# Excitons versus electron-hole plasma in monolayer transition metal dichalcogenide semiconductors

## TL;DR

This paper investigates the conditions under which excitons or electron-hole plasma dominate in monolayer transition metal dichalcogenides, revealing a tunable Mott transition influenced by dielectric environment and doping.

## Contribution

It introduces a comprehensive many-body theoretical framework to predict and control the exciton-to-plasma transition in 2D semiconductors, including the Mott transition point.

## Key findings

- Exciton-to-plasma ratio can be tuned by dielectric screening and doping.
- A Mott transition occurs at specific excitation densities.
- Predicted observable effects in photoemission and tunneling microscopy.

## Abstract

When electron-hole pairs are excited in a semiconductor, it is a priori not clear if they form a fermionic plasma of unbound particles or a bosonic exciton gas. Usually, the exciton phase is associated with low temperatures. In atomically thin transition metal dichalcogenide semiconductors, excitons are particularly important even at room temperature due to strong Coulomb interaction and a large exciton density of states. Using state-of-the-art many-body theory including dynamical screening, we show that the exciton-to-plasma ratio can be efficiently tuned by dielectric substrate screening as well as charge carrier doping. Moreover, we predict a Mott transition from the exciton-dominated regime to a fully ionized electron-hole plasma at excitation densities between $3\times10^{12}$ cm$^{-2}$ and $1\times10^{13}$ cm$^{-2}$ depending on temperature, carrier doping and dielectric environment. We propose the observation of these effects by studying excitonic satellites in photoemission spectroscopy and scanning tunneling microscopy.