Two assistant professors in the Department of Physics and Astronomy at the University of Notre Dame confirmed that the Berry curvature of electrons in solids can be detected optically by measuring how a material absorbs light in a new paper published in Physical Review Letters and wave featured on the cover of the Journal.

Berry phase and Berry curvature are quantum mechanical properties of electrons in solids that manifest when the electron is treated as a wave. They dictate how electrons in a given material respond to magnetic fields and whether the material allows current to flow specifically along the edge. Those electric currents are topologically protected from perturbations, allowing them to spread through the material without heat loss.

The research completed by Badih Assaf, an experimental physicist, and Yi-Ting Hsu, a theoretical physicist, holds potential implications for advanced sensing technologies and environmental modeling.

“A key breakthrough came when Hsu recalled a decades-old paper that hinted at this phenomenon in its appendix,” Assaf said. “By connecting our experimental results to those theoretical predictions, we were able to unlock the true significance of our data.”

Berry curvature is a complex quantum mechanical property of electrons. In a chunk of material, there are tons of electrons at the order of 10 to the power of 23 — that’s 1 followed by 23 zeroes) Hsu described. Physicists would like to predict how these electrons behave and how that will lead to macroscopic properties, or ones that can be observed and measured on a larger scale. By working together, the researchers were able to move from a theoretical framework, and a microscopic one, to actually observing the property through the interaction of light and magnetic materials.

While the research is fundamentally about deepening the understanding of quantum mechanics, the ability to detect Berry curvature using circularly polarized light opens up opportunities for developing advanced infrared sensors. These sensors could be used for environmental monitoring, such as detecting greenhouse gases like CO₂, or for thermal imaging in night vision technologies.

In addition to researchers at Notre Dame, other collaborators include researchers from the Institute of Semiconductor and Solid-State Physics, Johannes Kepler University, Linz, Austria; the Laboratoire National des Champs Magnétiques Intenses, CNRS, Université Grenoble Alpes, Grenoble, France; and the Department of Physics, Virginia Tech, Blacksburg, Virginia.

Originally published by Deanna Csomo Ferrell at science.nd.edu on February 20, 2025.