In-gap state and effect of light illumination in CuIr₂S ₄ probed by photoemission spectroscopy

We have studied disorder-induced in-gap states and effect of light illumination in the insulating phase of spinel-type CuIr₂S ₄ using ultraviolet photoemission spectroscopy (UPS). The Ir ³⁺/Ir⁴⁺ 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 Ir⁴⁺-Ir⁴⁺ dimer is very robust, although the long-range octamer order would be destructed by the photoexcitation. Present results suggest that the Ir⁴⁺-Ir ⁴⁺ bipolaronic hopping and disorder effects are responsible for the conductivity of CuIr₂S₄.