Tuning Superconductivity and Spin-Vortex Instabilities in CaKFe4As4 Through In-Plane Antisymmetric Strains

Lattice strains of appropriate symmetry have served as an excellent tool to explore the interaction of superconductivity in the iron-based superconductors with orthorhombic-nematic and stripe spin-density-wave (SSDW) order. In this Letter, we contribute to a broader understanding of the coupling of strain to superconductivity and competing normal-state orders by studying CaKFe4As4 under large, in-plane strains of B1g and B2g symmetry. In contrast to the majority of iron-based superconductors, pure CaKFe4As4 exhibits superconductivity with a relatively high transition temperature of Tc∼35K in proximity of a noncollinear, tetragonal, hedgehog spin-vortex crystal (SVC) order. Through experiments and calculations, we demonstrate an anisotropic in-plane strain response of Tc and the favored SVC configuration in CaKFe4As4. This supports a scenario, in which the change in spin fluctuations dominates the strain response of superconducting Tc. Overall, by suggesting moderate B2g strains as an effective parameter to change the stability of SVC and SSDW, we outline a pathway to a unified phase diagram of iron-based superconductivity. Published by the American Physical Society 2024