Magnetic field mixing and splitting of bright and dark excitons in monolayer MoSe ₂

Abstract

Monolayers of semiconducting transition metal dichalcogenides (TMDCs) with unique spin-valley contrasting properties and remarkably strong excitonic effects continue to be a subject of intense research interests. These model 2D semiconductors feature two fundamental intravalley excitons species -optically accessible ' bright' excitons with anti-parallel spins and optically inactive 'dark' excitons with parallel spins.For applications exploiting radiative recombination of bright excitons or long lifetime dark excitons, it is essential to understand the radiative character of the exciton ground state and establish the energy separation between the lowest energy bright and dark excitons. Here, we report a direct spectroscopic measure of dark excitons in monolayer MoSe 2 encapsulated in hexagonal boron nitride. By applying strong in-plane magnetic field, we induce mixing and splitting of bright and dark exciton branches, which enables an accurate spectroscopic determination of their energies. We confirm the bright character of the exciton ground state separated by a 1.5 meV gap from the higher energy dark exciton state, much smaller compared to the previous theoretical expectations. These findings provide critical information for further improvement of the accurate theoretical description of TMDCs electronic structure.

Measurement of the spin-forbidden dark excitons in MoS2 and MoSe2 monolayers

“…2b ) with Δ = −1.4 ± 0.1 meV and = 2.0 ± 0.2. These values are in good agreement with very similar measurements published very recently 21 . Note that the fit is very weakly sensitive to the value of δ for δ < 1 meV.…”

“…The high quality of the investigated samples allowed us to determine accurately the bright-dark energy splitting. We performed similar measurements in MoSe 2 MLs encapsulated in hBN where a bright-dark splitting of −1.3 meV is determined in agreement with a recent report 21 . Measurements performed in a tilted magnetic field (45°with respect to the ML plane) confirm the spin-forbidden nature of the observed transitions and allow us to provide a conceivable description of the neutral exciton fine structure.…”

Excitonic Transport and Intervalley Scattering Dynamics in Large‐Size Exfoliated MoSe2 Monolayer Investigated by Heterodyned Transient Grating Spectroscopy

“…This fluctuating Rashba potential strongly enhances the mixing of dark and bright exciton states in monolayer MoSe 2 , assisted by the very small energy difference between them. [92] The model has predicted an intervalley scattering time less than 1 ps, which is well in line with the current observations. Note that the theoretical calculation is based on a perfect single monolayer crystal, it does not account for a possible reducing the Rashba potential by the vacancy and impurity point defects.…”

Guide to optical spectroscopy of layered semiconductors

“…145 ). Brightening of (spin-) dark states due to an increased mixing of the spin-states in WSe 2 , WS 2 , MoSe 2 and MoS 2 monolayers can be observed in low-temperature magneto-PL experiments by applying strong (ideally several tens of tesla) in-plane magnetic fields 59,60,146 .…”