Revealing topological quantum states with STM-based spectroscopy

Topological insulators possess a massless Dirac dispersion with spin–momentum locking at the surface. The onset of a spontaneous magnetization or a broken time-reversal symmetry leads to the formation of an exchange gap in the Dirac band dispersion. In this work, we will present two examples to show how STM spectroscopy can be used to detect these signatures of topological quantum states. (1), Spin helical states on Bi2Te2Se. A multi-probe STM with spin-polarized tips allows us to perform in situ transport measurement to differentiate surface conductance from the bulk and spin-up chemical potential from the spin-down. As a result, a spin-momentum-locked current is revealed which shows ultra-high mobility and polarization. (2), Gapped surface states on MnBi2Te4. Quasi-particle interference patterns are used to probe local dispersions of both surface and bulk electronic structures. The theoretically predicted gapped surface states are evaluated with high spatial resolution. It is expected that tuning of the Fermi level in the exchange gap will result in the emergence of a quantum anomalous Hall effect.