Theory of spin-wave transport in ferromagnet-superconductor heterostructures: Negative refraction, perfect imaging and temperature-controlled spin-wave optics
We investigate spin-wave transport in ferromagnetic insulator-superconductor (FMI-SC) heterostructures and develop a general theoretical framework for spin-wave optics in these hybrid systems. We demonstrate that Meissner screening by the superconductor gives rise to a range of unconventional wave phenomena, including negative phase- and group-velocity refraction, and reflection and refraction laws that differ fundamentally from their optical counterparts. Within this framework, we derive the spin-wave Fresnel equations governing reflection and transmission at FMI-SC interfaces and show that the scattering properties exhibit a pronounced temperature dependence, enabling tunable spin-wave mirrors and refractive elements. Most strikingly, we find that superconducting screening can produce nearly straight isofrequency contours, far flatter than the kinked, intrinsically curved contours attainable in conventional dipolar spin-wave systems. We show that these straight contours enable functionalities such as perfect spin-wave imaging, efficient waveguiding, and interferometric elements, such as phase shifters and beam splitters, with unconventional properties. Our results establish FMI-SC heterostructures as a versatile platform for temperature-tunable spin-wave optics and interferometric magnonic devices.