Strongly interacting Hofstadter states in magic-angle twisted bilayer graphene

TL;DR

This study explores the complex interplay of strong correlations, topology, and magnetic fields in magic-angle twisted bilayer graphene, revealing novel Hofstadter states, symmetry-broken Chern insulators, and fractional quantum Hall phenomena.

Contribution

It uncovers the formation of symmetry-broken Chern insulators and fractional quantum Hall states in MATBG under magnetic fields, with insights into their phase transitions and topological properties.

Findings

Observation of symmetry-broken Chern insulator cascade.

FQH states form in Jain sequence and vanish at high magnetic fields.

Identification of a phase transition from composite fermions to Fermi liquid.

Abstract

Magic-angle twisted bilayer graphene (MATBG) hosts a multitude of strongly correlated states at partial fillings of its flat bands. In a magnetic field, these flat bands further evolve into a unique Hofstadter spectrum renormalized by strong Coulomb interactions. Here, we study the interacting Hofstadter states spontaneously formed within the topological magnetic subbands of an ultraclean MATBG device, notably including symmetry-broken Chern insulator (SBCI) states and fractional quantum Hall (FQH) states. The observed SBCI states form a cascade with their Chern numbers mimicking the main sequence correlated Chern insulators. The FQH states in MATBG form in Jain sequence; however, they disappear at high magnetic field, distinct from conventional FQH states which strengthen with increasing magnetic field. We reveal a unique magnetic field-driven phase transition from composite fermion phases to a dissipative Fermi liquid. Our theoretical analysis of the magnetic subbands hosting FQH states predicts non uniform quantum geometric properties far from the lowest Landau level. This points towards a more natural interpretation of these FQH states as in-field fractional Chern insulators of the magnetic subbands.