All-Optical Ultrafast Pure Spin Current in the Altermagnet Cr2SO

All-optical generation of pure spin current─the flow of spin in the absence of a corresponding charge flow─relies on symmetry based compensation between the two spin channels. 2D d-wave altermagnets, which in many respects are ideal spintronic materials due to their very low spin–orbit coupling, possess a magnetic point group and highly anisotropic valley manifolds that would appear to preclude such current compensation, excluding them as materials for the ultrafast generation of pure spin current. Here we show that infrared valley excitation combined with a THz pulse envelope allows the generation of large and nearly 100% pure spin currents in the 2D altermagnet Cr2SO. Our approach is based on a valley selection rule coupling linearly polarized light to opposite spin valleys, along with the intrinsic momentum shift that a co-occurring THz pulse imbues a valley spin excitation with. These results provide a practical and all-optical route to the generation of pure spin current in 2D d-wave altermagnets, opening a path to lightwave control of spin in an environment with very low intrinsic spin–orbit coupling.