Investigation of vortex phases and pinning regimes in <i>α</i>-FeSe superconductor from magneto-transport measurements

Superconductivity in the α phase of FeSe emerges at 10 K in the presence of Se deficiency, underscoring the role of stoichiometry in its electronic properties. Unlike the other Fe based superconductors, the simple structure of α-FeSe combined with the absence of doping-induced inhomogeneities and charged quasiparticle scattering provides a cleaner system for investigating its intrinsic flux dynamics. While most prior studies have focused on single-crystalline or thin-film FeSe to resolve intrinsic anisotropies of the vortex state, the present work investigates polycrystalline FeSe, where grain boundaries, random grain orientations, and intergranular coupling significantly influence vortex dynamics. To explore its diverse vortex phase diagram in polycrystalline α-FeSe, magnetic field-dependent electrical resistivity measurements were performed and analyzed by the thermally activated flux flow model. At low temperatures, vortices are frozen into a 2D vortex-glass phase, forming a pinned state. However, as temperature increases, vortex melting initiates, progressively transforming the system into a vortex liquid phase. The zero-temperature pinning potential (U0) exhibits an overall ∼H−p type nature across the full magnetic field range; however, a noticeably slower decay is observed below a crossover field, Hcr = 3 T (∼H−0.27), compared to the steeper decline at higher fields (∼H−0.62). This distinct crossover suggests the emergence of different vortex overlapping regimes, which are highly sensitive to the applied magnetic field. Beyond this threshold, a significant overlap of vortex cores enhances interactions, leading to the dominance of collective pinning. The microstructural effects of polycrystalline FeSe do not alter the intrinsic field dependence of U0 but significantly reduce its absolute magnitude. In particular, the suppressed U0 observed in our samples highlights the extrinsic influence of weak links and grain connectivity on vortex dynamics. These findings highlight how a polycrystalline microstructure affects the pinning properties and phase transitions in superconductors, which may be relevant from the application point of view.