Microwave Surface-Impedance Measurements of the Magnetic Penetration Depth in Single CrystalBa1−xKxFe2As2Superconductors…

We report high-sensitivity microwave measurements of the in-plane penetration depth ${\ensuremath{\lambda}}_{ab}$ and quasiparticle scattering rate $1/\ensuremath{\tau}$ in several single crystals of the hole-doped Fe-based superconductor ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($x\ensuremath{\approx}0.55$). While a power-law temperature dependence of ${\ensuremath{\lambda}}_{ab}$ with a power $\ensuremath{\sim}2$ is found in crystals with large $1/\ensuremath{\tau}$, 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.