Theory of phonon angular momentum transport across smooth interfaces between crystals

The discovery of chiral phonons—circularly polarized lattice vibrations—has opened new avenues in the study of phonon transport. These phonons carry spin angular momentum and play a key role in interactions with electron spins in ferromagnets and heavy metals. Studies on injecting these phonons into different materials via interfaces have thus gained attention.

While the phonon-to-electron spin conversion has been studied so far, direct phonon spin transport between crystals remains unexplored. In this study [1] we propose a theoretical framework for the diffusion of phonon angular momentum from a chiral to an achiral crystal, driven by a thermal gradient at low temperatures. We model the phonon transport within the Boltzmann formalism, applying a boundary condition based on the elastic power reflectance and transmittance. We demonstrate that the spin angular momentum diffuses through interfaces even in the absence of net heat flow across the interfaces. Time permitting, we will also address orbital angular momentum generated by boundary scattering, and show that the total phonon angular momentum, including spin and orbital components, is conserved at the interface.