Microstructural Engineering via Fe Deficiency in Fe(Se, Te) Superconductors: A Pathway to Enhanced Vortex Pinning

Defect engineering embodies a paradigm shift in materials science, repurposing structural imperfections into precise tools for property optimization. This approach is critically exemplified in type-II superconductors, where the deliberate introduction of defects, particularly those with dimensions comparable to the superconducting coherence length, creates effective flux-pinning centers and immobilizes magnetic flux vortices, directly enhancing the critical current density of superconductors. In this study, the microstructure of polycrystalline Fe(Se, Te) is engineered by employing a strategic Fe deficiency. The results exhibit that Fe deficiency facilitates the formation of nanoscale monoclinic Fe₇(Se, Te)₈ defects within the tetragonal superconducting matrix, substantially strengthening point pinning. Besides, the intrinsic Δκ pinning, driven by microscopic Se/Te compositional fluctuations, is restored by reducing interstitial Fe in Fe-deficient Fe(Se, Te). Benefiting from the coordination of the two mechanisms, Fe-deficient Fe(Se, Te) exhibits enhanced high-field flux pinning, marked by distinct second magnetization peaks, along with higher high-field J_c and flux pinning forces at elevated temperatures. Our work demonstrates a pathway for enhancing high-field flux pinning in polycrystalline Fe(Se, Te). It also reveals the feasibility of utilizing typically detrimental secondary phases to engineer beneficial pinning centers and provides critical guidance for enhancing the flux pinning capacity of type-II superconductors in high-field applications.