Mesoscale quantum conductance modulation in stanene topological edge states: role of size and defect spatial...

Topological insulators are generally believed to be highly immune to defects. However, its performance may become complicated depending on the specific application scenario. Taking monolayer mesoscale stanene ribbons as a model system, we investigated how the two realistic factors, the spatial distributions of vacancy defect and the device dimensions, would influence topological edge conductance. The electronic transport is calculated by an effective tight-binding model with parameters fitted through genetic algorithm. It is found that the edge states are more vulnerable than previously expected in the presence of random defects. For defect free ribbons, the calculated current is sharply localized at the edges with a typical ideal conductance of 2e²/h. It might be expected that the vacancy defect solely distributed in the edges will have larger effects on the edge conductance than those solely within the interior. On the contrary, the former have marginal effects as the current simply bypasses them, whereas the latter can lead to an appreciable decline in edge conductance via the hybridization of the interior defect states with the edge states. The most pronounced degradation of conductance occurs when defects are present at both edge and interior. The calculated local current reveals that the electrons traverse between edges through relay scattering among edge-interior defects, giving rise to back scattering and loop current. Increasing the ribbon width can help to reduce the inter-edge scattering and hence enhance the stability of the topological edge states. Whereas in a longer ribbon, the electrons will encounter more defects in the longer conduction path, increasing inter-edge scattering and weakening the topological edge conductance. Our study highlights the critical roles of spatial vacancy defect distribution and ribbon size in tuning the performance of topological devices.