  Atomic motion in nanopillars standing on the surface of a silicon membrane
generates vibrons, which are wavenumber-independent phonons that act as local
resonances. These vibrons couple with heat-carrying phonons traveling along the
base membrane causing a reduction in the in-plane lattice thermal conductivity.
In this work, we examine isolated silicon and gallium nitride nanopillars and
for each compare the vibrons density of states (DOS) to those of phonons in an
isolated uniform silicon membrane. We show that while the phonon-vibron DOS
conformity across the full spectrum is a key factor in reducing the thermal
conductivity of the assembled nanostructure, the presence of an intense vibron
population at more dominant low frequencies plays a competing role. We report
predictions from molecular dynamics simulations showing lower thermal
conductivities for a silicon membrane with gallium-nitride nanopillars compared
to a silicon membrane with silicon nanopillars.
