Symmetry broken Chern insulators and magic series of Rashba-like Landau level crossings in magic angle bilayer graphene (2007.13390v1)

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.

Symmetry Breaking and Topological Insights in Magic Angle Bilayer Graphene

The paper addresses the emergence of symmetry breaking phases in Magic Angle Twisted Bilayer Graphene (MATBG), exploring quantum phenomena induced by electronic interactions under the influence of external magnetic fields. It provides a meticulous examination of symmetry-broken Chern insulators and introduces a "magic series" concept related to Rashba-like Landau level crossings that add a new dimension to the MATBG phase diagram.

In MATBG, a specific twisting known as the magic angle (around 1.1°) results in ultra-flat bands that promote strong correlations leading to diverse quantum phases, such as correlated insulators, superconductivity, and orbital magnetism. Whereas previous studies focused on the role of external alignments like hexagonal boron nitride (hBN) inducing symmetry breaking, this research uniquely investigates intrinsic mechanisms. By leveraging magneto-transport and Hall measurements, the researchers observed wedge-like regions manifesting quantized Hall conductance with Chern numbers ranging from ±1 to ±4, tethered to integer fillings of the moiré unit cell.

These findings add significant insights into MATBG's zero-field phase diagram, revealing spin and valley polarized Chern insulators, similar to quantum Hall ferromagnets. This argues that symmetry breaking can be driven directly by electronic interactions without structural alignments, causing time-reversal symmetry to be broken and driving valley polarization. This perspective contrasts with models suggesting that symmetry breaking arises more typically from alignment with hBN substrates.

A pivotal aspect of the paper is its investigation of quantum magneto-oscillations in the higher energy dispersive bands beyond the flat-band regime, using Rashba-like Hamiltonian models. The introduced "magic series" of Landau level crossings presents a parameter-independent framework guiding the paper of these bands. This approach facilitates an extensive evaluation of the Rashba coupling, reflecting high consistency with experimental observations, and imposes constraints on the Bistritzer-MacDonald MATBG Hamiltonian parameters.

The implications both experimental and theoretical are considerable: the research provides concrete boundaries for understanding MATBG's electronic phases and facilitates precise parameter estimation for future theoretical models. The paper proposes sophisticated interpretations of the quantum phases permeating the MATBG system absent direct structural symmetry breaking.

Looking ahead, the findings suggest avenues for probing other emergent quantum states in MATBG, granting deeper comprehension of its correlated phases—potentially paving pathways for new quantum technologies harnessed from symmetry-broken electronic configurations. Moreover, further exploration into Rashba-type interactions in diverse materials might enhance predictions about topological and quantum transport phenomena in condensed matter physics.