Phonon-assisted luminescence in defect centers from many-body perturbation theory

Phonon-assisted luminescence is a key property of defect centers in
semiconductors, and can be measured to perform the readout of the information
stored in a quantum bit, or to detect temperature variations. The investigation
of phonon-assisted luminescence usually employs phenomenological models, such
as that of Huang and Rhys, with restrictive assumptions that can fail to be
predictive. In this work, we predict luminescence and study exciton-phonon
couplings within a rigorous many-body perturbation theory framework, an
analysis that has never been performed for defect centers. In particular, we
study the optical emission of the negatively-charged boron vacancy in 2D
hexagonal boron nitride, which currently stands out among defect centers in 2D
materials thanks to its promise for applications in quantum information and
quantum sensing. We show that phonons are responsible for the observed
luminescence, which otherwise would be dark due to symmetry. We also show that
the symmetry breaking induced by the static Jahn-Teller effect is not able to
describe the presence of the experimentally observed peak at 1.5 eV.