Unconventional sequence of correlated Chern insulators in magic-angle twisted bilayer graphene

TL;DR

This study uncovers a new sequence of correlated Chern insulators in magic-angle twisted bilayer graphene, revealing unexpected topological states at zero magnetic field due to broken translation symmetry, expanding the understanding of its phase diagram.

Contribution

It demonstrates the existence of unconventional Chern insulators in MATBG caused by broken translation symmetry, which splits bands and leads to novel incompressible states not explained by simple models.

Findings

Observation of incompressible states with unexpected Chern numbers at zero magnetic field.

Identification of broken translation symmetry doubling the moiré unit cell.

Expansion of the phase diagram of MATBG with new correlated phases.

Abstract

The interplay between strong electron-electron interactions and band topology can lead to novel electronic states that spontaneously break symmetries. The discovery of flat bands in magic-angle twisted bilayer graphene (MATBG) with nontrivial topology has provided a unique platform in which to search for new symmetry-broken phases. Recent scanning tunneling microscopy and transport experiments have revealed a sequence of topological insulating phases in MATBG with Chern numbers $C=\pm 3, \, \pm 2, \, \pm 1$ near moir\'e band filling factors $\nu = \pm 1, \, \pm 2, \, \pm 3$, corresponding to a simple pattern of flavor-symmetry-breaking Chern insulators. Here, we report high-resolution local compressibility measurements of MATBG with a scanning single electron transistor that reveal a new sequence of incompressible states with unexpected Chern numbers observed down to zero magnetic field. We find that the Chern numbers for eight of the observed incompressible states are incompatible with the simple picture in which the $C= \pm 1$ bands are sequentially filled. We show that the emergence of these unusual incompressible phases can be understood as a consequence of broken translation symmetry that doubles the moir\'e unit cell and splits each $C=\pm 1$ band into a $C=\pm 1$ band and a $C=0$ band. Our findings significantly expand the known phase diagram of MATBG, and shed light onto the origin of the close competition between different correlated phases in the system.