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

Minhao He,Yuhao Li,Takashi Taniguchi,Kenji Watanabe,Matthew Yankowitz,Ya-Hui Zhang,Zaiyao Fei,Xiaodong Xu

doi:10.1038/s41467-021-25044-1

Abstract

Flat band moiré 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 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.

Highlights

Flat band moiré superlattices have recently emerged as unique platforms for investigating the interplay between strong electronic correlations, nontrivial band topology, and multiple isospin ‘flavor’ symmetries
The maps are primarily confined to the flat conduction band (0 ≤ ν ≤ 4, where ν is the band filling factor as defined in Methods) and large ∣D∣, for which correlated states are observed at low temperatures
Our results reveal the richness of the correlated phase diagram of Twisted monolayer-bilayer graphene (tMBG), which can be tuned sensitively with the combination of ν, D, B, and θ

Summary

Introduction

Flat band moiré 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. 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. Twisted monolayer-bilayer graphene (tMBG) features lower crystal symmetry, and consequentially the lattice degeneracy is strongly lifted at the single-particle level in an external displacement field, D. The high tunability of the bands with the combination of twist angle, doping, D, and magnetic field makes tMBG an attractive platform for investigating the nature of closely competing correlated and topological ground states. Note 4 and Supplementary Table 1 provide a detailed summary of the correlated states observed in our three devices, as well as those previously reported in refs. 14–16

Methods

Results

Conclusion