Quasi-ballistic thermal transport from nanoscale interfaces observed using ultrafast coherent soft x-ray beams

Abstract

Heat dissipation from a nanoscale hot-spot is expected to be non-diffusive when a hot-spot is smaller than the phonon mean free path of the substrate. Our technique of observing diffraction of coherent soft x-ray pulses allows for very high resolution (∼pm) of thermally-induced surface distortion, as well as femtosecond time resolution of dynamics. We successfully model our experimental results with a diffusive transport model that is modified to include an additional boundary resistance. These results confirm the importance of considering ballistic transport away from a nanoscale heat source, and identify a means of correctly accounting for this ballistic transport.