Observation of a many-body excitonic complex in WSe₂ beyond the oxciton

Monolayers of Transition-metal dichalcogenide (TMD) semiconductors exhibit uniquely strong Coulomb interactions and valley-contrasting physics, capable of sustaining a wide range of excitonic many-body complexes. Recently, intriguing 6-particle (hexciton) and 8-particle (oxciton) complexes have been observed. These many-body complexes form due to the interaction of a photoexcited electron-hole pair with electrons in multiple electrostatically induced Fermi seas, distinguishable by either spin or momentum. The observation of further complexes is thus limited by either screening due to the Fermi seas or the physically achievable electrostatic doping. In this work, we use charge-tunable devices of hBN encapsulated monolayer WSe₂, capable of accessing previously uncharted charge doping regimes to study many-body complexes of up to several 10¹³ cm^-2. In reflection contrast, the B-hexciton surprisingly shows no decay for hole densities as high as 3*10¹³ cm^-2, suggesting that our device design has a mitigating effect on screening. When following the A-hexciton with increasing electron-doping, we observe the filling of the Q-valley, resulting in a new many-body complex beyond the oxciton. Our work expands the general understanding of exciton physics in van der Waals materials.