Phonon-mediated renormalization of exciton energies and absorption spectra in polar semiconductors

TL;DR

This study uses first-principles calculations to show how phonon-assisted screening affects exciton energies and absorption spectra in polar semiconductors, revealing shifts up to 50 meV due to vibrational effects.

Contribution

It introduces a method to explicitly include phonon-assisted screening in Bethe-Salpeter equation calculations for excitons in polar semiconductors.

Findings

Exciton binding energies are renormalized by tens of meV.

Absorption peaks shift by up to 50 meV due to vibrational screening.

Long-range Fröhlich coupling dominates vibrational screening effects.

Abstract

We investigate the influence of vibrational screening on the excitonic and optical properties of solids based on first-principles electronic-structure calculations. We solve the Bethe-Salpeter equation - the state-of-the-art description of excitons - by explicitly accounting for phonon-assisted screening effects in the screened Coulomb interaction. In the examples of the polar semiconductors ZnS, MgO, and GaN, the exciton binding energies at the absorption onset are found to be renormalized by a few tens of meV. Similar effects are also found for higher-lying unbound electron-hole pairs, leading to red-shifts of the absorption peaks by up to 50 meV. Our analysis reveals that vibrational screening is dictated by long-range Fr\"ohlich coupling involving polar longitudinal optical phonons, whereas the remaining vibrational degrees of freedom are negligible. Overall, by elucidating the influence of phonon screening on the excitonic states and absorption spectra of these selected ionic semiconductors, this study contributes to advancing the ab initio methodology and the fundamental understanding of exciton-phonon coupling in solids.