Non-local electrical detection of spin-polarized surface currents in the 3D topological insulator BiSbTeSe${}_2$

The spin-polarized surface states in topological insulators offer unique transport characteristics that make them distinguishable from trivial conductors. Here, we detect the impact of these surface states in the topological insulator BiSbTeSe${}_2$ by electrical means using a non-local transport configuration with ferromagnetic Co/Al${}_2$O${}_3$ electrodes. We show that the non-local measurement allows to probe the surface currents flowing along the whole surface, i.e.~from the top along the side to the bottom surface and back to the top surface along the opposite side. Increasing the temperature increases the interaction between bulk and surface states, which shortens this non-local current path along the surface and hence leads to a complete disappearance of the non-local signal at around 20K. Interestingly, we observe that the ratio between spin signal to background signal is much larger in the non-local geometry compared to the local one. Given that the observed ratio in the non-local geometry aligns well with expectations for spin-polarized surface states, our findings suggest that an as-yet unresolved mechanism diminishes the spin signal in the local geometry.