Material Characteristics Governing In-Plane Phonon-Polariton Thermal Conductance

The material dependence of phonon-polariton based in-plane thermal conductance is investigated by examining systems composed of air and several wurtzite and zinc-blende crystals. Phonon-polariton based thermal conductance varies by over an order of magnitude ($\sim 0.5-60$ nW/K), which is similar to the variation observed in the materials corresponding bulk thermal conductivity. Regardless of material, phonon-polaritons exhibit similar thermal conductance to that of phonons when layers become ultrathin ($\sim 10$ nm) suggesting the generality of the effect at these length-scales. A figure of merit is proposed to explain the large variation of in-plane polariton thermal conductance that is composed entirely of easily predicted and measured optical phonon energies and lifetimes. Using this figure of merit, in-plane phonon-polariton thermal conductance enlarges with increases in: (1) optical phonon energies, (2) splitting between transverse and longitudinal mode pairs, and (3) phonon lifetimes.