Bringing the Edge In: Manipulating Topological States Deep Inside Materials

Topological insulators are extraordinary materials that insulate on the inside but conduct electricity on their surface. A specialised class, known as higher-order topological insulators (HOTIs), features unique 'corner states'—energy signatures confined strictly to the material's outer boundaries. While these states hold immense potential for carrying information and energy, their utility has been hampered by a rigid constraint: they are physically pinned to the edges, limiting where and how they can be deployed.

In a significant leap forward, researchers have proposed and realised a method to liberate these states from the boundaries. They introduced the concept of higher-order topological point states (HOTPS), which exist at 'vacancies' or point-like holes created deep within the material. Theoretically, these inner voids were predicted to mimic the properties of corners, carrying a fractional charge of e/2.

The team validated this theory experimentally using acoustic and photonic metamaterials—artificial structures designed to manipulate sound and light waves—arranged in a Kekulé lattice. They successfully observed the predicted bound states at these inner vacancies. Furthermore, by varying the distance between two vacancies, they measured specific energy splitting, confirming that these states interact over a distance. This discovery implies that we can now create and position topological states anywhere within a material, offering a flexible new avenue for controlling particles in advanced technologies.