Competing correlated states and abundant orbital magnetism in twisted monolayer-bilayer graphene

TL;DR

This paper explores the rich correlated phases and orbital magnetism in twisted monolayer-bilayer graphene, revealing anomalous Hall effects, Chern insulators, and potential intervalley coherent states influenced by external fields and twist angles.

Contribution

It uncovers the prevalence of orbital magnetism and competing correlated states in tMBG, highlighting the role of external electric and magnetic fields in tuning topological and magnetic phases.

Findings

Orbital magnetism is abundant in tMBG's correlated phases.

Anomalous Hall effect observed near odd integer fillings.

Correlated Chern insulators stabilized by magnetic fields.

Abstract

Flat band moir\'e superlattices have recently emerged as unique platforms for investigating the interplay between strong electronic correlations, nontrivial band topology, and multiple isospin 'flavor' symmetries. Twisted monolayer-bilayer graphene (tMBG) is an especially rich system owing to its low crystal symmetry and the tunability of its bandwidth and topology with an external electric field. Here, we find that orbital magnetism is abundant within the correlated phase diagram of tMBG, giving rise to the anomalous Hall effect (AHE) in correlated metallic states nearby most odd integer fillings of the flat conduction band, as well as correlated Chern insulator states stabilized in an external magnetic field. The behavior of the states at zero field appears to be inconsistent with simple spin and valley polarization for the specific range of twist angles we investigate, and instead may plausibly result from an intervalley coherent (IVC) state with an order parameter that breaks time reversal symmetry. The application of a magnetic field further tunes the competition between correlated states, in some cases driving first-order topological phase transitions. Our results underscore the rich interplay between closely competing correlated ground states in tMBG, with possible implications for probing exotic IVC ordering.