Chiral Phonon-Driven Ultrafast Spin Currents in a Non-Magnetic Ferroelectric

The discovery that fundamental lattice vibrations can carry angular momentum allows non-magnetic materials to exhibit properties that until now have only been observable in magnetic materials. However, the role of these specific vibrations, commonly known as chiral phonons, in driving ultrafast spin dynamics has not yet been explored experimentally. Here, using broadband terahertz emission spectroscopy, we demonstrate that the non-magnetic ferroelectric ${\mathrm{In}}_2{\mathrm{Se}}_3$ generates signals consistent with a ultrafast spin current after excitation with an ultrafast laser pulse, which we attribute to a chiral phonon-induced spin Seebeck effect. We suggest that the transient heating creates an non-equilibrium phonon distribution, resulting in a net angular momentum and magnetization perpendicular to the thermal gradient, which is converted into a terahertz electromagnetic pulse via an inverse spin-Hall effect in a thin metal overlayer. The emitted field reverses sign upon flipping the sample orientation or replacing Pt with W, consistent with spin-current generation. Polarization-resolved measurements reveal controllable terahertz ellipticity, indicating an additional polarization-dependent ultrafast response. Our results establish chiral phonons as a route for generating ultrafast spin currents in non-magnetic materials as well as providing a pathway toward terahertz sources with switchable handedness.