Phonon screening and dissociation of excitons at finite temperatures from first principles

TL;DR

This paper presents a first-principles method to analyze how phonons influence exciton properties at finite temperatures, revealing their screening effects, temperature-dependent binding energies, and ultrafast dissociation dynamics in semiconductors.

Contribution

It introduces a parameter-free, ab initio approach to compute temperature-dependent phonon screening effects on excitons, extending previous models and validating them across various materials.

Findings

Phonon screening significantly affects exciton binding energies.

Excitons can dissociate rapidly due to phonon interactions at finite temperatures.

The developed method accurately predicts temperature-dependent exciton properties.

Abstract

The properties of excitons, or correlated electron-hole pairs, are of paramount importance to optoelectronic applications of materials. A central component of exciton physics is the electron-hole interaction, which is commonly treated as screened solely by electrons within a material. However, nuclear motion can screen this Coulomb interaction as well, with several recent studies developing model approaches for approximating the phonon screening to the properties of excitons. While these model approaches tend to improve agreement with experiment for exciton properties, they rely on several approximations that restrict their applicability to a wide range of materials, and thus far they have neglected the effect of finite temperatures. Here, we develop a fully first-principles, parameter-free approach to compute the temperature-dependent effects of phonon screening within the ab initio GW-Bethe Salpeter equation framework. We recover previously proposed models of phonon screening as well-defined limits of our general framework, and discuss their validity by comparing them against our first-principles results. We develop an efficient computational workflow and apply it to a diverse set of semiconductors, specifically AlN, CdS, GaN, MgO and SrTiO3. We demonstrate under different physical scenarios how excitons may be screened by multiple polar optical or acoustic phonons, how their binding energies can exhibit strong temperature dependence, and the ultrafast timescales on which they dissociate into free electron-hole pairs.