Lifting topological protection in a quantum spin Hall insulator by edge coupling

TL;DR

This study demonstrates experimentally that coupling of edges in a 2D topological insulator can partially lift topological protection, enabling new device functionalities like electronic interferometry.

Contribution

It provides direct STM/STS evidence that edge coupling in 2D TIs can modify topological protection, revealing new ways to control edge state properties.

Findings

Coupling of 2D TI edges alters quasi-particle interference patterns.

Isolated edges show no backscattering, coupled edges do.

Edge coupling enables active control of topological protection.

Abstract

The scientific interest in two-dimensional topological insulators (2D TIs) is currently shifting from a more fundamental perspective to the exploration and design of novel functionalities. Key concepts for the use of 2D TIs in spintronics are based on the topological protection and spin-momentum locking of their helical edge states. In this study we present experimental evidence that topological protection can be (partially) lifted by pairwise coupling of 2D TI edges in close proximity. Using direct wave function mapping via scanning tunneling microscopy/spectroscopy (STM/STS) we compare isolated and coupled topological edges in the 2D TI bismuthene. The latter situation is realized by natural lattice line defects and reveals distinct quasi-particle interference (QPI) patterns, identified as electronic Fabry-P\'erot resonator modes. In contrast, free edges show no sign of any single-particle backscattering. These results pave the way for novel device concepts based on active control of topological protection through inter-edge hybridization for, e.g., electronic Fabry-P\'erot interferometry.