Giant exciton-phonon coupling and zero-point renormalization in hexagonal monolayer boron nitride

Abstract

We report here a giant zero-point energy renormalization of 273 meV in the direct band gap at $\mathbf{K}$ in the Brillouin zone and a 571 meV of blueshifting in the position of the doubly-degenerate brightest excitonic peak in monolayer hexagonal boron nitride. The nonradiative exciton linewidth is found to be 97 meV at 0 K with a large coupling strength of 1.1 eV. This linewidth is found to be mainly dominated by the scattering from the longitudinal optical phonons near the degenerate LO-TO mode, with negligible contributions from other lower branches. Additionally, the band gap has a temperature dependent slope of $\ensuremath{-}0.53$ meV ${\mathrm{K}}^{\ensuremath{-}1}$, which we found to be in excellent agreement with the reported experimental data on large diameter boron nitride nanotubes. We obtained our results by solving a coupled electron-hole Bethe-Salpeter equation which includes the lattice vibrational dynamics, purely using an ab initio approach.