# Rotons in Optical Excitation Spectra of Monolayer Semiconductors

## TL;DR

This paper demonstrates that increasing electron density in monolayer semiconductors causes optical excitations to develop a roton-like dispersion, influenced by Coulomb interactions and screening, which may explain features in photoluminescence spectra.

## Contribution

It reveals the development of roton-like features in optical spectra of doped monolayer semiconductors due to electron density effects and long-range Coulomb interactions.

## Key findings

- Roton-like dispersion appears at finite momentum with increased electron density.
- The roton minimum is robust due to Coulomb interaction and dielectric screening.
- Roton emergence may explain unexplained photoluminescence features.

## Abstract

Optically generated excitons dictate the absorption and emission spectrum of doped semiconductor transition metal dichalcogenide monolayers. We show that upon increasing the electron density, the elementary optical excitations develop a roton-like dispersion, evidenced by a shift of the lowest energy state to a finite momentum on the order of the Fermi momentum. This effect emerges due to Pauli exclusion between excitons and the electron Fermi sea, but the robustness of the roton minimum in these systems is a direct consequence of the long-range nature of the Coulomb interaction and the nonlocal dielectric screening characteristic of monolayers. Finally, we show that the emergence of rotons could be related to hitherto unexplained aspects of photoluminescence spectra in doped transition metal dichalcogenide monolayers.