Domain walls as possible realization of edge state coupling in a quantum spin Hall insulator

The recently discovered monolayer system bismuthene/SiC(0001) is a promising candidate for the realization of a room temperature quantum spin Hall (QSH) effect. Previous experiments have established a large fundamental band gap (0.8 eV) and the existence of one-dimensional metallic edge states [1]. As expected for a QSH insulator, the electronic edge channels do not show any signs of backscattering from kinky edge sections that would manifest in interference phenomena. Notwithstanding, topological protection against defect scattering may become lifted when two helical edge channels are brought into direct proximity, resulting in quantum interference. By scanning tunneling microscopy we study phase-slip domain boundaries (DB) that form as result of bismuthene being a rt3 x rt3 R30° reconstruction on SiC(0001) as substrate. Kinks and disorder limit the longitudinal extent of these quasi one-dimensional topographic defects. By spectroscopic means we scrutinize quasi-particle interference along the DB that points towards a linear electronic dispersion strongly reminiscent of a Fabry-Pérot resonator. We discuss our findings as possible quantum interference between coupled helical edge states that are formed in the vicinity of a DB.

[1] F. Reis et al., Science 357, 287 (2017)