Direct observation of the lowest indirect exciton state in the bulk of hexagonal boron nitride

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 similar to 100 meV and, as we argue based on a comparison to our DFT calculations, is predominantly polarized along the Gamma K direction of the hexagonal Brillouin zone. Our data support the recent report by Cassabois et al. [Nat. Photonics 10, 262 (2016)] who indirectly inferred the existence of this mode from the photoluminescence signal, thereby establishing h-BN as an indirect semiconductor.