Elucidating the origin of planar defects that enhance critical current density in CaKFe4As4 single crystals
Elucidating the origin of planar defects that enhance critical current density in CaKFe4As4 single crystals
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DOI：
10.1088/1361-6668/abdba7
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发表时间：
2021-03-01
影响因子：
3.6
通讯作者：
Ishida, Shigeyuki
中科院分区：
文献类型：
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作者：
Ichinose, Ataru;Pyon, Sunseng;Ishida, Shigeyuki
CaKFe4As4是1144型铁基超导体（ibs）的新成员，由于其晶体结构与122型ibs相似，有望表现出卓越的超导性能。最近，在CaKFe4As4单晶中，已经报道了面内和面外临界电流密度（J(C)）之间存在明显的各向异性，以及高场下面内J(C)的特殊温度依赖性。J(C)的各向异性归因于平面缺陷的存在，这些缺陷被证明局限于ab平面。然而，这些缺陷的起源仍不清楚。为了阐明这些平面缺陷的起源，我们在此给出了高质量CaKFe4As4单晶的原子尺度微观结构分析结果。利用高分辨率扫描透射电子显微镜（STEM）以及电子能量损失谱，我们已经证明这些平面缺陷由一层或两层KFe2As2阶跃网络组成，该网络分布在周期性有序的KFe2As2和CaFe2As2单层上。此外，我们报告说，通过常规STEM观察到的暗对比区域是局部应变分布，这是由于单位细胞中Ca被K取代而产生的。
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.