Non-Hermitian theory of valley excitons in two-dimensional semiconductors

TL;DR

This paper develops a non-Hermitian theoretical framework for valley excitons in 2D semiconductors, revealing novel elliptically polarized states, symmetry-breaking phenomena, and topological effects influenced by optical pumping and decay.

Contribution

It introduces a non-Hermitian model incorporating decay and pumping, uncovering anomalous valley-polarized states and topological excitonic Hall transport in 2D materials.

Findings

Discovery of elliptically polarized valley excitonic states.

Identification of parity-time symmetry breaking effects.

Prediction of non-zero Berry curvature and topological transport.

Abstract

Electron-hole exchange interaction in two-dimensional transition metal dichalcogenides is extremely strong due to the dimension reduction, which promises valley-superposed excitonic states with linearly polarized optical emissions. However, strong circular polarization reflecting valley-polarized excitonic states is commonly observed in helicity-resolved optical experiments. Here we present a non-Hermitian theory of valley excitons by incorporating optical pumping and intrinsic decay, which unveils an anomalous valley-polarized excitonic state with elliptically polarized optical emission. This novel state arises from the non-Hermiticity induced parity-time ($\mathcal{PT}$)-symmetry breaking, which impedes the experimental observation of intervalley excitonic coherence effect. At large excitonic center-of-mass momenta, the $\mathcal{PT}$-symmetry is restored and the excitonic states recover their valley coherence. Interestingly, the linear polarization directions in optical emissions from these valley-superposed excitonic states are non-orthogonal and even become parallel at exceptional points. Our non-Hermitian theory also predicts a non-zero Berry curvature for valley excitons, which admits a topological excitonic Hall transport beyond the Hermitian predictions.