Atomically resolved wavefunction of integer and fractional quantum anomalous Hall states in twisted bilayer graphene

Moiré superlattices formed by stacking two-dimensional crystals offer a powerful platform for exploring strongly correlated and topological quantum phenomena. However, the absence of microscopic insights has long hindered a comprehensive understanding of their underlying nature and formation mechanisms. In this study, we utilize low-temperature, gate-tunable scanning tunneling microscopy and spectroscopy to directly visualize correlated and topological phases in hBN-aligned magic-angle twisted bilayer graphene. At zero magnetic field, well-defined Coulomb-gap features are observed at filling factors ν = +2 and + 3, indicative of two robust correlated insulating states. The ν = +3 state evolves with magnetic field, indicating a quantum anomalous Hall (QAH) insulator with Chern number C = +1. High-resolution imaging reveals a sublattice asymmetry within AA-stacked regions, providing direct evidence of valley polarization, while its magnetic hysteresis suggests a connection to orbital magnetism. At higher fields, a sequence of additional Chern and fractional Chern insulating states emerges, with their distinct spatial sublattice asymmetry patterns closely associated with the occupied valley. These results establish a direct microscopic correlation among sublattice polarization, valley occupation, and topological order in the moiré superlattice, offering a new spatially resolved perspective on the wavefunctions of correlated and topological ground states. Here, the authors use atomic-resolution scanning tunneling microscopy to identify a quantum anomalous Hall state and directly visualize its valley polarization and magnetic switching in hBN-aligned twisted bilayer graphene.