Magnetotransport studies of Fe vacancy-ordered FeSe nanowires.

We studied the electrical transport of FeSe single-crystal nanowires exhibiting √5 × √5 Fe-vacancy order and mixed valence of Fe. FeSe compound has been identified as the parent phase of FeSe superconductor. A first-order metal-insulator (MI) transition of transition temperature ∼ 28 K is observed at zero magnetic fields (). Colossal positive magnetoresistance emerges, resulting from the magnetic field-dependent MI transition. demonstrates anisotropic magnetic field dependence with the preferred orientation along the axis. At temperature < ∼17 K, the state of near-magnetic field-independent resistance, which is due to spin polarized even at zero fields, preserves under magnetic fields up to = 9 T. The Arrhenius law shift of the transition on the source-drain frequency dependence reveals that it is a nonoxide compound with the Verwey-like electronic correlation. The observation of the magnetic field-independent magnetoresistance at low temperature suggests it is in a charge-ordered state below ∼ 17 K. The results of the field orientation measurements indicate that the spin-orbital coupling is crucial in √5 × √5 Fe vacancy-ordered FeSe at low temperatures. Our findings provide valuable information to better understand the orbital nature and the interplay between the MI transition and superconductivity in FeSe-based materials.