Photoemission study of the metal-insulator transition inCuIr2S4

We have studied the electronic structure and its changes across the metal-insulator transition in the spinel-type compound ${\mathrm{CuIr}}_2$ ${\mathrm{S}}_4$ using photoemission and inverse-photoemission spectroscopy. Photoemission spectra near the Fermi level show a gap opening of ∼20 meV in the insulating phase, consistent with the transport activation energy. Core-level spectra indicate that the Cu ion is monovalent, and hence Ir is in the intermediate valence state of +3.5. Comparison between the spectra and band-structure calculation reveals that the Ir 5d density of states is strongly distorted, probably due to electron correlation in spite of the general belief of weak correlation in 5d-electron systems.