Strongly interacting Hofstadter states in magic-angle twisted bilayer graphene

Abstract

Magic-angle twisted bilayer graphene hosts a variety of strongly correlated states at partial fillings of its flat bands. In a magnetic field, these flat bands evolve into a Hofstadter spectrum renormalized by strong Coulomb interactions. Here we study the interacting Hofstadter states that spontaneously form within the topological magnetic sub-bands of an ultraclean magic-angle twisted bilayer graphene device, including symmetry-broken Chern insulator states and fractional quantum Hall states. The observed symmetry-broken Chern insulator states form a cascade, with their Chern numbers mimicking the main sequence of correlated Chern insulators. The fractional quantum Hall states form in a Jain sequence. However, they disappear at high magnetic field, in contrast to conventional fractional quantum Hall states that strengthen with increasing magnetic field. We reveal a magnetic-field-driven phase transition from composite fermion phases to a dissipative Fermi liquid. Our theoretical analysis of the magnetic sub-bands hosting the fractional quantum Hall states predicts non-uniform quantum geometric properties far from the lowest Landau level. This points towards a more natural interpretation of these states as in-field fractional Chern insulators of the magnetic sub-bands. Interacting electrons in Hofstadter bands can form symmetry-broken topological states. These are now revealed in magic-angle twisted bilayer graphene, and their properties are influenced by non-uniform quantum geometry.