  Understanding thermal transport from nanoscale heat sources is important for
a fundamental description of energy flow in materials, as well as for many
technological applications including thermal management in nanoelectronics,
thermoelectric devices, nano-enhanced photovoltaics and nanoparticle-mediated
thermal therapies. Thermal transport at the nanoscale is fundamentally
different from that at the macroscale and is determined by the distribution of
carrier mean free paths in a material, the length scales of the heat sources,
and the distance over which heat is transported. Past work has shown that
Fourier's law for heat conduction dramatically over-predicts the rate of heat
dissipation from heat sources with dimensions smaller than the mean free path
of the dominant heat-carrying phonons. In this work, we uncover a new regime of
nanoscale thermal transport that dominates when the separation between
nanoscale heat sources is small compared with the dominant phonon mean free
paths. Surprisingly, the interplay between neighboring heat sources can
facilitate efficient, diffusive-like heat dissipation, even from the smallest
nanoscale heat sources. This finding suggests that thermal management in
nanoscale systems including integrated circuits might not be as challenging as
projected. Finally, we demonstrate a unique and new capability to extract mean
free path distributions of phonons in materials, allowing the first
experimental validation of differential conductivity predictions from
first-principles calculations.
