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 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 WSe_{2} monolayer. Dictated by the crystal symmetry, the valley pseudospin texture over the extended exciton-momentum k_{ex} space exhibits rich structures, featured by the inherently full valley polarizations in the excitonic K_{ex}, K_{ex}^{′}, and Q_{ex,i} valleys and also by the contrasted valley depolarizations for the exciton states lying in the Γ_{ex}M_{ex,i}[over ¯] paths. Attractively, the superior valley polarizations of the intervalley dark excitons in 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.