Magnetotransport studies of Fe vacancy-ordered Fe4+δSe5 nanowires

We studied the electrical transport of Fe4+delta Se5 single-crystal nanowires exhibiting root 5 x root 5 Fe-vacancy order and mixed valence of Fe. Fe4+delta Se5 compound has been identified as the parent phase of FeSe superconductor. A first-order metal-insulator (MI) transition of transition temperature T-MI similar to 28 K is observed at zero magnetic fields (B). Colossal positive magnetoresistance emerges, resulting from the magnetic field-dependent MI transition. T-MI demonstrates anisotropic magnetic field dependence with the preferred orientation along the c axis. At temperature T < similar to 17 K, the state of nearmagnetic 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 similar to 17 K. The results of the field orientation measurements indicate that the spin-orbital coupling is crucial in root 5 x root 5 Fe vacancy-ordered Fe4+delta Se5 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.