Excitons in van der Waals materials: from monolayer to bulk hexagonal boron nitride

TL;DR

This paper develops a model to understand exciton properties in layered van der Waals materials, specifically analyzing hexagonal boron nitride, and explores how interlayer interactions influence exciton behavior.

Contribution

It introduces a simplified excitonic Hamiltonian based on single-layer building blocks and applies it to analyze exciton spectra in hBN from ab initio calculations.

Findings

Identification of the character of lowest-energy excitons in hBN

Analysis of interlayer hopping and exchange effects on exciton dispersion

Relation between exciton and plasmon excitations in layered materials

Abstract

We present a general picture of the exciton properties of layered materials in terms of the excitations of their single-layer building blocks. To this end, we derive a model excitonic hamiltonian by drawing an analogy with molecular crystals, which are other prototypical van der Waals materials. We employ this simplified model to analyse in detail the excitation spectrum of hexagonal boron nitride (hBN) that we have obtained from the {\it ab initio} solution of the many-body Bethe-Salpeter equation as a function of momentum. In this way we identify the character of the lowest-energy excitons in hBN, discuss the effects of the interlayer hopping and the electron-hole exchange interaction on the exciton dispersion, and illustrate the relation between exciton and plasmon excitations in layered materials.