Non-local electrical detection of spin-momentum-locked surface currents in the 3D topological insulator BiSbTeSe$_{2}$

The spin-polarized surface states in topological insulators offer unique transport characteristics which make them distinguishable from trivial conductors. Due to the topological protection, these states are gapless over the whole surface of the material. Here, we detect the surface states in the topological insulator BiSbTeSe$_{2}$ by electrical means using a non-local transport configuration. We unambiguously probe the spin-momentum locking of the topologically protected surface states by spin-sensitive electrical read-out using 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. This is in contrast to local transport configurations where only the surface states of the one face being in contact to the electrodes can be measured. Our results furthermore exclude the contribution of the bulk to the non-local transport at low temperatures. Increasing the temperature, on the other hand, increases the interaction between bulk and surface states, which shortens the non-local current path along the surface and hence leads to a complete disappearance of the non-local signal at around 20 K. All this demonstrates that the non-local signal at low temperatures is solely due to the topologically protected surface states.