dependence of the penetration depth and of the microwave conductivity of high-quality Ba(Fe_1-xCo_x)₂As₂ and (Ba_1-xK_x)Fe₂As₂ single crystals have been obtained by a 7.97-GHz superconducting coplanar resonator technique, in a perturbative approach. The proposed technique is suitable to obtain the absolute value of the penetration depth and shows high sensitivity to small samples. For the Co-doped BaFe₂As₂, the comparison of crystals with different shape factors allows distinguishing the in-plane and out-of-plane components of the penetration depth. Peaks in the normalized quasiparticle conductivity of both Co-doped and K-doped crystals are shown and discussed.