Insulators are materials in which electrons cannot move freely. Past theoretical studies predicted the existence of an unusual insulating state dubbed obstructed atomic insulator (OAI), in which electrons are localized inside a crystal, while their centers of charge lie in empty spaces between atoms, rather than on the atoms themselves.

Two independent research teams, one at Princeton University and Donostia International Physics Center (DIPC), and the other at Columbia University recently observed signatures of this long-theorized quantum state in two different transition metal dichalcogenides, niobium diselenide (NbSe₂) and tungsten diselenide (WSe₂). Their papers, both of which were published in Nature Physics, could open new possibilities for the study of topological quantum phenomena.