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

TL;DR

This study uses microwave measurements to explore the magnetic penetration depth and superfluid density in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ superconductors, revealing disorder-dependent behaviors and evidence for nodeless superconducting gaps.

Contribution

It provides the first detailed microwave analysis linking disorder levels to superfluid density and gap structure in Fe-based superconductors.

Findings

Power-law temperature dependence of penetration depth in disordered crystals.

Exponential temperature dependence indicating fully gapped superconductivity in cleaner samples.

Linear relation between scattering rate and quasiparticle 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.