> "That's the key finding," she said of the material's switchable vacancy order. "The idea of using vacancy order to control topology is the important thing. That just hasn't really been explored. People have generally only been looking at materials from a fully stoichiometric perspective, meaning everything's occupied with a fixed set of symmetries that lead to one kind of electronic topology. Changes in vacancy order change the lattice symmetry. This work shows how that can change the electronic topology. And it seems likely that vacancy order could be used to induce topological changes in other materials as well."

"Reversible non-volatile electronic switching in a near-room-temperature van der Waals ferromagnet" (2024) https://www.nature.com/articles/s41467-024-46862-z :

> Abstract: [...] Here, we report the observation of reversible and non-volatile switching between two stable and closely related crystal structures, with remarkably distinct electronic structures, in the near-room-temperature van der Waals ferromagnet Fe_{5−δ}GeTe_2. We show that the switching is enabled by the ordering and disordering of Fe site vacancies that results in distinct crystalline symmetries of the two phases, which can be controlled by a thermal annealing and quenching method. The two phases are distinguished by the presence of topological nodal lines due to the preserved global inversion symmetry in the site-disordered phase, flat bands resulting from quantum destructive interference on a bipartite lattice, and broken inversion symmetry in the site-ordered phase.