Critical current density challenges and vortex-state behavior in organic molecule-intercalated FeSe polycrystalline materials

Abstract High-field superconducting performance is governed by the critical current density ( J c ), which in type-II iron-based superconductors is often hampered by synthesis-induced microstructural features. Here, trapped-flux magnetization measurements were used to examine how intercalation-induced microstructure landscapes mediate the superconducting properties in the two-dimensional Li 0.6 (C 5 H 5 N) y Fe 2 Se 2 ( T c = 39 K). Two contrasting polycrystalline forms were examined: a single-phase granular powder and an impure pellet. Both samples exhibit a low first Josephson critical field of μ 0 H c1J of ∼2 mT, indicating weak intergranular connectivity. Consequently, their estimated J c based on the Bean critical state model is limited to ∼10 3 A cm −2 , an order of magnitude below single-crystal FeSe. This limitation stems from the strongly reducing intercalation route, which inherently yields granular products along with impurity phases. These impurities act as weak-link disruptors, suppressing J c , even though pelletization partially improves grain connectivity. Analysis of the normalized J c ( T ) suggests a vortex-glass state in both samples with distinct mean-free-path fluctuation-induced ( δ ℓ) pinning regimes. The single-phase powder exhibits single-vortex δ ℓ-pinning and a gradual suppression of J c up to ∼0.8 T c . In contrast, the pellet shows an impurity-driven large-bundle δ ℓ-pinning, where weakly pinned vortex bundles are readily thermally activated, leading to a much earlier reduction of J c near ∼0.5 T c . The results demonstrate that while intercalation of organic molecules increases T c in Fe-selenides, relatively weak intergrain coupling and impurity-driven pinning impede performance, necessitating improved densification and suppression of impurities through targeted microstructural engineering.