Exciton interference in hexagonal boron nitride

TL;DR

This paper provides a detailed analysis of exciton dispersion in bulk hexagonal boron nitride using ab initio methods, revealing interference effects between transition groups and offering new tools for understanding excitonic phenomena.

Contribution

It introduces a novel methodology for analyzing excitonic transitions and their interference effects, applicable to any material system.

Findings

Reproduces experimental exciton dispersion data accurately.

Identifies two groups of transitions ($KM$ and $MK'$) that interfere to shape excitonic peaks.

Demonstrates the potential to tune excitonic effects via transition interference.

Abstract

In this letter we report a thorough analysis of the exciton dispersion in bulk hexagonal boron nitride. We solve the ab initio GW Bethe-Salpeter equation at finite $\mathbf{q}\parallel \Gamma K$, and we compare our results with recent high-accuracy electron energy loss data. Simulations reproduce the measured dispersion and the variation of the peak intensity. We focus on the evolution of the intensity, and we demonstrate that the excitonic peak is formed by the superposition of two groups of transitions that we call $KM$ and $MK'$ from the k-points involved in the transitions. These two groups contribute to the peak intensity with opposite signs, each damping the contributions of the other. The variations in number and amplitude of these transitions determine the changes in intensity of the peak. Our results contribute to the understanding of electronic excitations in this systems along the $\Gamma K$ direction, which is the relevant direction for spectroscopic measurements. They also unveil the non-trivial relation between valley physics and excitonic dispersion in h--BN, opening the possibility to tune excitonic effects by playing with the interference between transitions. Furthermore, this study introduces analysis tools and a methodology that are completely general. They suggest a way to regroup independent-particle transitions which could permit a deeper understanding of excitonic properties in any system.