Microwave Surface-Impedance Measurements of the Magnetic Penetration Depth in Single Crystal Ba1-xKxFe2As2 Superconductors: Evidence for a Disorder-Dependent Superfluid Density

Abstract

We report high-sensitivity microwave measurements of the in-plane penetration depth ${\lambda }_{ab}$ and quasiparticle scattering rate $1/\tau$ in several single crystals of hole-doped Fe-based superconductor Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ ($x\approx 0.55$). While power-law temperature dependence of ${\lambda }_{ab}$ with the power $\sim 2$ is found in crystals with large $1/\tau$, we observe exponential temperature dependence of 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 level 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.

4 pages, 4 figures, 1 table. Accepted for publication in Phys. Rev. Lett. Changed title as suggested by the PRL editors

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