Photoemission study of the metal-insulator transition

We have studied the electronic structure of (Formula presented) which undergoes a metal-insulator transition as functions of composition (Formula presented) and temperature, by means of photoemission and inverse-photoemission spectroscopy. Spectral changes across the transition near the Fermi level (Formula presented) (particularly within (Formula presented) of (Formula presented) 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 (Formula presented) indicating significant correlation effects. Photoemission intensity just below (Formula presented) 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.