Precision microwave electrodynamic measurements of K- and Co-dopedBaFe2As2

Ba0:72K0:28Fe2As2 (hole-doped, Tc � 30 K) and Ba(Fe0:95Co0:05)2As2 (electron-doped, Tc �20 K), by cavity perturbation and broadband spectroscopy. SQUID magnetometry was used to confirm the quality and homogeneity of the samples under study. Through cavity perturbation techniques, the temperature dependence of the in-plane London penetration depth ��(T), and therefore the superfluid phase stiffness � 2 (0)/� 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 � 2 (0)/� 2 (T) � T n and n � 2.5. In the three samples we studied, a weak feature near the sensitivity limit of our measurements appears near T/Tc = 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 ! 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 ��(T).