STM spectroscopy of a gate-switchable moiré quantum anomalous Hall insulator

Twisting and stacking atomically-thin materials provides a versatile platform for investigating emergent quantum phases of matter driven by strong correlation and non-trivial topology. Novel phenomena such as correlated insulating states, unconventional superconductivity, and the quantum anomalous Hall (QAH) effect have been observed in different twisted moiré systems, but a full understanding of their underlying microscopic mechanisms remains a challenge. We have used scanning tunneling microscopy and spectroscopy to explore the interplay between correlation, topology, and local atomic structure in determining the behavior of a QAH insulator made from twisted monolayer-bilayer graphene (a sandwich of monolayer and bernal-stacked bilayer graphene with a small twist between them). We observe local spectroscopic signatures of correlated insulating states having total Chern number C_tot = +2 and -2 at ¾-filling of the conduction moiré mini-band and have characterized their evolution in an out-of-plane magnetic field. We have determined the relationship between topological behavior, local twist angle, and local hetero-strain, and show that C_tot can be switched between +2 and -2 via electrostatic gating only over a limited range of twist angle and strain. Electrical control of the Chern number results from a competition between the orbital magnetization of bulk bands and chiral edge states that is highly sensitive to distortion of the moiré superlattice.