  Prevailing nanostructuring strategies focus on increasing phonon scattering
and reducing the mean-free-path of phonons across the spectrum. In nanoporous
Si materials, for example, boundary scattering reduces thermal conductivity
drastically. In this work, we identify an unusual anticorrelated specular
phonon scattering effect which can result in additional reductions in thermal
conductivity of up to ~ 80% for specific nanoporous geometries. We further find
evidence that this effect has its origin in heat trapping between large pores
with narrow necks. As the heat becomes trapped between the pores, phonons
undergo multiple specular reflections such that their contribution to the
thermal conductivity is partly undone. We find this effect to be wave-vector
dependent at low temperatures. We use large-scale molecular dynamics
simulations, wave packet analysis, as well as an analytical model to illustrate
the anticorrelation effect, evaluate its impact on thermal conductivity, and
detail how it can be controlled to manipulate phonon transport in nanoporous
materials.
