Title: Dark trions and biexcitons in WS2 and WSe2 made bright by e-e scattering,
Abstract: The direct band gap character and large spin-orbit splitting of the valence
band edges (at the K and K' valleys) in monolayer transition metal
dichalcogenides have put these two-dimensional materials under the spot-light
of intense experimental and theoretical studies. In particular, for Tungsten
dichalcogenides it has been found that the sign of spin splitting of conduction
band edges makes ground state excitons radiatively inactive (dark) due to spin
and momentum mismatch between the constituent electron and hole. One might
similarly assume that the ground states of charged excitons and biexcitons in
these monolayers are also dark. Here, we show that the intervalley
K$\leftrightarrows$K' electron-electron scattering mixes bright and dark states
of these complexes, and estimate the radiative lifetimes in the ground states
of these "semi-dark" trions and biexcitons to be ~ 10ps, and analyse how these
complexes appear in the temperature-dependent photoluminescence spectra of WS2
and WSe2 monolayers.