Displacement field-controlled fractional Chern insulators and charge density waves in a graphene/hBN moir\'e superlattice

TL;DR

This study demonstrates how displacement fields can control topological phases and charge density waves in a graphene/hBN moiré superlattice, revealing tunable fractional Chern insulators and phase transitions in a highly adaptable system.

Contribution

It introduces a method to manipulate topological and trivial phases in moiré graphene using displacement fields, highlighting the emergence of fractional Chern insulators and charge density waves.

Findings

Observation of integer and fractional Chern insulators at specific filling factors

Persistence of the v=1/3 state down to 0.2 T magnetic field

Mapping of a phase diagram showing field-tuned topological and trivial states

Abstract

Rhombohedral multilayer graphene, with its flat electronic bands and concentrated Berry curvature, is a promising material for the realization of correlated topological phases of matter. When aligned to an adjacent hexagonal boron nitride (hBN) layer, the graphene develops narrow minibands with non-trivial topology. By tuning an externally-applied electric displacement field, the conduction electrons can either be pushed towards or away from the moir\'e superlattice. Motivated by the recent observation of the fractional quantum anomalous Hall effect (FQAHE) in the moir\'e-distant case, we study the opposite moir\'e-proximal case, where the superlattice potential is considerably stronger. We explore the physics within the moir\'e conduction bands through capacitance measurements that allow us to determine the inverse electronic compressibility and extract energy gaps of incompressible states. We observe integer and fractional Chern insulator states at superlattice filling factors v = 1, 2/3, and 1/3 with Streda slopes of -1, -2/3, and -1/3, respectively. Remarkably, the v = 1/3 state persists down to a magnetic field of 0.2 T. In addition, we also observe numerous trivial and topological charge density waves. We map out a phase diagram that is highly sensitive to both displacement and magnetic fields, which tune the system between various ground states by modifying the band dispersion and the structure of the electronic wavefunctions. This work demonstrates displacement field control of topological phase transitions in the moir\'e-proximal limit of rhombohedral pentalayer graphene, creating a highly-tunable platform for studying the interplay between intrinsic band topology and strong lattice effects.