Hybrid magnon -- Nambu-Goldstone excitations in topological superconductor/ferromagnetic insulator thin-film heterostructures

We address a previously unexplored type of dynamical proximity effect that occurs in s-wave topological superconductor/ferromagnetic insulator (TS/FI) heterostructures. It is predicted that magnons in the FI and the Nambu-Goldstone (NG) collective superconducting phase mode in the TS are coupled, forming composite magnon-NG excitations. The mechanism of this coupling is associated with the complete spin-momentum locking of electrons in the helical surface state of the TS. The strength of the magnon-NG coupling is strongly anisotropic with respect to the mutual orientation of the magnon wave vector and the equilibrium magnetization of the FI. This effect provides a mechanism for the interconversion of spin signals and the spinless signals carried by collective superconducting excitations, thereby giving new impetus to the development of superconducting spintronics.