A New Class of Quantum Material Lets Light and Magnetism Talk Directly to Each Other

Researchers at the City College of New York have identified a class of quantum materials, only a few atoms thick, in which light and magnetism interact directly — a departure from the long-standing arrangement in which the two properties are handled by separate parts of a device and coaxed into talking through an intermediary.

The work, published in Nature Materials under the title "Excitons in van der Waals magnetic materials," centers on two quasiparticles. An exciton forms when an incoming photon energizes an electron and leaves behind a positively charged hole; the pair stays bound as an electrically neutral particle that couples strongly to light. A magnon is a collective magnetic wave rippling through a material's ordered spin structure. In most systems these live in different worlds. In van der Waals magnetic semiconductors, the researchers found, excitons and magnetic moments emerge from the same electronic orbitals — which is why they can couple as tightly as they do.

"An exciton is not just a passive light-driven excitation sitting on top of the magnetism," said lead author Pratap Chandra Adak. "It can sense the spin order and magnons, and under the right conditions, even help control the magnetic state itself." That two-way street is the finding that matters: light has long been used to read magnetic order, but an exciton that can push back on the spin state turns light from a probe into a control knob.

Senior author Vinod M. Menon framed the shift in the field's trajectory, saying research has moved from "detecting magnetism in atomically thin crystals to actively exploring how magnetic order can control light-matter interactions."

The team examined several material platforms, including chromium triiodide, nickel phosphorus trisulfide and chromium sulfur bromide — layered crystals held together by weak van der Waals forces, which is what allows them to be thinned down to a few atomic layers while retaining magnetic order.

The applications the researchers sketch follow from the coupling. Magneto-photonic memory and data readout, all-optical logic devices, tunable light-emitting devices and magneto-optic lasers all depend on light and magnetism influencing one another quickly and in a small volume. So do quantum transducers, which convert microwave signals into optical ones — a conversion step that quantum computing badly needs, since superconducting qubits speak microwave while long-distance quantum networks speak light. The CCNY work was supported by DARPA and the Gordon and Betty Moore Foundation, with co-authors from the Technical University of Munich, the National Laboratory of the Rockies, Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau and the University of Washington.