# Fundamental exciton linewidth broadening in monolayer transition metal dichalcogenides

## TL;DR

This paper revises the understanding of exciton linewidth broadening in monolayer TMDS by introducing a modified light cone concept and estimating a fundamental radiative linewidth, revealing the significant role of phonons and setting limits on linewidth measurements.

## Contribution

It proposes a new light cone framework and a self-consistent method to distinguish radiative and non-radiative linewidth components in monolayer TMDS.

## Key findings

- Estimated a fundamental radiative linewidth of 1.54 meV.
- Found a large linewidth broadening (~0.27 meV) due to zero-point phonons.
- Identified a fundamental limit on non-radiative linewidth broadening at 0 K.

## Abstract

Monolayer Transition Metal Dichalcogenides (TMDS) are highly luminescent materials despite being sub-nanometer thick due to the ultra-short ($<1$ ps) radiative lifetime of the strongly bound bright excitons hosted by these materials. The intrinsically short radiative lifetime results in a large broadening in the exciton band with a magnitude that is about two orders greater than the spread of the light cone itself. The situation calls for a need to revisit the conventional light cone picture. We present a modified light cone concept which places the light line $(\hbar cQ)$ as the generalized lower bound for allowed radiative recombination. A self-consistent methodology, which becomes crucial upon inclusion of large radiative broadening in the exciton band, is proposed to segregate the radiative and the non-radiative components of the homogeneous exciton linewidth. We estimate a fundamental radiative linewidth of $1.54\pm0.17\ $meV, owing purely to finite radiative lifetime in the absence of non-radiative dephasing processes. As a direct consequence of the large radiative limit, we find a surprisingly large ($\sim 0.27 $ meV) linewidth broadening due to zero-point energy of acoustic phonons. This obscures the precise experimental determination of the intrinsic radiative linewidth and sets a fundamental limit on the non-radiative linewidth broadening at $T = 0$ K.