Two different heat-transport mechanisms are known in solids: in crystals, heat carriers
propagate and scatter like particles, as described by the Peierls-Boltzmann equation for phonon wavepackets [Peierls, Ann. Phys., 1989]; in glasses, instead, carriers behave in a wave-like fashion, diffusing via a Zener-like tunneling between quasi-degenerate vibrational eigenstates, as described by the Allen-Feldman equation [PRL 62, 1989]. Recently, it has been shown that these two conduction mechanisms emerge as limiting cases from a unified transport equation based on the Wigner phase-space formulation of quantum mechanics [Simoncelli et al, PRX 12, 2022], which covers, on the same footing, solids ranging from crystals to glasses. Importantly, materials with an intermediate degree of disorder exist [Simoncelli et al., arXiv:2405.13161, accepted in PNAS], and these are not described by either the Peierls or the Allen-Feldman limits.

This project will employ the recently developed unified theory of thermal transport and machine-learning simulation methods to deepen current understandings of the microscopic physics underlying thermal transport in hybrid crystal-glass materials with controlled degree of disorder in bond topology and geometry, which are employed as a thermal barriers in zero-carbon jet engines.