Electronic properties of twisted bilayer graphene suspended and encapsulated with hexagonal boron nitride

TL;DR

This study investigates how hexagonal boron nitride (hBN) substrates influence the electronic, optical, and topological properties of twisted bilayer graphene (TBG), revealing significant modifications depending on whether TBG is suspended or encapsulated.

Contribution

The paper provides a detailed atomistic analysis of the effects of hBN substrates on TBG's properties, highlighting the sensitivity and potential for tuning electronic behaviors in moiré superlattices.

Findings

hBN significantly alters TBG's bandwidth, band gap, and optical conductivity

alignment with hBN affects the topological response to electric fields

suspended and encapsulated TBG exhibit different electronic responses to external electric fields

Abstract

The recent observed anomalous Hall effect in magic angle twisted bilayer graphene (TBG) aligned to hexagonal boron nitride (hBN) and unconventional ferroelectricity in Bernal bilayer graphene sandwiched by hBN present a new platform to tune the correlated properties in graphene systems. In these graphene-based moir\'e superlattices, the aligned hBN substrate plays an important role. In this paper, we analyze the effects of hBN substrate on the band structure of the TBG. By means of an atomistic tight-binding model we calculate the electronic properties of TBG suspended and encapsulated with hBN. Interestingly, we found that the physical properties of TBG are extremely sensitive to the presence of hBN and they may be completely different if TBG is suspended or encapsulated. We quantify these differences by analysing their electronic properties, optical conductivity and band topology. We found that the narrow bandwidth, band gap, local density of states and optical conductivity are significantly modified by the aligned hBN substrates. Interestingly, these electronic properties can be used as a signature of the alignment in experiment. Moreover, the TBG/hBN superlattices in the presence or absence of the two-fold rotation symmetry response differently to the external electric field. For the TBG suspended in the hBN, application of an electric field results in the charge unevenly distributed between graphene layers, which can be used to tune the strength of the valley Hall effect or the anomalous Hall effect. Such rich topological phase diagram in these systems may be useful for experiments.