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 temper ature, by means of photoemission and inverse-photoemission spectroscop y. Spectral changes across the transition near the Fermi level (E-F) ( particularly within similar to 100 meV of E-F) have been interpreted a s 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 w eight transfer over a wider energy range of similar to 0.5-1 eV, indic ating significant correlation effects. Photoemission intensity just be low EF remains high in the insulating phases, indicating that the carr ier number is large at high temperatures and that the activation-type transport is due to the activated mobility rather than the activated c arrier number. [S0163-1829(98)07536-5].