Fine structure of negatively charged and neutral excitons in monolayer MoS$_{2}$

Abstract: We present experimental and theoretical results on the high-quality single-layer MoS${2}$ which reveal the fine structure of charged excitons, i.e., trions. In the emission spectra we resolve and identify two trion peaks, T${1}$ and T${2}$, resembling the pair of singlet and triplet trion peaks (T$_S$ and T${T}$) in tungsten-based materials. However, in polarization-dependent photoluminescence measurements we identify these peaks as novel intra- and inter-valley singlet trions, constituting the trion fine structure distinct from that already known in bright and dark 2D materials with large conduction-band splitting induced by the spin-orbit coupling. We show that the trion energy splitting in MoS${2}$ is a sensitive probe of inter- and intra-valley carrier interaction. With additional support from theory we claim that the existence of these singlet trions combined with an anomalous excitonic g-factor and the characteristic temperature dependence of the emission spectra together suggest that monolayer MoS${2}$ has a dark excitonic ground state, despite having "bright" single-particle arrangement of spin-polarized conduction bands.