Elucidating the origin of planar defects that enhance critical current density in CaKFe4As4 single crystals

Abstract

CaKFe4As4 is a new member of the 1144-type iron-based superconductors (IBSs) which is expected to show remarkable superconducting properties because its crystal structure is similar to that of 122-type IBSs. Recently, substantial anisotropy between the in-plane and out-of-plane critical current densities (J C), with peculiar temperature dependence of in-plane J C at high fields, has been reported in CaKFe4As4 single crystals. The anisotropy of J C was attributed to the presence of planar defects, which were shown to be confined to the ab plane. However, the origin of these defects remains unclear. To elucidate the origin of these planar defects, we herein present our results on atomic-scale microstructure analysis of high-quality CaKFe4As4 single crystals. Using high-resolution scanning transmission electron microscopy (STEM), along with electron energy loss spectroscopy, we have demonstrated that these planar defects consist of one- or two-layer KFe2As2 step network, which is spread over the periodically ordered KFe2As2 and CaFe2As2 monolayers. Further, we report that the dark contrast regions observed via conventional STEM are local strain distributions, which originate from the substitution of Ca by K in a unit cell.