Emergence of composite many-body exciton states in ${\mathrm{WS}}_2$ and ${\mathrm{MoSe}}_2$ monolayers

When doped with a high density of mobile charge carriers, monolayer transition-metal dichalcogenide (TMD) semiconductors can host new types of composite many-particle exciton states that do not exist in conventional semiconductors. Such multiparticle bound states arise when a photoexcited electron-hole pair couples not to just a single Fermi sea that is quantum-mechanically distinguishable (as in the case of conventional charged excitons or trions), but rather couples simultaneously to multiple Fermi seas, each having distinct spin and valley quantum numbers. Composite six-particle "hexciton" states were recently identified in electron-doped ${\mathrm{WSe}}_2$ monolayers, but under suitable conditions they should also form in all other members of the monolayer TMD family. Here we present spectroscopic evidence demonstrating the emergence of many-body hexcitons in charge-tunable ${\mathrm{WS}}_2$ monolayers (at the A-exciton) and ${\mathrm{MoSe}}_2$ monolayers (at the B-exciton). The roles of distinguishability and carrier screening on the stability of hexcitons are discussed.