Cascade of zero-field Chern insulators in magic-angle bilayer graphene.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Mechanics

Background:

• Strong electron-electron interactions and symmetry breaking significantly influence material topological properties.
• In magic-angle twisted bilayer graphene (MATBG), broken C2z or C2zT symmetry leads to a non-zero Chern number in the flat band.
• Electron interactions can lift SU(4) degeneracy, creating Chern insulator states.

Purpose of the Study:

• To experimentally observe and characterize zero-field Chern insulators in hBN-aligned MATBG.
• To investigate the role of broken C2z symmetry in inducing topological states.
• To explore Chern insulator states at various odd-integer and valence band fillings.

Main Methods:

• Fabrication of hexagonal boron nitride (hBN)-aligned magic-angle twisted bilayer graphene (MATBG).
• Electrical transport measurements to probe electronic properties and identify topological phases.
• Analysis of Hall resistance and longitudinal resistance under varying magnetic fields and doping levels.

Main Results:

• Observation of a complete sequence of zero-field Chern insulators at odd-integer fillings (ν = ±1, ±3) with distinct chiralities.
• First-time observation of Chern states at hole fillings (ν = -1, -3) with opposite chirality to electron fillings.
• Identification of a zero-field symmetry-broken Chern insulator (C = -1) at valence band filling ν = -7/2.
• Demonstration of quantized Hall resistance and vanishing longitudinal resistance at ν = -3 under magnetic field, indicating stabilized incommensurate Chern insulator states.
• Observation of a Streda-formula violation around the ν = -3 state.

Conclusions:

• The study provides the first experimental evidence of zero-field Chern insulators in the flat valence band of MATBG with broken C2z symmetry.
• These findings complete the topological framework for MATBG under broken C2z symmetry.
• The results highlight the crucial interplay between electron interactions, symmetry breaking, and topological phenomena in twisted bilayer graphene.