Anomalous local enhancement of thermal conductivity under nanoscale hotspot: spatial inhomogeneity and phonon nonequilibrium in silicon

The miniaturization of electronic devices gives rise to localized heat accumulation and thus nanoscale hotspot, triggering degenerated heat dissipation. However, the phonon transport mechanism and quantification of inhomogeneous nonequilibrium thermal conductivity remain elusive. Here, through molecular dynamics simulations of silicon thin film under nanoscale hotspot, we reveal a dramatic spatial inhomogeneity of cross-plane thermal conductivity κ_z under nanoscale hotspot excitation. Contrary to the conventional picture of phonon nonequilibrium simply suppressing heat transport, we find a non-monotonic spatial evolution: κ_z drops by up to 68% immediately beneath the hotspot (L_hot=10.9 nm), but subsequently recovers to exceed the uniform-heating baseline by 12% at a distance of ∼40 nm, producing a maximum κ_z gradient of 3.2 W·m^-1·K^-1·nm^-1 — 2.3 times that of uniform heating. Spectral decomposition indicates that longitudinal acoustic (LA) phonons dominate the anomalous local thermal conductivity enhancement. Modal temperature calculations reveal that the spatial relaxation of hotspot-induced LA phonon nonequilibrium matches well with the transition of thermal conductivity from suppression to anomalous enhancement. The magnitude of the spatial inhomogeneity diminishes with rising temperature (from 3.05 W·m^-1·K^-1·nm^-1 at 300 K to 1.25 W·m^-1·K^-1·nm^-1 at 600 K) due to increased anharmonic scattering. These findings challenge the common assumption of spatially uniform thermal conductivity in hotspot systems and provide a quantitative, spectrally-resolved benchmark for thermal management of nanoscale devices.