  The coupled nonequilibrium dynamics of electrons and phonons in monolayer
MoS2 is investigated by combining first-principles calculations of the
electron-phonon and phonon-phonon interaction with the time-dependent Boltzmann
equation. Strict phase-space constraints in the electron-phonon scattering are
found to influence profoundly the decay path of excited electrons and holes,
restricting the emission of phonons to crystal momenta close to few
high-symmetry points in the Brillouin zone. As a result of momentum selectivity
in the phonon emission, the nonequilibrium lattice dynamics is characterized by
the emergence of a highly-anisotropic population of phonons in reciprocal
space, which persists for up to 10 ps until thermal equilibrium is restored by
phonon-phonon scattering. Achieving control of the nonequilibrium dynamics of
the lattice may provide unexplored opportunities to selectively enhance the
phonon population of two-dimensional crystals and, thereby, transiently tailor
electron-phonon interactions over sub-picosecond time scales.
