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

TL;DR

This study investigates the temperature dependence of the in-plane magnetic penetration depth in superconducting CaKFe$_4$As$_4$, revealing multiple gaps and complex order parameter symmetry through muon-spin rotation and ARPES comparison.

Contribution

It provides new insights into the multigap structure and order parameter symmetry of CaKFe$_4$As$_4$ using combined $bc$SR and ARPES data, highlighting additional bands with small gaps.

Findings

Confirmation of multiple superconducting gaps.

Identification of additional bands not seen in ARPES.

Complex $s^b1$ order parameter symmetry.

Abstract

The temperature dependence of the in-plane magnetic penetration depth ($\lambda_{ab}$) in an extensively characterized sample of superconducting CaKFe$_4$As$_4$ ($T_{\rm c}\simeq35$ 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.