Kineo-Elasticity and Nonreciprocal Phonons by Rashba-induced Interfacial Spin-Lattice Coupling

We identify a previously unrecognized spin-lattice coupling that is allowed in the presence of broken inversion symmetry that can be considered as a lattice analogue to the electronic Rashba spin-orbit coupling. In the low-frequency regime with magnons integrated out, the interfacial spin-lattice coupling is shown to engender a kineo-elastic term in the phonon Lagrangian that couples the strain on the lattice to its velocity and thereby gives rise to a nonreciprocity in transverse phonon velocity. We further analyze the full magnon-phonon spectrum and uncover directional hybridization and absorption, leading to asymmetric phonon propagation lengths for opposite directions. Our results indicate that such interfacial spin-lattice coupling can serve as an efficient route to achieve nonreciprocal phonon propagation properties in magnetic heterostructures with strong Rashba spin-orbit coupling.