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

In the calculations of lattice thermal conductivity (κL), 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 κL is found to be significantly overestimated (~115.84pt0exW0.16em0exm^-10.16em0exK^-1 at 300 K). With the incorporation of the four-phonon scattering processes, the κL reduces to 24.6 W0.16em0exm^-10.16em0exK^-1, which is found to be closer to the experimentally measured κL of 34.5 W0.16em0exm^-10.16em0exK^-1. Four-phonon scattering significantly impacts the carrier lifetime (τ) of the low-energy out-of-plane acoustic mode (ZA) phonons and thereby, suppresses its contribution in κL from 64% (for three-phonon scattering) to 16% (for both three- and four-phonon scatterings). The unusually high four-phonon scattering rate (τ₄^-1) 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 τ₄^-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.