Phonon thermal transport in strained monolayer graphene: Role of four-phonon and normal scattering

We present a first-principles investigation of phonon thermal transport in monolayer graphene under biaxial tensile strain at room temperature. Surprisingly, when four-phonon (4ph) scattering is included, we observe a remarkable strain-induced enhancement of lattice thermal conductivity ($\ensuremath{\kappa}$)---nearly doubling at 5% strain---in stark contrast to the strain-insensitive $\ensuremath{\kappa}$ predicted by three-phonon (3ph) theory. This unexpected enhancement arises from the strain-induced suppression of both 3ph and 4ph scattering, primarily due to the linearization of the flexural acoustic (ZA) phonon branch, which severely restricts the phase space for ZA-involved scattering processes. Our calculations further show that the role of 4ph scattering in reducing $\ensuremath{\kappa}$ diminishes substantially with strain. Notably, the ZA phonons are found to persist as the primary heat carriers even under large strain, though their relative contribution to $\ensuremath{\kappa}$ decreases considerably. Additionally, we unveil that tensile strain markedly weakens nonresistive normal processes, thereby suppressing nondiffusive phonon transport. These findings provide fundamental insights into the strain-engineered phonon-phonon interactions and thermal transport in graphene.