Convergent thermal conductivity in strained monolayer graphene

The strain dependence of thermal conductivity (\ensuremath{\kappa}) in monolayer graphene, with reports of enhancement, suppression, or even divergence, has been highly controversial. To address this open question, we have systematically investigated the effects of tensile strain on the \ensuremath{\kappa} of graphene using the exact solution of the Peierls-Boltzmann transport equation based on the first-principles interatomic force constants combined with machine learning assisted molecular dynamics simulations. In contrast to previous studies, we find that the \ensuremath{\kappa} in the strained graphene is convergent after considering four-phonon scattering, which is dominant for the long-wavelength flexural phonons because of its much weaker frequency dependence (${\ensuremath{\tau}}_{4}^{\ensuremath{-}1}\ensuremath{\propto}{\ensuremath{\omega}}^{\ensuremath{\beta}}$ with \ensuremath{\beta} 2) compared to the three-phonon scattering case (${\ensuremath{\tau}}_{3}^{\ensuremath{-}1}\ensuremath{\propto}{\ensuremath{\omega}}^{\ensuremath{\beta}}$ with \ensuremath{\beta} > 2). Furthermore, \ensuremath{\kappa} exhibits nonmonotonic variations with increasing strain up to 8% due to the competition between phonon lifetime, group velocity, and heat capacity of acoustic phonons. Our results deepen the fundamental understanding of thermal transport in strained graphene and offer insights for tuning the thermal properties of two-dimensional materials through strain engineering.