  A nanophononic metamaterial is a new type of nanostructured material that
features an array, or a forest, of intrinsically distributed resonating
substructures. Each substructure exhibits numerous local resonances, each of
which may hybridize with the phonon dispersion of the underlying host material
causing significant reductions in the group velocities and consequently a
reduction in the lattice thermal conductivity. In this paper, molecular
dynamics simulations are utilized to investigate both the dynamics and the
thermal transport properties of a nanophononic metamaterial configuration
consisting of a freely suspended silicon membrane with an array of silicon
nanopillars standing on the surface. The simulations yield results consistent
with earlier lattice-dynamics based predictions which showed a reduction in the
thermal conductivity due to the presence of the local resonators. Using a
spectral energy density approach, in which only simulation data is utilized and
no a priori information on the nanostructure resonant phonon modes is provided,
we show direct evidence of the existence of resonance hybridizations as an
inherent mechanism contributing to the slowing down of thermal transport in
this system.
