Effects of dynamical lattice screening on excitonic and optical properties of polar compounds

The Bethe-Salpeter equation (BSE) framework is very common for predicting optical and excitonic properties in materials. Typically, only high-frequency electronic dielectric screening is considered in the screened electron-hole Coulomb interaction. In materials hosting polar phonon modes that induce macroscopic electric fields, there is an additional dynamical lattice screening effect. To describe this contribution, we use the Shindo approximation and the Fröhlich model to predict exciton binding energies in NaCl, MgO, AlN, and GaN. Dynamical lattice screening exerts a small influence on the excitonic properties of the high band gap materials NaCl and MgO. In AlN, the predicted exciton binding energy reduces from 145 to 112 meV. In GaN, dynamical lattice screening drastically reduces the exciton binding energy from 52 to 30 meV, in good agreement with experiments. The optical spectra for all compounds are predicted to be in good agreement with experimental spectra under the inclusion of effective dynamical lattice screening, and we find that it is a critical contributor to the screened electron-hole interaction in polar materials with low exciton binding energies and high longitudinal optical phonon frequencies.