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.

Two RIKEN physicists have established new theoretical limits for experimentally measurable quantities by viewing solids through a lens of quantum geometry. Their results shed light both on the physics of solids and on quantum mechanics.

The usual approach to studying a solid in physics is to consider all the interactions acting between its atoms or molecules and then use the laws of quantum mechanics to determine the solid’s properties. But a new methodology involves considering the “quantum geometry” of a solid. It entails studying the geometric structures that arise not in physical space, but in the space of quantum states.

One of the key concepts in this approach is the quantum geometric tensor—a matrix that contains information about the distances and curvatures of quantum states.

An international team of researchers, including scientists from HZDR and Fritz Haber Institute of the Max Planck Society, for the first time directly observed how angular momentum is transferred and conserved within a crystal lattice. Using intense terahertz laser pulses, the researchers were able to selectively control these processes, which unveiled a surprising effect: During the angular momentum transfer, the direction of rotation reverses—caused by the rotational symmetry of the material.

The results, published in Nature Physics, provide new insights into the foundation of magnetism and open up possibilities for tailored control of quantum materials.

Conserved quantities such as energy, momentum, and angular momentum determine the fundamental laws of nature. In a closed system, these quantities are always conserved: they cannot be created or destroyed, only transformed or transferred. While angular momentum is familiar in everyday life through rotating carousels or riding a bicycle, it plays a central role at the quantum level—for example, as the fundamental origin of magnetism.