Tunable Excitons in Rhombohedral Trilayer Graphene

TL;DR

This paper investigates how an external electric field can open a bandgap in rhombohedral trilayer graphene and explores the tunable excitonic optical responses using a combined theoretical approach.

Contribution

It introduces a semi-analytical method to study excitonic effects in rhombohedral trilayer graphene under external electric fields.

Findings

External electric field opens a tunable bandgap.

Excitonic resonances can be controlled via the electric field.

Excitonic states become more localized with increasing field.

Abstract

Trilayer graphene is receiving an increasing level of attention due to its stacking--dependent magnetoelectric and optoelectric properties, and its more robust ferromagnetism relative to monolayer and bilayer variants. Additionally, rhombohedral stacked trilayer graphene presents the possibility of easily opening a gap via either an external electric field perpendicular to the layers, or via the application of external strain. In this paper, we consider an external electric field to open a bandgap in rhombohedral trilayer graphene and study the excitonic optical response of the system. This is done via the combination of a tight binding model with the Bethe--Salpeter equation, solved semi--analytically and requiring only a simple numerical quadrature. We then discuss the valley--dependent optical selection rules, followed by the computation of the excitonic linear optical conductivity for the case of a rhombohedral graphene trilayer encapsulated in hexagonal boron nitride. The tunability of the excitonic resonances via an external field is also discussed, together with the increasing localization of the excitonic states as the field increases.