Evidence for Two Gaps and Breakdown of the Uemura Plot in Ba${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$ Single Crystals

Abstract

We report a detailed investigation on the lower critical field $H_{c1}$ of the superconducting Ba${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$ (FeAs-122) single crystals. A pronounced kink is observed on the $H_{c1}(T)$ curve, which is attributed to the existence of two superconducting gaps. By fitting the data $H_{c1}(T)$ to the two-gap BCS model in full temperature region, a small gap of ${\Delta }_a(0)=2.0\pm 0.3$ meV and a large gap of ${\Delta }_b(0)=8.9\pm 0.4$ meV are obtained. The in-plane penetration depth ${\lambda }_{ab}(0)$ is estimated to be 105 nm corresponding to a rather large superfluid density, which points to the breakdown of the Uemura plot in FeAs-122 superconductors.

5 pages, 4 figures

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