Intrinsic defect structures of polycrystalline CaKFe₄As₄ superconductors

We investigated the defect structures of polycrystalline CaKFe₄As₄ (CaK1144) superconductors by scanning transmission electron microscopy (STEM). The STEM studies revealed the presence of a one-layer CaFe₂As₂ (∼1 nm size) defect along the ab-plane, as observed in single crystalline CaK1144. Step-like CaFe₂As₂ defects are also observed. These nanoscale defects generate fine-sized stacking faults, a lattice mismatch, and stress field defects in the matrix of CaK1144 owing to the different sizes. Correlation of the defects in polycrystalline and single crystalline samples suggests that the defects type and their density depend on the synthesis conditions. A self-field critical current density (J_c) of 15.2 kA cm^-2 was obtained at 5 K, and the curves were sustained above 30 K with a considerable J_c value of 1.4 kA cm^-2. We investigated the relationship between the observed intrinsic defects and the behavior of the field dependence of J_c. The intrinsically intergrown planar defects, even in polycrystalline samples, are expected to be advantageous for various high-field applications of bulk CaK1144 superconductors.