  Wave effects of phonons can give rise to controllability of heat conduction
beyond that by particle scattering at surfaces and interfaces. In this work, we
propose a new class of 3D nanostructure: a silicon-nanowire-cage (SiNWC)
structure consisting of silicon nanowires (SiNWs) connected by
nano-cross-junctions (NCJs). We perform equilibrium molecular dynamics (MD)
simulations, and find an ultralow value of thermal conductivity of SiNWC, 0.173
Wm-1K-1, which is one order lower than that of SiNWs. By further modal analysis
and atomistic Green's function calculations, we identify that the large
reduction is due to significant phonon localization induced by the phonon local
resonance and hybridization at the junction part in a wide range of phonon
modes. This localization effect does not require the cage to be periodic,
unlike the phononic crystals, and can be realized in structures that are easier
to synthesize, for instance in a form of randomly oriented SiNWs network.
