Abstract
Higher-order correlated excitonic states arise from the mutual interactions of excitons, which generally requires a significant exciton density and therefore high excitation levels. Here, we report the emergence of two biexcitons species, one neutral and one charged, in monolayer tungsten diselenide under moderate continuous-wave excitation. The efficient formation of biexcitons is facilitated by the long lifetime of the dark exciton state associated with a spin-forbidden transition, as well as improved sample quality from encapsulation between hexagonal boron nitride layers. From studies of the polarization and magnetic field dependence of the neutral biexciton, we conclude that this species is composed of a bright and a dark excitons residing in opposite valleys in momentum space. Our observations demonstrate that the distinctive features associated with biexciton states can be accessed at low light intensities and excitation densities.
Highlights
Higher-order correlated excitonic states arise from the mutual interactions of excitons, which generally requires a significant exciton density and high excitation levels
By comparing the emission of the different exciton species under strong magnetic fields and as a function of doping level, we deduce that the neutral biexciton is formed predominantly from the combination of a dark and bright exciton residing in opposite valleys in momentum space
The 20-meV binding energy matches that of the neutral biexciton in MoSe2 monolayers observed by Hao et al using two-dimensional coherent spectroscopy[8] and of WSe2 in pump-probe spectroscopy[29], even though both works describe bright–bright excitons, which would suggest the biexciton binding energy is only weakly sensitive to the spin configuration of the two constituent excitons
Summary
Higher-order correlated excitonic states arise from the mutual interactions of excitons, which generally requires a significant exciton density and high excitation levels. The Coulomb attraction between electrons and holes in these systems leads to the formation of tightly bound excitons, which causes a reduction in the optical band gap by a large fraction of an electron volt and yields very short intrinsic radiative lifetimes[2,3,4,5]. Both trion (charged exciton) and biexciton (exciton molecule) states have been observed in TMDCs6–8. We report the observation of a neutral and charged biexciton composed of both bright and dark excitons
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