Spin radiation of electrons, excitons, and phonons

In the celebrated Stern-Gerlach experiment, an inhomogeneous static magnetic field separates a beam of charge-neutral atoms with opposite spins, thereby driving a ``spin current'' normal to the propagation direction. Here, we generalize it to the dynamic scenario by demonstrating a spin transfer from an ac inhomogeneous magnetic field to electrons or charge-neutral excitons and phonons. We predict that parametric pumping can efficiently radiate the angular momentum of local ac magnetic sources to their dc spin currents with van der Waals semiconductors as prototypes. This mechanism brings a unified and efficient paradigm in the spin transport of distinct mobile carriers.