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

TL;DR

This study investigates in-gap states and the impact of light on the insulating phase of CuIr$_2$S$_4$ using photoemission spectroscopy, revealing robust Ir$^{4+}$-Ir$^{4+}$ dimers and disorder effects influencing conductivity.

Contribution

It provides new insights into the robustness of Ir$^{4+}$-Ir$^{4+}$ dimers and the role of disorder and bipolarons in the conductivity of CuIr$_2$S$_4$.

Findings

In-gap states with softgap observed below the transition temperature.

Light illumination does not increase spectral weight at the Fermi level.

Ir$^{4+}$-Ir$^{4+}$ dimers are robust against photo-excitation.

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$_4$.