Nonlocal Electrical Detection of Spin-Polarized Surface Currents in the 3D Topological Insulator BiSbTeSe₂

The spin-polarized surface states in topological insulators offer unique transport characteristics that make them distinguishable from trivial conductors. Herein, the impact of these surface states in the topological insulator BiSbTeSe₂ is detected by electrical means using a nonlocal (NL) transport configuration with ferromagnetic Co/Al₂O₃ electrodes. It is shown that the NL measurement allows probing of 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 NL current path along the surface and hence leads to a complete disappearance of the NL signal at around 20 K. Interestingly, it is observed that the ratio between spin signal to background signal is much larger in the NL geometry compared to the local one. Given that the observed ratio in the NL geometry aligns well with expectations for spin-polarized surface states, the findings suggest that an as-yet unresolved mechanism diminishes the spin signal in the local geometry.