# Exciton spectrum in two-dimensional transition metal dichalcogenides: The role of Diracness

## TL;DR

This paper explains deviations from the hydrogenic exciton spectrum in two-dimensional transition metal dichalcogenides by highlighting the relativistic Dirac nature of electrons, which alters exciton properties and matches experimental observations.

## Contribution

It demonstrates that the relativistic Dirac equation for electrons in these materials explains the non-hydrogenic exciton spectrum and its reduced level spacing.

## Key findings

- Relativistic Dirac electrons cause deviations from hydrogenic exciton spectra.
- The exciton level spacing is significantly reduced due to Diracness.
- The model aligns with spectroscopic and ab-initio results.

## Abstract

The physics of excitons, electron-hole pairs that are bound together by their mutual Coulomb attraction, can to great extent be understood in the framework of the quantum-mechanical hydrogen model. This model has recently been challenged by spectroscopic measurements on two-dimensional transition-metal dichalchogenides that unveil strong deviations from a hydrogenic spectrum. Here, we show that this deviation is due to the particular relativistic character of electrons in this class of materials. Indeed, their electrons are no longer described in terms of a Schroedinger but a massive Dirac equation that intimately links electrons to holes. Dirac excitons therefore inherit a relativistic quantum spin-1/2 that contributes to the angular momentum and thus the exciton spectrum. Most saliently, the level spacing is strongly reduced as compared to the hydrogen model, in agreement with spectroscopic measurements and ab-initio calculations.