On 4 November, Charles Kane and Eugene Mele received the 2019 Breakthrough Prize in Fundamental Physics for their trailblazing work related to topological insulators. The University of Pennsylvania researchers published two papers in 2005 about the quantum spin Hall effect in graphene. They predicted that spin-orbit coupling would open a bandgap in graphene without an external magnetic field. As a result, the material would behave as a bulk insulator but with unidirectional conducting edge states corresponding to modes in the bandgap. Kane and Mele realized that graphene’s band topology protects those edge states, which puts it into a class of materials called topological insulators (see the article by Xiao-Liang Qi and Shou-Cheng Zhang, Physics Today, January 2010, page 33).

Now some researchers are looking beyond quantum systems to mechanical systems that also behave as topological insulators. By observing and tinkering with the emergent edge states in classical systems, researchers are investigating phenomena that also hold true in harder-to-manipulate quantum systems. Mechanical topological insulators could also lead to applications in controlling and directing the flow of energy, such as impact absorption, sound isolation, and acoustic lensing.

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