Emergence of Chern insulating states in non-Magic angle twisted bilayer graphene

TL;DR

This study investigates how twist angle variations in bilayer graphene influence topological insulating states, revealing the emergence and evolution of Chern insulators and Hofstadter butterfly phases as the angle deviates from the magic angle.

Contribution

It provides the first detailed magneto-transport analysis of non-magic angle twisted bilayer graphene, mapping the transition from Chern insulators to Hofstadter phases with changing twist angle.

Findings

Chern insulating states emerge at 1.25° with Chern number |C|=4-|v|

Chern insulators vanish and evolve into Hofstadter butterfly states for angles >1.25°

Magnetic flux in moiré unit cell influences quantum Hall phases

Abstract

Twisting two layers into a magic angle (MA) of ~1.1{\deg} 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{\deg} at one end to 1.43{\deg} at the other. For {\theta}=1.25{\deg}, 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 moir\'e unite cell. When {\theta}>1.25{\deg}, the Chern insulator from flat band disappears and evolves into fractal Hofstadter butterfly quantum Hall insulator where magnetic flux in one moir\'e unite cell matters. Our observations will stimulate further theoretical and experimental investigations on the relationship between electron interactions and non-trivial band topology.