  In a typical semiconductor material, the majority of heat is carried by long
wavelength, long mean-free-path phonons. Nanostructuring strategies to reduce
thermal conductivity, a promising direction in the field of thermoelectrics,
place scattering centers of size and spatial separation comparable to the
mean-free-paths of the dominant phonons to selectively scatter them. The
resultant thermal conductivity is in most cases well predicted using
Matthiessens rule. In general, however, long wavelength phonons are not as
effectively scattered as the rest of the phonon spectrum. In this work, using
large-scale Molecular Dynamics simulations, Non-Equilibrium Greens Function
simulations, and Monte Carlo simulations, we show that specific nanoporous
geometries, which create narrow constrictions in the passage of phonons, lead
to anticorrelated heat currents in the phonon spectrum. This results in
super-suppression of long-wavelength phonons due to heat trapping, and
reductions in the thermal conductivity well below what is predicted by
Matthiessens rule.
