Robust fully - gapped $s_\pm$ superconductivity in CaKFe$_4$As$_4$ single crystals from penetration depth and STM spectroscopy
ORAL
Abstract
Low - temperature variation of the London penetration depth, $\Delta \lambda (T)$, and tunneling conductance in single crystals of stoichiometric iron - based superconductor CaKFe$_4$As$_4$ (CaK1144) show nodeless superconductivity with two effective gaps in the range of 6 - 10 meV and 1-4 meV. Substantial rate of $T_c$ suppression and robust exponential low-temperature behavior of $\Delta \lambda (T)$ upon electron irradiation provide a strong argument for a sign-changing $s_{\pm}$ pairing in this compound. Overall, the observed behavior and extracted parameters are quite similar to a slightly overdoped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (BaK122), e.g. $x=$0.54, $T_c \approx$ 34 K. Considering the results obtained on BaK122 across the superconducting ``dome" and varying disorder [1], we conclude that $s_{\pm}$ superconductivity is a robust pairing state in iron - based superconductors, independent of the degree of substitutional or lattice damage disorder. [1] K. Cho {\it et.~al}, Sci. Adv. {\bf 2}, 1600807 (2016).
*This work was supported by the USDOE/OS/BES Materials Science and Engineering Division.
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