Electrical manipulation and STM visualization of 1D chiral edge states in a moiré quantum anomalous Hall insulator

The quantum anomalous Hall (QAH) effect reflects an interplay between magnetism and non-trivial topology and is characterized by non-zero Chern number and chiral edge-states that carry dissipationless current along topologically defined interfaces. The discovery of the QAH effect in van der Waals moiré heterostructures provides a new opportunity for studying this exotic 2D state of matter. Here I will discuss our scanning tunneling microscopy and spectroscopy (STM/STS) measurements of twisted monolayer-bilayer graphene (tMBLG), a system that has recently been discovered to exhibit the QAH effect as well as gate-switchable orbital magnetism in transport measurements.¹ I will first discuss local spectroscopy measurements of the correlated insulating states at band-fillings of ν = 2 and ν = 3 electrons per moiré unit cell, where the ν = 3 state shows a total Chern number of |C| = 2. Under a small magnetic field, we are able to detect switching of the sign of the Chern number for the ν = 3 state induced by electrostatic gating, a result of the competition between bulk and edge orbital magnetization of the QAH state.² Utilization of this gate-switchable Chern number allows us to directly visualize the chiral edge-state of a moiré QAH insulator for the first time through gate-dependent dI/dV mapping. We are additionally able to manipulate the spatial location and chirality of QAH edge-states by controlling local carrier concentration with the STM tip. I will discuss the implications of our experimental results for future electronic devices as well as the important role of local moiré strain in determining the correlated and topological properties of tMBLG.

¹ H. Polshyn et al., Nature, 588, 66-70 (2020)

² C. Zhang, T. Zhu et al., arXiv: 2210.06506 (2022)