Resonant inelastic x-ray scattering study of charge ordering in CuIr2S4

We present Ir L$_{3}$-edge resonant inelastic x-ray scattering (RIXS) spectra and resonant x-ray emission spectra (RXES) on the thiospinel, CuIr$_2$S$_4$, which has been attracting much interest due to intriguing metal-insulator transitions. At room temperature CuIr$_2$S$_4$ is metallic, but goes through a metal insulator transition at T$_{\rm{MI}} \sim$ 226 K due to the formation of charge order (CO) of Ir$^{3+}$ and Ir$^{4+}$ together with spin dimerization between Ir$^{4+}$ ions. By exposing the sample to x-ray below T $ = $ 50 K, the crystal symmetry goes from triclinic to tetragonal, accompanied by reduced resistivity. The RIXS signal was dominated by a broad and strong feature around 3 eV, arising from t$_{2g}$ to e$_{g}$ transition, but we were able to observe a clear signature of opening of the insulating gap across the metal-insulator transition. In addition, we also found that this gap is partially filled in the irradiation-induced phase. The emission spectra reveals the existence of an excited Ir-5d \emph{t$_{2g}$} state, which is hidden in the Ir L$_3$-edge XAS of CuIr$_2$S$_4$. The result indicates that the electronic reconstruction that takes place in the irradiation-induced phase comes from the Ir$^{3+}$ while the Ir$^{4+}$ dimers are unchanged.