Disorder effects in the bipolaron system studied by photoemission spectroscopy

¹ Department of Physics, University of Tokyo - Bunkyo-ku, Tokyo, 113-0033, Japan
² Department of Physics and Department of Complexity Science and Engineering University of Tokyo - Bunkyo-ku, Tokyo 113-0033, Japan
³ Department of Applied Physics, University of Tokyo Bunkyo-ku, Tokyo 113-0033, Japan
⁴ Department of Advanced Materials Science, University of Tokyo Bunkyo-ku, Tokyo 113-0033, Japan

Received: 29 January 2002
Accepted: 3 July 2002

Abstract

We have performed a photoemission study of Ti₄O₇ around its two transition temperatures so as to cover the metallic, high-temperature insulating (“bipolaron-liquid”) and low-temperature insulating (bipolaron-crystal) phases. While the spectra of the low-temperature insulating phase show a finite gap at the Fermi level, the spectra of the high-temperature insulating phase are gapless, which is interpreted as a soft Coulomb gap due to dynamical disorder. We suggest that the spectra of the high-temperature disordered phase of Fe₃O₄, which exhibits a charge order-disorder transition (Verwey transition), can be interpreted in terms of a Coulomb gap.