Emergence of Chern Insulating States in Non-Magic Angle Twisted Bilayer Graphene

Twisting two layers into a magic angle (MA) of similar to 1.1 degrees is found essential to create low energy flat bands and the resulting correlated insulating, superconducting, and magnetic phases in twisted bilayer graphene (TBG). While most of previous works focus on revealing these emergent states in MA-TBG, a study of the twist angle dependence, which helps to map an evolution of these phases, is yet less explored. Here, we report a magneto-transport study on one non-magic angle TBG device, whose twist angle theta changes from 1.25 degrees at one end to 1.43 degrees at the other. For theta = 1.25 degrees we observe an emergence of topological insulating states at hole side with a sequence of Chern number | C | = 4 - | v |, where v is the number of electrons (holes) in moire unite cell. When theta > 1.25 degrees, the Chern insulator from flat band disappears and evolves into fractal Hofstadter butterfly quantum Hall insulator where magnetic flux in one moire unite cell matters. Our observations will stimulate further theoretical and experimental investigations on the relationship between electron interactions and non-trivial band topology.

Automated summary

Twisting two layers of graphene into a magic angle of around 1.1 degrees is crucial for creating low-energy flat bands and related insulating, superconducting, and magnetic phases. This study explores the dependence of these phases on the twist angle, revealing the emergence of topological insulating states and the evolution into fractal Hofstadter butterfly quantum Hall insulators in TBG devices with varying twist angles.