Abstract
We have studied the electronic structure of the spinel CuIr2S4 using synchrotron-radiation photoemission spectroscopy. The phase transition from a high temperature paramagnetic metal to a low temperature diamagnetic insulator at similar to 230 K is clearly observed through the significant line shape change of the Ir 4f photoemission spectra and the similar to 0.09 eV gap opening of the valence band spectra. The photon energy dependence of the Ir 4f photoemission spectra enables us to characterize the satellite peaks in the metallic phase of CuIr2S4, providing compelling experimental evidence for the Ir3+-Ir4+ charge disproportionation in CuIr2S4.