Electrical detection of the surface spin polarization of the candidate topological Kondo insulator SmB6

The Kondo insulator compound $\mathrm{Sm}{\mathrm{B}}₆$ has emerged as a strong candidate for the realization of a topologically nontrivial state in a strongly correlated system, a topological Kondo insulator, which can be a novel platform for investigating the interplay between nontrivial topology and emergent correlation-driven phenomena in solid-state systems. Electronic transport measurements on this material, however, so far showed only the robust surface-dominated charge conduction at low temperatures, lacking evidence of its connection to the topological nature by showing, for example, spin polarization due to spin-momentum locking. Here, we find evidence for surface-state spin polarization by electrical detection of a current-induced spin chemical-potential difference on the surface of a $\mathrm{Sm}{\mathrm{B}}₆$ single crystal. We clearly observe that a surface-dominated spin voltage, which is proportional to the projection of the spin polarization onto the contact magnetization, is determined by the direction and magnitude of the charge current and is strongly temperature dependent due to the crossover from surface to bulk conduction. We estimate the lower bound of the surface-state net spin polarization as 25% based on the quantum transport model, providing direct evidence that $\mathrm{Sm}{\mathrm{B}}₆$ supports metallic spin helical surface states.