Theory of excitonic polarons: From models to first-principles calculations

Abstract

Excitons are neutral excitations that are composed of electrons and holes bound together by their attractive Coulomb interaction. The electron and the hole forming the exciton also interact with the underlying atomic lattice, and this interaction can lead to a trapping potential that favors exciton localization. The quasi-particle thus formed by the exciton and the surrounding lattice distortion is called excitonic polaron. Excitonic polarons have long been thought to exist in a variety of materials, and are often invoked to explain the Stokes shift between the optical absorption edge and the photo-luminescence peak. However, quantitative ab initio calculations of these effects are exceedingly rare. In this manuscript, we present a theory of excitonic polarons that is amenable to first-principles calculations. We first apply this theory to model Hamiltonians for Wannier excitons experiencing Fröhlich or Holstein electron-phonon couplings. We find that, in the case of Fröhlich interactions, excitonic polarons only form when there is a significant difference between electron and hole effective masses. Then, we apply this theory to calculating excitonic polarons in lithium fluoride ab initio. The key advantage of the present approach is that it does not require supercells, therefore it can be used to study a variety of materials hosting either small or large excitonic polarons. This work constitutes the first step toward a complete ab initio many-body theory of excitonic polarons in real materials.

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• Identification of large polarons and exciton polarons in rutile and anatase polymorphs of titanium dioxide
• Polarons in the Cubic Generalized Fröhlich Model: Spontaneous Symmetry Breaking
• Polaron catastrophe within quantum acoustics
• How to choose efficiently the size of the Bethe-Salpeter Equation Hamiltonian for accurate exciton calculations on supercells
• Exciton pairs coupled via the long-living phonons and their superfluorescent markers