Precision microwave electrodynamic measurements of K- and Co-doped BaFe2As2

We have studied the microwave electrodynamics of single-crystal iron-based superconductors Ba0.72K0.28Fe2As2 (hole doped, T-c approximate to 30 K) and Ba(Fe0.95Co0.05)(2)As-2 (electron doped, T-c approximate to 20 K), by cavity perturbation and broadband spectroscopy. Meissner curves were used to confirm the quality and homogeneity of the samples under study. Through cavity perturbation techniques, the temperature dependence of the inplane London penetration depth Delta lambda(T), and therefore the superfluid phase stiffness lambda(2)(0)/lambda(2)(T) was measured. Down to 0.4 K, the data do not show the exponential saturation at low temperatures expected from a singly, fully gapped superconductor. Rather, both the electron- and the hole-doped systems seem to be best described by a power-law behavior with lambda(2)(0)/lambda(2)(T) similar to T-n and n approximate to 2.5. In the three samples we studied, a weak feature near the sensitivity limit of our measurements appears near T/T-c= 0.04, hinting at a corresponding low-energy feature in the superconducting density of states. The data can also be relatively well described by a simple two-gap s-wave model of the order parameter but this yields parameters which seem unrealistic and dependent on the fit range. Broadband surface resistance measurements reveal a sample-dependent residual loss whose origin is unclear. The data from the Ba0.72K0.28Fe2As2 samples can be made to scale as omega(2) if the extrinsic loss is treated as an additive component, indicating large scattering rates. Finally, the temperature dependence of the surface resistance at 13 GHz obeys a power law very similar to those observed for Delta lambda(T)