Strongly Correlated Chern Insulators in Magic-Angle Twisted Bilayer Graphene

TL;DR

This paper reports the discovery of a sequence of strongly correlated topological phases, specifically Chern insulators with various Chern numbers, in magic-angle twisted bilayer graphene using a novel local spectroscopic technique, revealing the role of electron interactions.

Contribution

The study introduces a new STM-based method to detect topological phases in MATBG and demonstrates that strong electron-electron interactions can produce multiple Chern insulators beyond weakly interacting predictions.

Findings

Detection of Chern insulators with C = ±1, ±2, ±3 in MATBG

Strong correlations can induce topological phases without magnetic fields

Observation of phases stabilized by modest magnetic fields

Abstract

Interactions among electrons and the topology of their energy bands can create novel quantum phases of matter. Most topological electronic phases appear in systems with weak electron-electron interactions. The instances where topological phases emerge only as a result of strong interactions are rare, and mostly limited to those realized in the presence of intense magnetic fields. The discovery of flat electronic bands with topological character in magic-angle twisted bilayer graphene (MATBG) has created a unique opportunity to search for new strongly correlated topological phases. Here we introduce a novel local spectroscopic technique using a scanning tunneling microscope (STM) to detect a sequence of topological insulators in MATBG with Chern numbers C = $\pm$ 1, $\pm$ 2, $\pm$ 3, which form near $\nu$ = $\pm$ 3, $\pm$ 2, $\pm$ 1 electrons per moir\'e unit cell respectively, and are stabilized by the application of modest magnetic fields. One of the phases detected here (C = +1) has been previously observed when the sublattice symmetry of MATBG was intentionally broken by hexagonal boron nitride (hBN) substrates, with interactions playing a secondary role. We demonstrate that strong electron-electron interactions alone can produce not only the previously observed phase, but also new and unexpected Chern insulating phases in MATBG. The full sequence of phases we observed can be understood by postulating that strong correlations favor breaking time-reversal symmetry to form Chern insulators that are stabilized by weak magnetic fields. Our findings illustrate that many-body correlations can create topological phases in moir\'e systems beyond those anticipated from weakly interacting models.