Many-body Hexcitons and Oxcitons in Monolayer Semiconductors

Hydrogen-like bound states of photoexcited electron-hole pairs in semiconductors -- that is, excitons -- have been a focus of considerable study for more than half a century. In undoped direct-gap semiconductors, neutral excitons comprise the photogenerated electron and hole in the conduction and valence bands (CB and VB), respectively, and typically manifest as discrete optical resonances below the free-particle band-gap energy. More interesting states arise when electron-hole (e-h) pairs are photoexcited into doped semiconductors containing a Fermi-sea of mobile carriers. Most well-known are the charged excitons (or trions) that emerge in lightly electron- or hole-doped semiconductors. In the simplest picture, a trion is a three-particle complex consisting of a carrier from the Fermi sea bound to the photoexcited e-h pair. Trions appear as an additional optical resonance below the neutral exciton, and the energy difference between the two provides a measure of the binding energy between the exciton and the resident carrier.

In the archetypal monolayer semiconductor WSe₂, the distinct ordering of spin-polarized valleys (low-energy pockets) in the CB allows for studies of not only simple neutral excitons and charged excitons (i.e., trions), but also more complex many-body states that are predicted at higher electron densities [1]. I will discuss magneto-optical measurements of electron-rich WSe2 monolayers [2], and interpret the spectral lines that emerge at high electron doping as optical transitions of 6-body exciton states (``hexcitons'') and 8-body exciton states (``oxcitons''). These many-body states emerge when a photoexcited electron-hole pair interacts simultaneously with multiple Fermi seas, each having distinguishable spin and valley quantum numbers.

[1] Dinh Van Tuan and Hanan Dery, Composite excitonic states in doped semiconductors, Phys. Rev. B 106, L081301 (2022).

[2] Dinh Van Tuan, Su-Fei Shi, Xiaodong Xu, Scott A. Crooker and Hanan Dery, Hexcitons and oxcitons in monolayer WSe₂, Phys. Rev. Lett. 129, 076801 (2022).