Tensile strain induced enhancement of lattice thermal conductivity and its origin in two-dimensional SnC

Recently, higher-order four-phonon scattering has been revealed significant in the phonon thermal transport in crystalline materials with wide phononic band gap or with horizontal mirror symmetry. In this work, by utilizing first-principles calculations combined with phonon Boltzmann transport theory, we explore the fourphonon scattering and its effect on the lattice thermal conductivity KL of two-dimensional (2D) semiconductor SnC-that possesses both wide phononic gap and mirror symmetry. Our calculations show that four-phonon scattering significantly reduces the intrinsic KL of SnC from 43 to 19 W/(m K) at room temperature, by 54%. This reduction is found to come mainly from the out-of-plane, flexural acoustic (ZA) phonons, whose lifetimes are dominated by four-phonon scattering over three-phonon scattering due to the mirror symmetry-induced selection rules. Moreover, we predict based on four-phonon theory that applying 2% tensile strain increases the KL of SnC by a factor of five at room temperature. Through analyzing the modal contribution to KL, we attribute this huge improvement of KL primarily to the in-plane transverse acoustic (TA) phonons, whose scattering rates are extremely sensitive to strain. This fundamentally originates from the quadratic ZA phonon dispersion, which is linearized by the tensile strain in the long-wavelength limit, resulting in severe suppression of three-phonon and four-phonon processes involving ZA modes. Specifically, the linearized ZA branch causes the three-phonon scattering rate of TA modes to decrease rapidly from the original constant in the long-wavelength limit, and the corresponding four-phonon counterparts also decrease significantly. These findings highlight the importance of strain in modulating phonon-phonon scattering and thermal transport in 2D planar materials.