Accessing the intrinsic spin transport in a topological insulator with a four-probe scanning tunneling microscope

The electrical transport through topologically protected surface states in topological insulators is expected to exhibit superior mobility from prohibited backscattering and spin-polarized current from spin-momentum locking. However, access to the intrinsic transport properties of surface states remains an experimental challenge, due to the extrinsic effects such as device geometry and environmental contaminations. Here, we directly access the intrinsic surface conductance of topological insulators by using a four-probe scanning tunneling microscope to tune the crossover of bulk-to-surface conductance. By controlling the probe-spacing and temperature, we realize 100 % surface conductance on the bulk single crystal of Bi₂Te₂Se. It allows us to measure more than an order of magnitude higher surface carrier mobility than the bulk, and a spin polarization approaching theoretically predicted value. A scattering-free spin transport is revealed at micrometer scale through topological surface states. In this manner, we achieve a direct and quantitative measurement of the intrinsic spin-polarized transport associated with topological surface states.