Excitonic Effects in Hexagonal Boron Nitride Multilayer Systems*

Hexagonal boron nitride (hBN) is a layered material and an insulator displaying strong excitonic effects.
We investigate theoretically the effects of layer stacking on the optical properties of few-layer hBN systems.
Using both ab initio simulations and a tight-binding model for an effective Hamiltonian describing the excitons, we show that the fine structure of the optical absorption spectra is characterized by the the splitting between low-lying surface excitons, that are mostly localized on the outer layers, and inner, bulk-like excitons.
Moreover, in the case of single-layer hBN, we study the correction to the first excitonic peak due to exciton-phonon interaction.
We compare a single-phonon, dynamical method [Marini, PRL 101, 2008] based on the calculation of the electron-phonon self-energy with the static, multi-phonon Williams-Lax theory [Zacharias et al, PRL 115, 2015] applied to the Bethe-Salpeter equation.
We find a strong redshift of the exciton peak (around 0.5 eV), but a relatively small broadening.
In the case of bulk hBN, we perform finite-differences simulations on static atomic displacements to investigate the cross section of phonon-assisted indirect absorption and excitation of finite-q excitons.