Disorder Effects in the Bipolaron System Ti$_{4}$O$_{7}$ Studied by Photoemission Spectroscopy

TL;DR

This study uses photoemission spectroscopy to investigate how disorder influences the electronic structure of Ti$_{4}$O$_{7}$ across its phase transitions, revealing a soft Coulomb gap in the high-temperature disordered phase.

Contribution

It provides new insights into the disorder-induced Coulomb gap in Ti$_{4}$O$_{7}$ and relates it to similar phenomena in Fe$_{3}$O$_{4}$ during the Verwey transition.

Findings

Low-temperature phase shows a finite gap at the Fermi level.

High-temperature phase exhibits a gapless spectrum interpreted as a Coulomb gap.

Disorder plays a key role in the electronic properties of Ti$_{4}$O$_{7}$.

Abstract

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.