In-gap state and effect of light illumination in CuIr${}_2$S${}_4$ probed by photoemission spectroscopy

Abstract

We have studied disorder-induced in-gap states and effect of light illumination in the insulating phase of spinel-type CuIr${}_2$S${}_4$ using ultra-violet photoemission spectroscopy (UPS). The Ir${}^{3+}$/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 Ir${}^{4+}$-Ir${}^{4+}$ dimer is very robust although the long-range octamer order would be destructed by the photo-excitation. Present results suggest that the Ir${}^{4+}$-Ir${}^{4+}$ bipolaronic hopping and disorder effects are responsible for the conductivity of CuIr${}_2$S${}_4$.

14 pages, 5 figures

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• Band Jahn-Teller Instability and Formation of Valence Bond Solid in a Mixed-Valent Spinel Oxide LiRh2O4
• Electronic structure of BaIrO3: A first principle study using local spin-density approximations