In-plane magnetic penetration depth of superconducting CaKFe4As4

Abstract

The temperature dependence of the in-plane magnetic penetration depth (${\ensuremath{\lambda}}_{ab}$) in an extensively characterized sample of superconducting ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}\phantom{\rule{0.28em}{0ex}}({T}_{\mathrm{c}}\ensuremath{\simeq}35\phantom{\rule{0.28em}{0ex}}\mathrm{K})$ was investigated using muon-spin rotation ($\ensuremath{\mu}\mathrm{SR}$). A comparison of ${\ensuremath{\lambda}}_{ab}^{\ensuremath{-}2}(T)$ measured by $\ensuremath{\mu}\mathrm{SR}$ with the one inferred from angle-resolved photoemission spectroscopy (ARPES) data confirms the presence of multiple gaps at the Fermi level. An agreement between $\ensuremath{\mu}\mathrm{SR}$ and ARPES requires the presence of additional bands, which are not resolved by ARPES experiments. These bands are characterized by small superconducting gaps with an average zero-temperature value of ${\mathrm{\ensuremath{\Delta}}}_{0}=2.4(2)\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$. Our data suggest that in ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$ the ${s}^{\ifmmode\pm\else\textpm\fi{}}$ 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.