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

TL;DR

This study reveals that stoichiometric CaKFe$_4$As$_4$ exhibits nodeless, two-gap superconductivity with high critical temperature, supporting the idea that multigap $s_{ ext{±}}$ superconductivity is fundamental in iron-based high-temperature superconductors.

Contribution

It demonstrates that two-gap $s_{ ext{±}}$ superconductivity occurs in stoichiometric CaKFe$_4$As$_4$, independent of substitutional disorder, highlighting its role in high-temperature superconductivity.

Findings

CaKFe$_4$As$_4$ has a $T_c$ of about 35.8 K.

The superconductor exhibits two effective gaps of 6-9 meV and 1-4 meV.

Superconductivity is nodeless and multigap, similar to overdoped Ba$_{1-x}$K$_x$)Fe$_2$As$_2$.

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.