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In materials known as graphene nanoribbons, topological states can be precisely engineered and probed, providing an experimental platform for studying electronic topology. See Letters p.204 & p.209
For more than a decade, two-dimensional sheets of carbon atoms known as graphene have captured researchers' imaginations. Last year, it was predicted that electronic states in narrow strips of graphene - dubbed graphene nanoribbons - could have different topologies depending on the width of the strip1. On pages 204 and 209, respectively, Rizzo et al.2 and Gröning et al.3 report experiments that confirm this prediction. Their results show that graphene nanoribbons provide a flexible and highly precise platform for designing and fabricating materials that have what is known as a non-trivial topology. The authors suggest that such materials could be used to realize desired exotic topological states for quantum technologies.
We learn in school that materials can differ starkly in their electrical properties. The difference between conductors and insulators is rooted in the states that are available to the electrons in these materials. In conductors, such as metals, electrons can move freely because available states exist at arbitrarily low energies. By contrast, the electrons in insulators are effectively localized, and do not conduct electricity unless they are provided with sufficient energy to overcome an energy gap.
This understanding of conductors and insulators was an early triumph for the application of quantum theory to materials. However, over the past decade or so, researchers have learnt that this picture needs to be amended in fundamental ways. This realization...