Brightening dark excitons and trions in systems with a Mexican-hat energy dispersion: example of InSe

TL;DR

This paper explores how dark excitons and trions in 2D materials with Mexican-hat dispersion, like monolayer InSe, can be brightened via phonon coupling, impacting optoelectronic applications.

Contribution

It introduces a variational approach to study momentum-dark and bright exciton states in Mexican-hat dispersion systems, highlighting the role of van Hove singularities and phonon interactions.

Findings

Momentum-dark states dominate due to van Hove singularities.

Phonon coupling can activate dark excitons, making them optically bright.

Results suggest potential for enhanced optoelectronic device performance.

Abstract

We investigate the properties of momentum-dark excitons and trions formed in two-dimensional (2D) materials that exhibit an inverted Mexican-hat-shaped dispersion relation, taking monolayer InSe as an example. We employ variational techniques to obtain the momentum-dark and bright ground-states (non-zero and zero quasiparticle momenta, respectively). These states are of particular interest due to their peaks in the quasiparticle density of states, the largest contribution comes from the momentum-dark ground state due to the presence of a van Hove singularity (VHS). These momentum-dark systems require a physical process to provide the necessary momentum to become bright. We study the brightening of this state due to coupling with phonons and compute the resulting photoluminescence spectrum. This work opens new avenues of research, such as exploiting dark excitons in solar cells and other semiconductor-based optoelectronic devices.