Exciton and trion formation in systems with van Hove singularities

TL;DR

This paper explores how van Hove singularities in electronic band structures influence the formation and properties of excitons and trions, highlighting the potential to engineer 2D materials for specific optical applications.

Contribution

It demonstrates that higher-order van Hove singularities can significantly alter exciton and trion dispersion, providing new insights into material engineering for optoelectronic devices.

Findings

Trion formation is favored in narrower Mexican-hat dispersions.

Higher-order VHS induce similar singularities in exciton and trion dispersions.

HOVHS can modify the density of states, affecting optical properties.

Abstract

We investigate the role of van-Hove singularities (VHS) in a system's electronic band structure on the formation and properties of excitons and trions. We consider (i) the different parameters of a Mexican-hat-type dispersion of the valence band, which hosts a VHS at the band edge, and (ii) the presence of regular VHS or higher-order VHS (HOVHS). We find that for a given spin-degenerate Mexican-hat-shaped valence band, where a trion and exciton can form, trion formation becomes more favourable as the Mexican-hat dispersion becomes narrower. Also, we show that if the electronic band structure contains an HOVHS, then both the exciton and trion dispersion will also contain such a singularity. Therefore, a HOVHS in the valence band can suitably change the density of states (DOS) of the bound-state particle and lead to the generation of new states, which could impact the optical properties of the system. Our work provides a pathway to how 2D quantum materials, which host such singularities, can be engineered to favour a particular bound-state thus opening new avenues for these materials into potential applications.