Verwey transition as evolution from electronic nematicity to trimerons via electron-phonon coupling

Abstract

Understanding the driving mechanisms behind metal-insulator transitions (MITs) is a critical step towards controlling material's properties. Since the proposal of charge-order-induced MIT in magnetite Fe3O4 in 1939 by Verwey, the nature of the charge order and its role in the transition have remained elusive-a longstanding challenge in the studies of complex oxides. Recently, a trimeron order was discovered in the low-temperature monoclinic structure of Fe3O4; however, the expected transition entropy change in forming trimeron at the Verwey transition is greater than the observed value, which arises a reexamination of the ground state in the high-temperature phase. Here we use electron diffraction to unveil that a nematic charge order on particular Fe sites emerges in the high-temperature cubic structure of bulk Fe3O4, and that upon cooling, a competitive intertwining of charge and lattice orders leads to the emergence of the Verwey transition. Moreover, MeV ultrafast electron diffraction (UED) provides a dynamic measure of the strong coupling between photoexcited electrons and the X3 phonon modes. Our findings discover a new type of electronic nematicity in correlated materials and offer novel insights into the Verwey transition mechanism in Fe3O4 via the electron-phonon coupling.