Tensile strain enhances four-phonon scattering and suppresses thermal conductivity in monolayer MoSe

Mechanical strain provides an effective means of tuning thermal conductivity in two-dimensional (2D) materials, but its effect on higher-order phonon scattering in fractional-layer materials remains unclear. Here, we investigate the tensile strain-dependent thermal conductivity of fractional-layer MoSe₂, structurally corresponding to monolayer MoSe, using first-principles calculations and the phonon Boltzmann transport equation with three-phonon (3ph) and four-phonon (4ph) scattering. At 300 K, the thermal conductivity calculated with only 3ph scattering decreases from 3.69 to 2.33 W m^-1 K^-1 as the biaxial tensile strain increases from 0% to 3%. When 4ph scattering is included, the corresponding value decreases from 1.35 to 0.46 W m^-1 K^-1. Consequently, the relative reduction induced by 4ph scattering increases from approximately 63% to 80%. The increasing importance of 4ph scattering is associated with the systematic expansion of the 4ph weighted phase space and the redistribution of mode-resolved anharmonicity, while the 3ph response remains strongly branch dependent. Tensile strain also weakens the Mo-Se bonding, softens the phonon spectrum, and lowers the group velocities of heat-carrying acoustic phonons, resulting in the reduction in thermal conductivity. Our findings clarify the distinct strain responses of 3ph and 4ph scattering and highlight the importance of 4ph scattering in fractional-layer thermal transport.