Many-body exciton and inter-valley correlations in heavily electron-doped WSe₂ monolayers

In monolayer transition-metal dichalcogenide semiconductors, many-body correlations can manifest in optical spectra when photoexcited electron-hole pairs (excitons) are introduced into a 2D Fermi sea of mobile carriers. At low background electron and hole densities, the formation of negatively and positively charged excitons (X^-, X⁺) is well documented.  However, in WSe₂ monolayers an additional strong absorption resonance, often called X^-' , emerges at higher electron doping.  Its origin is not understood.  Here we investigate the nature of the X^-' state, via polarized absorption spectroscopy of electrostatically-gated WSe₂ monolayers in high magnetic fields to 60 T. Field-induced filling and emptying of the lowest optically-active Landau level in the K' valley leads to repeated quenching of the corresponding optical absorption. Surprisingly, however, these quenchings are accompanied by changes in the absorption to higher-lying Landau levels in both the K' and K valleys, which are unoccupied. These results cannot be reconciled within a single particle picture, and demonstrate the many-body nature and inter-valley correlations of the X^-' quasiparticle state.