Detection of Spin-Momentum Locking in Topological Insulators with Spin-Polarized Four-Probe STM

The spin-momentum locking in topological insulators (TIs) does not only provide a topological protection against backscattering but also ensures a spin-polarized surface conductance. The electrical detection of such a spin-momentum-locking, however, remains elusive. We developed a spin-polarized four-probe STM by combining spin-polarized STM and four-probe STM to achieve spin-sensitive multi-probe characterization. Here we report its applications in detecting spin current on pristine TI surfaces. By examining the potential difference between a ferromagnetic probe and a nonmagnetic probe as a function of the spatial separation of them, not only can we differentiate surface and bulk conductance [Nano Lett. 16, 2213 92016)], but also isolate the non-vanishing spin-dependent electrochemical potential from the Ohmic contribution [Phys. Rev. Lett. 119, 137202 (2017)]. The measured spin chemical potentials directly come from the 2D charge current. In this manner, we achieved a direct and quantitative measurement of spin current generation efficiency, which opens a new avenue to access the intrinsic spin transport associated with pristine topological surface states.