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

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 ($\mathcal{P}\mathcal{T}$)-symmetry breaking, which impedes the experimental observation of intervalley excitonic coherence effect. At large excitonic center-of-mass momenta, the $\mathcal{P}\mathcal{T}$-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.

Published in Phys. Rev. Lett. 133, 236902 (2024)

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Cited by in corpus (1)

• Breakdown of Non-Bloch Bulk-Boundary Correspondence and Emergent Topology in Floquet Non-Hermitian Systems