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

TL;DR

This paper investigates the magnetic field-induced phases of magic angle twisted bilayer graphene, revealing a sequence of Chern insulators driven by interactions and analyzing Landau level crossings to constrain theoretical models.

Contribution

It provides the first detailed magneto-transport study of symmetry-breaking phases in MATBG and introduces a new series of Landau level crossings for parameter estimation.

Findings

Identification of a sequence of Chern insulators with various Chern numbers.

Evidence that these phases are driven by electronic interactions breaking time-reversal symmetry.

Discovery of a Rashba-like dispersion and a new series of Landau level crossings.

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 quantized Hall conductance with Chern numbers C = +(-)1, +(-)2, +(-)3, +(-)4 which nucleate from integer fillings of the moire unit cell v = +(-)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-oscillation in the yet unexplored higher energy dispersive bands with a Rashba-like dis-persion. 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. Over-all, 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.