Symmetry governed valley-pseudospin textures of the full-zone excitonic bands of transition-metal dichalcogenide monolayers

TL;DR

This paper investigates the symmetry and valley-pseudospin textures of excitonic bands in transition-metal dichalcogenide monolayers, revealing conditions for valley polarization preservation and potential applications in valley-based photonics.

Contribution

It provides a comprehensive symmetry-guided analysis of finite-momentum excitons across the full Brillouin zone, highlighting their immunity to exchange-induced depolarization and skyrmion-like valley textures.

Findings

Finite-momentum excitons are generally immune to valley depolarization.

Valley-pseudospin textures form skyrmion-like structures in momentum space.

High valley polarization is transferable to optical polarization in phonon-assisted photoluminescence.

Abstract

Preserving a high degree of valley polarization of excitons in photo-excited transition-metal dichalcogenide monolayers (TMD-MLs) is desirable for the valley-based photonic applications, but widely recognized as a hard task hindered by the intrinsic electron-hole exchange interaction. In this study, we present a comprehensive investigation of valley-polarized finite-momentum excitons in WSe$_2$-MLs over the entire Brillouin zone by solving the density-functional-theory(DFT)-based Bethe-Salpeter equation (BSE) under the guidance of symmetry analysis. We reveal that finite-momentum excitons are actually in general well immune from the exchange-induced valley depolarization, except for those with specific exciton momenta directionally coincide with the axes associated with the $3\sigma_v$ and $3C_2'$ symmetries in TMD-MLs. Governed by the symmetries, the valley pseudo-spin texture of the full-zone exciton band in the momentum space is locally featured by individual skyrmion-like structures where highly valley-polarized finite-momentum exciton states are centred. Remarkably, we show that the high degrees of valley polarizations of the finite-momentum exciton states are excellently well transferable to the optical polarizations in the resulting phonon-assisted photo-luminescences, suggesting the prospective usefulness of those inter-valley excitons in valley-based photonics.