Symmetry-broken Chern insulators and Rashba-like Landau-level crossings in magic-angle bilayer graphene

Flat bands in magic-angle twisted bilayer graphene (MATBG) have recently emerged as a rich platform to explore strong correlations(1), superconductivity(2-5) and magnetism(3,6,7). However, the phases of MATBG in a magnetic field and what they reveal about the zero-field phase diagram remain relatively uncharted. Here 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 moire unit cell v = +/- 3, +/- 2, +/- 1 and 0, respectively. We interpret these phases as spin- and valley-polarized many-body Chern insulators. 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. We further study the yet unexplored higher-energy dispersive bands with a Rashba-like dispersion. The analysis of Landau-level crossings enables a parameter-free comparison to a newly derived 'magic series' of level crossings in a magnetic field and provides constraints on the parameters of the Bistritzer-MacDonald MATBG Hamiltonian. Overall, our data provide direct insights into the complex nature of symmetry breaking in MATBG and allow for the quantitative tests of the proposed microscopic scenarios for its electronic phases.

Automated summary

The study presents a rich sequence of quantized Hall conductance regions in magic-angle twisted bilayer graphene (MATBG), driven by specific electronic interactions, revealing the complex nature of symmetry breaking in MATBG. Analysis of Landau level crossings provides constraints on the parameters of the MATBG Hamiltonian and allows for quantitative tests of proposed microscopic scenarios for its electronic phases.