Phonon Heat Conduction in Corrugated Silicon Nanowires Below the Casimir Limit

hh; The thermal conductance of straight and corrugated monocrystalline silicon nanowires has been measured between 0.3~K and 5~K. The difference in the thermal transport between corrugated nanowires and straight ones demonstrates a strong reduction in the mean free path of the phonons. This averaged mean free path is remarkably smaller than the smaller diameter of the nanowire, evidencing a phonon thermal transport reduced below the Casimir limit. Monte Carlo simulations highlight that this effect can be attributed to significant multiple scattering of ballistic phonons occuring on the corrugated surfaces. This result suggests an original approach to transforming a monocrystalline material into a phonon glass.