Direct Observation of Massless Excitons and Linear Exciton Dispersion

TL;DR

This study experimentally confirms the existence of massless excitons with linear dispersion in monolayer hexagonal boron nitride, aligning with theoretical predictions and advancing understanding of 2D excitonic physics.

Contribution

First direct experimental observation of massless excitons with linear dispersion in 2D materials, validating theoretical models and clarifying the band gap of monolayer hBN.

Findings

Observed linear exciton dispersion in monolayer hBN

Identified the lowest dipole-allowed transition at 6.6 eV

Confirmed theoretical predictions with ab initio calculations

Abstract

Excitons -- elementary excitations formed by bound electron-hole pairs -- govern the optical properties and excited-state dynamics of materials. In two-dimensions (2D), excitons are theoretically predicted to have a linear energy-momentum relation with a non-analytic discontinuity in the long wavelength limit, mimicking the dispersion of a photon. This results in an exciton that behaves like a massless particle, despite the fact that it is a composite boson composed of massive constituents. However, experimental observation of massless excitons has remained elusive. In this work, we unambiguously experimentally observe the predicted linear exciton dispersion in freestanding monolayer hexagonal boron nitride (hBN) using momentum-resolved electron energy-loss spectroscopy. The experimental result is in excellent agreement with our theoretical prediction based on ab initio many-body perturbation theory. Additionally, we identify the lowest dipole-allowed transition in monolayer hBN to be at 6.6 eV, illuminating a long-standing debate about the band gap of monolayer hBN. These findings provide critical insights into 2D excitonic physics and open new avenues for exciton-mediated superconductivity, Bose-Einstein condensation, and high-efficiency optoelectronic applications.