Defect-induced vortex pattern variations in a CaKFe 4 As 4 single crystal

The current-carrying capability of superconductors, crucial for technological applications, is fundamentally limited by vortex motion, making effective vortex pinning paramount. While intrinsic defects can serve as potent pinning centers, their specific influence on vortex behavior remains poorly understood, particularly at the microscopic level. The stoichiometric superconductor ${\mathrm{CaKFe}}₄{\mathrm{As}}₄$, free from extrinsic disorder, offers an ideal platform to address this knowledge gap. Here, we employ low-temperature magnetic force microscopy to directly correlate distinct intrinsic planar defects---${\mathrm{CaFe}}₂{\mathrm{As}}₂$ and ${\mathrm{KFe}}₂{\mathrm{As}}₂$ intergrowths---with vortex states in ${\mathrm{CaKFe}}₄{\mathrm{As}}₄$. We reveal that ${\mathrm{CaFe}}₂{\mathrm{As}}₂$ intergrowths induce significantly stronger pinning and more disordered vortex configurations than ${\mathrm{KFe}}₂{\mathrm{As}}₂$ intergrowths, with the vortex lattice exhibiting a squarelike symmetry at higher fields. Continuous field-cooling experiments uncover an inhomogeneous pinning landscape associated with ${\mathrm{CaFe}}₂{\mathrm{As}}₂$ intergrowths, corroborated by microscale variations in deduced pinning energy and force per unit length, indicative of nanoscale superfluid density fluctuations. Crucially, correlating these microscopic observations with macroscopic critical current density yields a characteristic disorder length scale ($\ensuremath{\sim}177\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$) consistent with the lateral dimensions of ${\mathrm{CaFe}}₂{\mathrm{As}}₂$ intergrowths. This study establishes a direct microscopic link between specific defect structures and vortex pinning mechanisms in ${\mathrm{CaKFe}}₄{\mathrm{As}}₄$, providing fundamental insights essential for optimizing critical currents through targeted defect engineering in superconductors.