# Direct Observation of the Lowest Indirect Exciton State in the Bulk of Hexagonal Boron Nitride

## TL;DR

This study combines electron energy-loss spectroscopy and first-principles calculations to directly observe the lowest indirect exciton state in bulk hexagonal boron nitride, confirming its indirect semiconductor nature.

## Contribution

It provides the first direct experimental observation and detailed characterization of the lowest indirect exciton in bulk h-BN, supported by theoretical calculations.

## Key findings

- Identified a sharp, narrow mode with ~100 meV bandwidth
- Confirmed the exciton is predominantly polarized along the ΓK direction
- Supported the classification of h-BN as an indirect semiconductor

## Abstract

We combine electron energy-loss spectroscopy and first-principles calculations based on density-functional theory (DFT) to identify the lowest indirect exciton state in the in-plane charge response of hexagonal boron nitride (h-BN) single crystals. This remarkably sharp mode forms a narrow pocket with a dispersion bandwidth of $\sim 100~\text{meV}$ and, as we argue based on a comparison to our DFT calculations, is predominantly polarized along the $\Gamma\text{K}$-direction of the hexagonal Brillouin zone. Our data support the recent report by Cassabois \emph{et al.} (Nat. Photon. \textbf{10}, 262) who indirectly inferred the existence of this mode from the photoluminescence signal, thereby establishing h-BN as an indirect semiconductor.