In-plane magnetic penetration depth of superconducting CaKFe${}_4$As${}_4$

Abstract

The temperature dependence of the in-plane magnetic penetration depth (${\lambda }_{ab}$) in an extensively characterized sample of superconducting CaKFe${}_4$As${}_4$ ($T_{\mathrm{c}}\simeq 35$ K) was investigated using muon-spin rotation ($\mu$SR). A comparison of ${\lambda }_{ab}^{-2}(T)$ measured by $\mu$SR with the one inferred from ARPES data confirms the presence of multiple gaps at the Fermi level. An agreement between $\mu$SR and ARPES requires the presence of additional bands, which are not resolved by ARPES experiments. These bands are characterized by small supercondcting gaps with an average zero-temperature value of ${\Delta }_0=$ 2.4(2) meV. Our data suggest that in CaKFe${}_4$As${}_4$ the $s^{\pm }$ order parameter symmetry acquires a more sophisticated form by allowing a sign change not only between electron and hole pockets, but also within pockets of similar type.

6 pages, 4 figures. Accpeted for publication in Phys.Rev.B (Rapid)