Helical edge transport at electrically defined topological domain boundaries

In two-dimensional topological insulators, helical edge channels normally emerge at the physical edges of the sample, where crystal defects and/or inhomogeneous electrostatic potentials inevitably form and make the identification of genuine transport properties difficult. Here we propose a new setup in which helical edge channels are induced not at the physical edges but at topological domain boundaries that are created by local control of the topological property. We use InAs/InGaSb quantum wells whose topological index can be varied by dual gating [1]. By switching topological indices of two adjacent regions independently, we observed electrical conduction at the boundary only when the two regions are topologically distinct, demonstrating helical edge transport therein. Furthermore, under a magnetic field, we observed a conductance dip, which directly represents an energy gap opening at the Kramers degeneracy point in the edge dispersion. This feature, one of the most direct pieces of evidence for the spin-helical nature [2], was observed for the first time in semiconductor quantum wells by eliminating the effects of physical edge irregularities.

[1] H. Irie et al., Phys. Rev. Mater. 4, 104201 (2020). [2] S. Wu et al., Science 359, 76 (2018).