Title: Fine Structure and Lifetime of Dark Excitons in Transition Metal Dichalcogenide Monolayers,
Abstract: The intricate interplay between optically dark and bright excitons governs
the light-matter interaction in transition metal dichalcogenide monolayers. We
have performed a detailed investigation of the "spin-forbidden" dark excitons
in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field
Bz. In agreement with the theoretical predictions deduced from group theory
analysis, magneto-photoluminescence experiments reveal a zero field splitting
$\delta=0.6 \pm 0.1$ meV between two dark exciton states. The low energy state
being strictly dipole forbidden (perfectly dark) at Bz=0 while the upper state
is partially coupled to light with z polarization ("grey" exciton). The first
determination of the dark neutral exciton lifetime $\tau_D$ in a transition
metal dichalcogenide monolayer is obtained by time-resolved photoluminescence.
We measure $\tau_D \sim 110 \pm 10$ ps for the grey exciton state, i.e. two
orders of magnitude longer than the radiative lifetime of the bright neutral
exciton at T=12 K.