Correlated Charge Density Wave Insulators in Chirally Twisted Triple Bilayer Graphene

TL;DR

This paper introduces chirally twisted triple bilayer graphene as a tunable flat-band system exhibiting novel correlated charge density wave insulators at fractional moiré fillings, expanding the understanding of moiré electronics.

Contribution

It demonstrates the existence of a new type of correlated charge density wave insulators in twisted graphene structures, with experimental and theoretical evidence.

Findings

Observation of insulating states at half-integer moiré fillings.

Enhanced half-integer states under weak magnetic field.

Identification of charge density wave insulators via Hartree-Fock calculations.

Abstract

Electrons residing in flat-band system can play a vital role in triggering spectacular phenomenology due to relatively large interactions and spontaneous breaking of different degeneracies. In this work we demonstrate chirally twisted triple bilayer graphene, a new moir\'e structure formed by three pieces of helically stacked Bernal bilayer graphene, as a highly tunable flat-band system. In addition to the correlated insulators showing at integer moir\'e fillings, commonly attributed to interaction induced symmetry broken isospin flavors in graphene, we observe abundant insulating states at half-integer moir\'e fillings, suggesting a longer-range interaction and the formation of charge density wave insulators which spontaneously break the moir\'e translation symmetry. With weak out-of-plane magnetic field applied, as observed half-integer filling states are enhanced and more quarter-integer filling states appear, pointing towards further quadrupling moir\'e unit cells. The insulating states at fractional fillings combined with Hartree-Fock calculations demonstrate the observation of a new type of correlated charge density wave insulators in graphene and points to a new accessible twist manner engineering correlated moir\'e electronics.