  The Kondo insulator compound SmB6 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 SmB6 single crystal. We clearly
observe 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 15
percent based on the quantum transport model providing direct evidence that
SmB6 supports metallic spin helical surface states.
