  The multivalley band structure of monolayer transition metal dichalcogenides
(TMDs) gives rise to intravalley and intervalley excitons. Much knowledge of
these excitons has been gained, but fundamental questions remain, such as how
to describe them all in a unified picture with their correlations, how are
those from different valleys coupled to form the intervalley biexciton? To
address the issues, we derive an exciton Hamiltonian from interpair
correlations between the constituent carriers-fermions of two excitons.
Identifying excitons by irreducible representations of their point symmetry
group, we find their pairwise interaction depending on interacting excitons'
symmetry. It is generally repulsive, except for the case excitons from
different valleys, which attract each other to form the intervalley biexciton.
We establish a semianalytical relationship between the biexciton binding energy
with exciton mass and dielectric characteristics of the material and
surroundings. Overall, by providing insight into the nature of diverse excitons
and their correlations, our theoretical model captures the exciton interaction
properties permitting an inclusive description of the structure and energy
features of the intervalley biexciton in monolayer TMDs.21
