Photoemission study of the metal-insulator transition in NiS2-xSex

We have studied the electronic structure of NiS2-xSex, which undergoes a metal-insulator transition as functions of composition x and temperature, by means of photoemission and inverse-photoemission spectroscopy. Spectral changes across the transition near the Fermi level (E-F) (particularly within similar to 100 meV of E-F) have been interpreted as due to a "semimetallic" closure of the band gap in going from the insulating phase to the antiferromagnetic metallic phase. On the other hand, there is also composition- and temperature-dependent spectral weight transfer over a wider energy range of similar to 0.5-1 eV, indicating significant correlation effects. Photoemission intensity just below EF remains high in the insulating phases, indicating that the carrier number is large at high temperatures and that the activation-type transport is due to the activated mobility rather than the activated carrier number