In-gap state and effect of light illumination inCuIr2S4probed by photoemission spectroscopy

Abstract

We have studied disorder-induced in-gap states and effect of light illumination in the insulating phase of spinel-type ${\text{CuIr}}_{2}{\text{S}}_{4}$ using ultraviolet photoemission spectroscopy (UPS). The ${\text{Ir}}^{3+}/{\text{Ir}}^{4+}$ charge-ordered gap appears below the metal-insulator transition temperature. However, in the insulating phase, in-gap spectral features with softgap are observed in UPS just below the Fermi level $({E}_{F})$, corresponding to the variable range hopping transport observed in resistivity. The spectral weight at ${E}_{F}$ is not increased by light illumination, indicating that the ${\text{Ir}}^{4+}{\text{-Ir}}^{4+}$ dimer is very robust, although the long-range octamer order would be destructed by the photoexcitation. Present results suggest that the ${\text{Ir}}^{4+}{\text{-Ir}}^{4+}$ bipolaronic hopping and disorder effects are responsible for the conductivity of ${\text{CuIr}}_{2}{\text{S}}_{4}$.