PHOTOEMISSION-STUDY OF THE METAL-INSULATOR-TRANSITION IN CUIR2S4

We have studied the electronic structure and its changes across the me tal-insulator transition in the spinel-type compound CuIr2S4 using pho toemission and inverse-photoemission spectroscopy Photoemission spectr a near the Fermi level show a gap; opening of similar to 20 meV in the insulating phase, consistent with the transport activation energy. Co re-level spectra indicate that the Cu ion is monovalent, End hence Ir is in the intermediate valence state of +3.5. Comparison between the s pectra and band-structure calculation reveals that the Ir 5d density o f states is strongly distorted, probably due to electron correlation i n spite of the general belief of weak correlation in 5d-electron syste ms.