Many topological phases are characterized and identified by states at the boundary of a crystalline material that are distinct from those in the bulk. As an example, a topological insulator hosts a metallic surface state whereas its bulk is insulating. Now, two papers —one by Gaurav Bahl and coworkers and the other by Jian-Hua Jiang and coworkers—report that in addition to boundaries, topological states also exist at crystallographic defects in the bulk. They show that disclination defects trap fractional charges, characteristic of topological crystalline insulators. The Gaurav Bahl group realizes these states in microwave-frequency metamaterials whereas the Jian-Hua Jiang group uses photonic crystals. These results demonstrate the feasibility of probing topological states at bulk defects that are not easily detectable at the boundary of materials.
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