Thermal phonon coherence in superlattices and the role of phonon scattering phase space

Phonon coherence in heterostructures like superlattices is a topic of intense interest due to the potential to realize new materials that do not occur naturally. Most prior works have focused on how coherence can open up phonon bandgaps and modify group velocities. Here, we show that the first-order impact of phonon coherence on thermal transport is instead the phonon scattering phase space, or the space that defines which modes can interact in three-phonon scattering processes. Ab-initio calculations show that the thermal transport properties of heterogeneous solids can be made drastically different, both higher and lower, than those of the constituents by manipulating this phase space. Our work provides a new viewpoint into how phonon coherence can be exploited to realize materials with exceptional thermal properties for applications such as thermoelectricity.