Spectroscopy of a Tunable Moir\'e System with a Correlated and Topological Flat Band

TL;DR

This study uses gate-tuned scanning tunneling spectroscopy to demonstrate the tunability of the band structure, correlations, and topology in twisted double bilayer graphene, revealing a correlated insulator gap and topological properties of its flat band.

Contribution

It provides direct spectroscopic evidence of tunable band structure, correlations, and topology in twisted double bilayer graphene using GT-STS, aligning with continuum model predictions.

Findings

Observation of a correlated insulator gap at partial flat band filling

Detection of valley polarization and Chern band splitting under magnetic field

Confirmation of band structure tunability with electric field

Abstract

Moir\'e superlattices created by the twisted stacking of two-dimensional crystalline monolayers can host electronic bands with flat energy dispersion in which interaction among electrons is strongly enhanced. These superlattices can also create non-trivial electronic band topologies making them a platform for study of many-body topological quantum states. Among the moir\'e systems realized to date, there are those predicted to have band structures and properties which can be controlled with a perpendicular electric field. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a tunable moir\'e system, for which partial filling of its flat band, transport studies have found correlated insulating states. Here we use gate-tuned scanning tunneling spectroscopy (GT-STS) to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of both electronic correlations and topology for its flat band. Our spectroscopic experiments show excellent agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands that respond strongly to the field with a large effective g-factor. Our experiments advance our understanding of the properties of TDBG and set the stage for further investigations of correlation and topology in such tunable moir\'e systems.