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

Abstract

Flat-bands in magic angle twisted bilayer graphene (MATBG) have recently emerged as a rich platform to explore strong correlations, superconductivity and mag- netism. However, the phases of MATBG in magnetic field, and what they reveal about the zero-field phase diagram remain relatively unchartered. Here we use magneto- transport and Hall measurements to reveal a rich sequence of wedge-like regions of quan- tized Hall conductance with Chern numbers C = ±1, ±2, ±3, ±4 which nucleate from inte- ger fillings of the moiré unit cell 𝜈 = ±3, ±2, ±1, 0 correspondingly. We interpret these phases as spin and valley polarized Chern insulators, equivalent to quantum Hall ferro- magnets. The exact sequence and correspondence of Chern numbers and filling factors suggest that these states are driven directly by electronic interactions which specifically break time-reversal symmetry in the system. We further study quantum magneto-oscil- lation in the yet unexplored higher energy dispersive bands with a Rashba-like disper- sion. Analysis of Landau level crossings enables a parameter-free comparison to a newly derived “magic series” of level crossings in magnetic field and provides constraints on the parameters w0 and w1 of the Bistritzer-MacDonald MATBG Hamiltonian. Overall, our data provides direct insights into the complex nature of symmetry breaking in MATBG and allows for quantitative tests of the proposed microscopic scenarios for its electronic phases.