When atoms crystallize, their energy levels split to fill wide bands. That spreading of the levels sets the crystal’s electronic properties, but it’s not the only factor. How the atoms arrange in space also counts, and in some rare cases, a crystal can possess certain symmetries that bestow the bands with “topological” features. Like electrons, phonons move about a crystal in ways prescribed by a band structure, which can also have topological features. Now, Jiade Li of China’s Institute of Physics and his collaborators have used a type of electron spectroscopy to demonstrate that graphene, which has already been shown to have topological electrons, has topological phonons [1].

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