Electric-field-induced insulator to Coulomb glass transition via oxygen vacancy migration in Ca-doped BiFeO3

The BiFeO3 (BFO) is an interesting playground to explore correlated electronic conduction. It is found that a non-rigid polaronic band is created by hole doping as a result of a strong electron-lattice coupling. We also show strong evidence for the disorder-driven formation of a Coulomb glass state. These results can be harmonized in a framework of the spatial inhomogeneity of polaronic charge density, suggesting a unique electronic conduction mechanism applicable to systems with coexistence of strong electron correlation, electron-lattice interaction, and randomness.