Quantitatively analyzing microscopic mechanism of phonon coupling in nanostructured diamond

By combining nonequilibrium molecular dynamics, the spectral phonon temperature method, and the phonon weak-coupling model, we quantify the coupling between longitudinal and transverse phonon mode groups in nanostructured diamond. At 300 K and a system length of approximately 35 nm, the selected LP and TP modes along Γ-X retain a resolvable temperature difference, yielding a coupling length of 130 ± 34 nm and indicating weak coupling. From 300 to 700 K, the coupling length decreases overall with increasing temperature, whereas above 700 K the temperature difference becomes too small for stable PWCM fitting. The coupling length increases with system length, which may mainly arise from the size dependence of thermal conductivity. These results demonstrate that distinct phonon mode groups in high-thermal-conductivity diamond can remain in local thermal nonequilibrium over nanoscale distances. These results characterize branch-resolved thermal nonequilibrium in nanostructured diamond