Full-zone valley polarization landscape of finite-momentum exciton in transition metal dichalcogenide monolayers

Abstract

In this study we present a theoretical investigation of the full-zone landscape of finite-momentum dark excitons in ${\mathrm{WSe}}_{2}$-MLs by solving the density-functional-theory (DFT)-based Bethe-Salpeter equation (BSE) under the guidance of symmetry analysis. The studies reveal the comprehensive valley-polarization landscape of finite-momentum exciton of ${\mathrm{WSe}}_{2}$ monolayer. Dictated by the crystal symmetry, the valley pseudospin texture over the extended exciton-momentum ${\mathbit{k}}_{\text{ex}}$ space exhibits rich structures, featured by the inherently full valley polarizations in the excitonic ${K}_{\text{ex}}$, ${K}_{\text{ex}}^{\ensuremath{'}}$, and ${Q}_{\text{ex},i}$ valleys and also by the contrasted valley depolarizations for the exciton states lying in the $\overline{{\mathrm{\ensuremath{\Gamma}}}_{\text{ex}}{M}_{\text{ex},i}}$ paths. Attractively, the superior valley polarizations of the intervalley dark excitons in ${\mathrm{WSe}}_{2}$-MLs are shown almost fully transferable to the optical polarization in the phonon-assisted photoluminescences because of the native suppression of exchange-induced depolarization in the second-order optical processes. The analysis of phonon-assisted photoluminescences accounts for the recently observed brightness, high degree of optical polarization, and long lifetime of the intervalley dark exciton states in tungsten-based TMD-MLs.