Charged Biexciton Formation with Many-Body-Induced Valley Polarization in a Monolayer Semiconductor

In the realm of many-body physics, the study of low-dimensional excitonic complexes has emerged as a compelling area of research, offering tunable modifications and highlighting quasiparticle interactions as key drivers for advancing valleytronics. Building upon extensive studies of simpler few-body systems such as excitons and trions, here we present a comprehensive exploration of the valley dynamics in more complex five-body charged biexciton (XX–) in monolayer WS2 using helicity-resolved ultrafast spectroscopy. We observe a near-unity degree of valley polarization at a moderate temperature of ∼150 K, which persists substantially longer than the population lifetime. Intriguingly, this polarization reveals an unexpected positive correlation with external disturbances such as temperature and pump fluence─behaviors distinct from conventional few-body systems. These phenomena are attributed to the inherent suppression of valley-exchange interactions in XX–, combined with its dual formation mechanisms: direct optical excitation and indirect conversion mediated by trion–trion interactions. Our results demonstrate that multibody excitonic complexes are stable candidates for maintaining valley polarization and could enable valleytronic applications that utilize many-body correlations.