1D topological insulator found at edges of germanene nanoribbons

Researchers from Utrecht University and the University of Twente have probed the limits of topological insulation. Making ever-thinner nanoribbons of germanene, the germanium equivalent of the 2D material graphene, they observed that the material stopped being a 2D topological insulator at a width of about 2 nanometers, forming a 1D topological insulator at the edges instead. This insight could be useful for quantum computing and the next generation of low-energy electronics.

Two-dimensional topological insulators like germanene are insulating in their interior but have conductive edges, where electricity flows without resistance. This prompts a simple question: what happens when strips of germanene get narrower? Ultimately, a one-dimensional line would result, but does that coincide with a single line of atoms? Or do the topological properties disappear sooner, when the ribbon is still multiple atoms wide?

The researchers found that the nanoribbons maintain their topological edge states down to a critical width of about two nanometers. Below this width, the edge states disappear and new quantum states localized at the ends of the nanoribbons emerge. These bear a resemblance to a 1D topological insulator.