  Monolayer films of transition metal dichalcogenides (in particular, MoS2,
MoSe2, WS2, and WSe2) can be considered as ideal systems for the studies of
high-temperature electron-hole liquids. The quasi-two-dimensional nature of
electrons and holes ensures their stronger interaction as compared to that in
bulk semiconductors. The screening of the Coulomb interaction in monolayer
heterostructures is significantly reduced, since it is determined by the
permittivities of the environment (e.g., vacuum and substrate), which are much
lower than those characteristic of the films of transition metal
dichalcogenides. The multivalley structure of the energy spectrum of charge
carriers in transition metal dichalcogenides significantly reduces the kinetic
energy, resulting in the increase in the equilibrium density and binding energy
of the electron-hole liquid. The binding energy of the electron-hole liquid and
its equilibrium density are determined. It is shown that the two-dimensional
Coulomb potential should be used in the calculations for the electron-hole
liquid.
