Exciton-phonon coupling in bulk hexagonal boron nitride from first principles: phonon-assisted UV absorption and emission spectra

We present an ab initio method to calculate phonon-assisted absorption and emission spectra when strong excitonic effects are present. We apply the method to bulk hexagonal BN, a material with indirect band gap which exhibits strong luminescence in the UV range. We first calculate the excitons at the wave vector q_i of the indirect gap with the Bethe-Salpeter equation. The coupling of these excitons with the various phonon modes at q_i is investigated with group theory and then expressed in terms of a product of the mean square displacement of the atoms and the second derivative of the optical response function with respect to atomic displacement along the phonon eigenvectors. The derivatives are calculated numerically with a finite difference scheme in a supercell commensurate with q_i. We use detailed balance arguments to obtain the intensity ratio between emission and absorption processes. Our results[1] explain recent luminescence experiments[2] and reveal the exciton-phonon coupling channels responsible for the emission lines.

[1] arXiv:1810.08976 (2018)
[2] Nat. Photonics 10, 262 (2016)