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Electrical spin injection into graphene from a topological insulator in a van der Waals heterostructure

All-electrical (magnetic-material-free) spin injection is one of the outstanding goals in spintronics. Topological insulators (TIs) have been recognized as a promising electrically controlled spin source thanks to the strong spin-orbit coupling and in particular, the spin-momentum locked topological surface states (TSS) supporting helically spin polarized currents. Many TI materials such as Bi-based chalcogenides are also layered 2D materials and can be incorporated into van der Waals (vdW) coupled heterostructures, opening the possibility of the utilization of TIs for electrical spin injection into other 2D materials. Here, we demonstrate electrical injection of helically spin-polarized current into graphene through a 3D TI in a mechanically stacked heterostructure between Bi2Te2Se (a TI) and chemical vapor deposition (CVD)-grown graphene, using the spin potentiometric measurement. When a dc current is flowing from the TI to graphene, we detect a striking step-like voltage change (spin signal) in both the TI and graphene using ferromagnetic (FM) probes. The sign of the spin signal can be reversed by reversing the direction of the dc bias current, and the corresponding amplitude of the spin signal increases linearly with the bias current, indicative of a current-induced helical spin polarization in both TI and graphene. In contrast, the graphene itself exhibits usual nonlocal spin valve signal when the spins are injected using an FM electrode. We discuss possible origins of the helical spin polarization injected into the graphene in our TI/graphene heterostructure that may include TSS as well as the spin-orbit coupled Rashba states. Our findings show electrical injection of a spin helical current into graphene through a TI and demonstrate TIs as potential spin sources for future spintronic devices wherein spin manipulation is achieved electrically.