Magnetic field driven quantum phases in magic angle twisted bilayer graphene

The discovery of magic angle twisted bilayer graphene (MATBG) has opened a plethora of grand new opportunities in the field of topology, superconductivity, and other strongly correlated effect. For MATBG with a small twist angle close to θ = 1.1°, the electronic bands are flattened by the periodic potential of the moiré bands and isolated from higher-energy dispersive bands. These flat electronic bands in MATBG have recently emerged as a rich platform to explore strong correlations. However, the phases of MATBG in a magnetic field and what they reveal about the zero-field phase diagram remain relatively uncharted. We report a rich sequence of wedge-like regions of quantized Hall conductance with Chern numbers C = ±1, ±2, ±3 and ±4, which nucleate from integer fillings of the moiré unit cell ν = ±3, ±2, ±1 and 0, respectively. The exact sequence and correspondence of the Chern numbers and filling factors suggest that these states are directly driven by electronic interactions, which specifically break the time-reversal symmetry in the system. Additionally, we studied the detailed magnetotransport behaviour of the Hofstadter spectrum of MATBG. We observed the re-entrance of insulating states at ν = +2, ±3 of the moiré unit cell of MATBG upon applying an external magnetic field close to the full flux quantum Φ/Φ₀ = 1 of the superlattice unit cell and interaction-driven Fermi-surface reconstructions at other fillings, which are identified by new sets of Landau levels originating from these.