Precision microwave electrodynamics of K and Co - doped BaFe$_2$As$_2$

We present a detailed cavity perturbation measurement of the microwave electrodynamics of single crystal iron-based superconductors. The measurements, down to $\sim$0.4K, were performed on Ba$_{0.72}$K$_{0.28}$Fe$_2$As$_2$ hole-doped with $T_c\sim$30K, and BaFe$_{1.9}$Co$_{0.1}$As$_2$ electron-doped with $T_c\sim$20K. Our results show that the temperature dependence of the in-plane London penetration depth $\Delta\lambda(T)$ does not exhibit the exponential saturation at low temperatures expected from a fully-gapped superconductor. In addition we present broadband surface resistance measurements of two FeAs samples and discuss the behavior of the in-plane superfluid density $\rho_{ab}(T)$ in light of existing theoretical models proposed for the iron pnictides superconductors.