Microwave surface-impedance measurements of the magnetic penetration depth in single crystal Ba1-xKxFe2As2 superconductors: Evidence for a disorder-dependent superfluid density

We report high-sensitivity microwave measurements of the in-plane penetration depth λab and quasiparticle scattering rate 1/τ in several single crystals of the hole-doped Fe-based superconductor Ba1-xKxFe2As2 (x≈0.55). While a power-law temperature dependence of λab with a power ∼2 is found in crystals with large 1/τ, we observe an exponential temperature dependence of the superfluid density consistent with the existence of fully opened two gaps in the cleanest crystal we studied. The difference may be a consequence of different levels of disorder inherent in the crystals. We also find a linear relation between the low-temperature scattering rate and the density of quasiparticles, which shows a clear contrast to the case of d-wave cuprate superconductors with nodes in the gap. These results demonstrate intrinsically nodeless order parameters in the Fe arsenides.