When thin becomes thinner – Germanene nanoribbons pave the way for quantum computing

Because of these properties, topological insulators are candidate materials for quantum computing and for the next generation of low-energy electronics. “But as we try to make devices smaller and more efficient, there are key questions that remained unanswered”, says Pantelis Bampoulis, one of the researchers.

“Like, what is the smallest size a topological material retains its two-dimensional properties? And what happens when we go smaller?”, Bampoulis continues. The researchers addressed these questions in their latest research using nanoribbons made from germanene. Germanene is an atomically thin layer of germanium atoms with unique topological properties.

“In our work, we made germanene nanoribbons. These are structures that are just a few nanometres wide but hundreds of nanometres long. With germanene nanoribbons, we studied both theoretically and experimentally how the topological edge states change as the ribbons get narrower and narrower”, explains Dennis Klaassen, PhD student supervised by Bampoulis and first author of the study.

The researchers found that the nanoribbons maintain their topological edge states down to a critical width of about two nanometres. Below this width, something remarkable happens. The edge states you normally find in germanene nanoribbons disappeared, and instead, new quantum states localised at the ends of the nanoribbons appeared. These end states are protected by fundamental symmetries and indicate the emergence of a one-dimensional topological insulator.

The end states are very stable against defects and other local impurities. This makes them perfect for use in quantum applications, for example, in the development of error-resistant qubits. “Interestingly, these states are similar to Majorana zero modes, which are elusive particles that have fascinated scientists ever since their prediction. Although we do not address Majorana zero modes, our study provides a template for exploring such phenomena in a one-dimensional material with strong spin-orbit coupling”, says Bampoulis. ‘On top of that, the fabrication procedure allows us to make dense arrays of topological edge states where current could flow without dissipation, fulfilling a major requirement for low-energy electronics’, says Klaassen.

Dennis Klaassen and Dr Pantelis Bampoulis are researchers in the Physics of Interfaces and Nanomaterials research group (PIN; Faculty of S&T / MESA+). Together with researchers from Utrecht University, Radboud University Nijmegen, Fudan University (China) and the University of Tokyo (Japan), they recently published their findings in an article entitled ‘Realization of a one-dimensional topological insulator in ultrathin germanene nanoribbons’ in the scientific journal Nature Communications.