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Two-dimensional Fröhlich interaction in transition-metal dichalcogenide monolayers: Theoretical modeling and first-principles calculations

We perform ab initio calculations of the coupling between electrons and small-momentum polar-optical phonons in monolayer transition metal dichalcogenides of the 2H type: MoS2, MoSe2, MoTe2, WS2, and WSe2. The so-called Fröhlich interaction is fundamentally affected by the dimensionality of the system. In a plane-wave framework with periodic boundary conditions, this coupling is affected by the spurious interaction between the 2D material and its periodic images. To overcome this, we perform density functional perturbation theory calculations with a truncated Coulomb interaction in the out-of-plane direction. We show that the 2D Fröhlich interaction is much stronger than assumed in previous ab initio studies. We provide analytical models depending on the effective charges and dielectric properties of the materials to interpret our ab initio calculations. Screening is shown to play a fundamental role in the phonon-momentum dependency of the polar-optical coupling, with a crossover between two regimes depending on the dielectric properties of the material relative to its environment. The Fröhlich interaction is screened by the dielectric environment in the limit of small phonon momenta and sharply decreases due to stronger screening by the monolayer at finite momenta. The small-momentum regime of the ab initio Fröhlich interaction is reproduced by a simple analytical model, for which we provide the necessary parameters. At larger momenta, however, direct ab initio calculations of electron-phonon interactions are necessary to capture band-specific effects. We compute and compare the carrier relaxation times associated to the scattering by both LO and A1 phonon modes. While both modes are capable of relaxing carriers on timescales under the picosecond at room temperature, their absolute and relative importance vary strongly depending on the material, the band, and the substrate.