Revealing topological surface states with STM-based spectroscopy*

Topological insulators possess Dirac surface states with spin–momentum locked band dispersion. Breaking time-reversal symmetry can create an exchange gap and open a door to many exotic quantum states. STM spectroscopy is uniquely suitable for detecting the topological surface states. In this talk, we will present two examples to illustrate how STM spectroscopy resolves spin helical states on Bi₂Te₂Se and gapped surface states on MnBi₂Te₄. First, a multi-probe STM with spin-polarized tips offers in situ transport spectroscopy allowing differentiations of surface conductance and spin-momentum locked chemical potential from contributions of the bulk states [1]. A spin-polarized transport is revealed with ultra-high mobility and spin polarization [2]. Second, quasi-particle interference STM is used to probe local dispersions of both surface and bulk bands, which allows access to the gapped surface states with high spatial resolution [3]. The revealed surface gap is near the Fermi level and resides the bulk band gap, providing the key to access to other emergence phenomena such as quantum anomalous Hall effect and axion insulator state.
[1] SM Hus et al, PRL 119, 137202 (2017).
[2] W Ko et al, PRL 121, 176801 (2018).
[3] W Ko et al, PRB 102, 115402 (2020).