Observation of room-temperature ballistic thermal

In ballistic thermal conduction, the wave characteristics of phonons allow the transmission of energy without dissipation. However, the observation of ballistic heat transport at room temperature is challenging because of the short phonon mean free path. Here we show that ballistic thermal conduction persisting over 8.3 mm can be observed in SiGe nanowires with low thermal conductivity for a wide range of structural variations and alloy concentrations. We find that an unexpectedly low percentage (∼0.04%) of phonons carry out the heat conduction process in SiGe nanowires, and that the ballistic phonons display properties including non-additive thermal resistances in series, unconventional contact thermal resistance, and unusual robustness against external perturbations. These results, obtained in a model semiconductor, could enable wave-engineering of phonons and help to realize heat waveguides, terahertz phononic crystals and quantum phononic/thermoelectric devices ready to be integrated into existing silicon-based electronics. T he presence of phonon scattering processes and the associated complex interferences of phonons limit the wave character-istics within a phonon mean free path, l. The short mean free path (l, 0.1 mm) at room temperature for most materials has hindered the observation of ballistic thermal conduction. According to the kinetic theory of phonons, the thermal conduc-tivity k is given by k =