Fe-Based Superconductors: Growth, Control, and Applications

High pressure growth and impurity effects on the superconducting properties of stoichiometric CaKFe₄As₄

CaKFe₄As₄ (1144) provides a high transition temperature (T_c) of ~36 K and a high critical current density (J_c) of ~10⁷-10⁸ A/cm² for iron-based superconductors (FBS) as a stoichiometric compound. However, the preparation of high-quality and large amounts of 1144 bulk is challenging due to the narrow growth temperature range (~940-960°C) by the conventional synthesis process at ambient pressure (CSP). Hence, it is needed to find an alternative method to improve the quality of 1144 bulks. Recent studies suggest that the high-pressure and high-temperature synthesis (HP-HTS) method can be a unique way to enhance the superconducting properties as well as to improve the sample quality and density. To understand the high-pressure growth effect on the superconducting properties of CaKFe₄As₄, we have synthesized various parent 1144 bulks by both CSP and HP-HTS methods under different growth conditions, and these samples were well characterized by structural, microstructural, transport, and magnetic measurements to investigate their superconducting properties. Interestingly, 1144 bulks prepared by HP-HTS have an enhanced T_c by ~2 K and J_c value by one order of magnitude in the whole magnetic field range of 9 T compared to that (10³ A/cm²) of 1144 samples prepared by CSP. Structural and microstructural analysis suggest that sample quality, sample density, and grain connections are improved by HP-HTS, which could be a reason for the improvement of J_c and its pinning properties. Additionally, the influence of impurities on the superconducting properties of CaKFe₄As₄ has also been studied under CSP and HP-HTS processes. Interestingly, the onset transition temperature (T_c^onset) of this 1144 family is robust with the presence of the common 122 (CaFe₂As₂ or KFe₂As₂) impurity phases, which is a different behavior compared to other FBS families. However, these impurity phases lead to the phase separation during the formation of the superconducting (1144) phase.