We have performed a photoemission study of Ti$_{ 4}$O$_{ 7}$ 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 Fe$_{ 3}$O$_{ 4}$, which exhibits a charge order-disorder transition (Verwey transition), can be interpreted in terms of a Coulomb gap.
Categories: cond-mat.str-el