Phonon heat conduction in corrugated silicon nanowires below the Casimir limit

The thermal conductance of straight and corrugated monocrystalline silicon nanowires has been measured between 0.3 K and 5 K. It is demonstrated that the corrugation strongly reduces the thermal transport by reducing the mean free path of the phonons. The experimental 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 occurring on the corrugated surfaces. This result suggests an original approach to transforming a monocrystalline material into a phonon glass.