Phonon-assisted photoluminescence of finite-momentum excitons of transition-metal dichalcogenide monolayers
Ping-Yuan Lo1*, Guan-Hao Peng1, Wei-Hua Li1, Yi Yang1, Shun-Jen Cheng1
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
* Presenter:Ping-Yuan Lo,
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 [1]. However, it is widely considered a hard task to make use of the optically accessible valley degree of freedom in practice, because excitons in TMD-MLs are severely valley-depolarized due to the intrinsic electron-hole exchange interaction [2]. Here, we present a comprehensive study of the valley-polarized finite-momentum excitons in WSe2-MLs over the entire first Brillouin zone by solving the density-functional-theory(DFT)-based Bethe-Salpeter equation (BSE) [3]. We find that the valley-polarization of finite-momentum excitons are generally very robust against the exchange-induced valley depolarization. By representing the full-zone exciton band with the valley-pseudospin texture, we reveal and visualize the quantum nature of the excitonic valley degree of freedom, which exhibits the skyrmion-like structure surrounding highly valley-polarized exciton states in the momentum space. Finally, we show that the high degree of valley-polarization of finite-momentum exciton states are well-transferrable to the optical polarizations through the phonon-assisted photoluminescence, suggesting the prospective utilities of inter-valley excitons in valley-based photonics.

[1] G. Wang, A. Chernikov, M. M. Glazov, T. F. Heinz, X. Marie, T. Amand, and B. Urbaszek, Rev. Mod. Phys. 90, 021001 (2018).
[2] H. Yu, G.-B. Liu, P. Gong, X. Xu, and W. Yao, Nat. Commun. 5, 3876 (2014).
[3] G.-H. Peng, P.-Y. Lo, W.-H. Li, Y.-C. Huang, Y.-H. Chen, C.-H. Lee, C.-K. Yang, and S.-J. Cheng, Nano Lett. 19, 2299 (2019).

Keywords: transition-metal dichalcogenides, 2D materials, 2D theory and simulation