Nodeless multiband superconductivity in stoichiometric single crystalline CaKFe${}_4$As${}_4$

Abstract

Measurements of the London penetration depth and tunneling conductance in single crystals of the recently discovered stoicheometric, iron - based superconductor, CaKFe${}_4$As${}_4$ (CaK1144) show nodeless, two effective gap superconductivity with a larger gap of about 6-9 meV and a smaller gap of about 1-4 meV. Having a critical temperature, $T_{c,onset}\approx$35.8 K, this material behaves similar to slightly overdoped Ba${}_{1-x}$K${}_x$)Fe${}_2$As${}_2$ (e.g. $x=$0.54, $T_c\approx$ 34 K)---a known multigap $s_{\pm }$ superconductor. We conclude that the superconducting behavior of stoichiometric CaK1144 demonstrates that two-gap $s_{\pm }$ superconductivity is an essential property of high temperature superconductivity in iron - based superconductors, independent of the degree of substitutional disorder.