摘要
We studied the electrical transport of Fe <sub>4+δ</sub> Se <sub>5</sub> single-crystal nanowires exhibiting √5 × √5 Fe-vacancy order and mixed valence of Fe. Fe <sub>4+δ</sub> Se <sub>5</sub> compound has been identified as the parent phase of FeSe superconductor. A first-order metal-insulator (MI) transition of transition temperature T <sub>MI</sub> ∼ 28 K is observed at zero magnetic fields (B). Colossal positive magnetoresistance emerges, resulting from the magnetic field-dependent MI transition. T <sub>MI</sub> demonstrates anisotropic magnetic field dependence with the preferred orientation along the c axis. At temperature T < ∼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 B = 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 T ∼ 17 K. The results of the field orientation measurements indicate that the spin-orbital coupling is crucial in √5 × √5 Fe vacancy-ordered Fe <sub>4+δ</sub> Se <sub>5</sub> 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.