Understanding the role of four-phonon scattering in the lattice thermal transport of monolayer MoS2

In the calculations of lattice thermal conductivity (kappa L), vital contributions stemming from four-phonon scattering is often neglected. The significance of four-phonon scattering in the thermal transport properties of monolayer (ML) MoS2 has been unraveled using first-principles calculations combined with the Boltzmann transport equation. If only three-phonon scattering processes are considered then the kappa L is found to be significantly overestimated (similar to 115.8 W m-1 K-1 at 300 K). With the incorporation of the four-phonon scattering processes, the kappa L reduces to 24.6 W m-1 K-1, which is found to be closer to the experimentally measured kappa L of 34.5 W m-1 K-1. Four-phonon scattering significantly impacts the carrier lifetime (tau) of the low-energy out-ofplane acoustic mode (ZA) phonons and thereby, suppresses its contribution in kappa L from 64% (for three-phonon scattering) to 16% (for both three- and four-phonon scatterings). The unusually high four-phonon scattering rate (tau -1 4 ) of the ZA phonons is attributed to the distinctive quadratic dispersion, along with the combined effects of the acoustic-optical frequency gap, strong anharmonicity, and the selection rule imposed by reflection symmetry. The strong coupling between the dispersion characteristics of the ZA mode and the tau 4-1 is discovered through the application of mechanical strain. The strain induced increase in the linearity of the ZA mode dispersion dramatically reduces the significance of the four-phonon scattering in ML-MoS2, both qualitatively and quantitatively. These findings offer valuable insights into the thermal transport phenomena of ML-MoS2, as well as other 2D materials.