  Interactions between two excitons can result in the formation of bound
quasiparticles, known as biexcitons. Their properties are determined by the
constituent excitons, with orbital and spin states resembling those of atoms.
Monolayer transition metal dichalcogenides (TMDs) present a unique system where
excitons acquire a new degree of freedom, the valley pseudospin, from which a
novel intervalley biexciton can be created. These biexcitons comprise two
excitons from different valleys, which are distinct from biexcitons in
conventional semiconductors and have no direct analogue in atomic and molecular
systems. However, their valley properties are not accessible to traditional
transport and optical measurements. Here, we report the observation of
intervalley biexcitons in the monolayer TMD MoS2 using ultrafast pump-probe
spectroscopy. By applying broadband probe pulses with different helicities, we
identify two species of intervalley biexcitons with large binding energies of
60 meV and 40 meV. In addition, we also reveal effects beyond biexcitonic
pairwise interactions in which the exciton energy redshifts at increasing
exciton densities, indicating the presence of many-body interactions among
them.
