  Density functional theory based calculations have been performed for solving
the phonon Boltzmann transport equation to investigate the thermal transport
properties of monolayer (ML) ZnO under in-plane isotropic biaxial tensile
strain. The in-plane lattice thermal conductivity ($\kappa_{\text{L}}$) of
ML-ZnO increases dramatically in response to the biaxial tensile strain ranging
from 0% to 10%, conflicting with the general belief. The strain-induced
stiffening of the ZA phonon mode and the resulting concomitant increase in
group velocity and decrease in phonon population is found to play a significant
role behind the unusual enhancement of $\kappa_{\text{L}}$. The mode resolved
analysis shows the tensile strain driven competitive behavior between different
phonon properties, mainly the group velocity and phonon lifetimes, being
responsible for the observed unusual enhancement in $\kappa_{\text{L}}$.
Additionally, the phonon scattering calculations show the importance of
inclusion of 4-phonon scattering in the thermal transport calculations
suggesting the significance of higher-order anharmonicity in ML-ZnO. A
strikingly high 4-phonon scattering strength in ML-ZnO primarily results from
the strong anharmonicity, quadratic ZA mode dispersion, large frequency gap in
phonon dispersion, and reflection symmetry induced selection rule. The
incorporation of 4-phonon scattering significantly alters the transport
characteristics of all the phonon modes, in general and ZA phonons, in
particular. At large strains, a linear dispersion of the ZA mode and closure of
the frequency gap is observed, which results in a significant reduction of
4-phonon scattering strength in ML-ZnO.
