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20.05.2025 12:43

Luminous Magnets: Quasiparticles Discovered on the Surface of Semiconductor Magnets

Katja Lesser Presse- und Öffentlichkeitsarbeit
Würzburg-Dresdner Exzellenzcluster ct.qmat

Alexey Chernikov and his team specialize in detecting optical quasiparticles using ultrafast microscopy. Together with international colleagues, they recently visualized a completely new quantum phenomenon: luminous quasiparticles – known as excitons – appearing on the surface of a semiconductor magnet. Until now, it was believed they could only exist within such materials. The team made this discovery when studying ultra-thin crystal layers – each just a few atoms thick – of the antiferromagnetic quantum semiconductor chromium sulfide bromide (CrSBr). Their findings have now been published in Nature Materials.

Quasiparticles in Semiconductor Magnets
The quantum materials studied by Alexey Chernikov – Professor of Ultrafast Microscopy and Photonics of the Cluster of Excellence ct.qmat at the Universities of Würzburg and Dresden – and his team are often only a few atomic layers thick. Their research focuses on exploring luminous quasiparticles. These quasiparticles form when a light pulse excites an electron, leaving behind a positively charged hole. The electron and hole bind together through electrostatic attraction, behaving as a new, independent particle-like entity. These excitons play a crucial role in light absorption and emission, as well as in the transmission of energy and quantum information. Scientists are also exploring their potential for light storage.

Excitons are typically found in non-magnetic materials, since most magnets are metallic and cannot form stable excitons. But chromium sulfide bromide (CrSBr) – an antiferromagnetic quantum semiconductor – defies that rule. This unique material combines magnetic order with semiconducting properties. Moreover, its crystal layers are only loosely held together by van der Waals forces, allowing them to be obtained as ultra-thin films just a few atoms thick. At low temperatures, the magnetic moments – called spins – in adjacent layers align in opposite directions. The structure of luminescent excitons depends on this magnetic order, which means scientists can precisely manipulate light absorption and emission using magnetic fields.

Capturing Excitons
To visualize these quasiparticles, Chernikov’s team uses advanced optical methods, allowing them to detect excitons in individual atomic layers less than a nanometer thick. For reference, a nanometer is one-millionth of a millimeter – just slightly more than the distance between two atoms.

“In the lab, we didn’t just see excitons deep inside the material – we also found them right on the surface,” says Professor Chernikov, who works at ct.qmat’s Dresden branch. “That was a critical step in understanding these fascinating and unusual quantum structures. Surface excitons reflect and emit light in a slightly different color than those inside the material, and thus we were able to see them.” The idea of looking for surface excitons came from conversations of the research team with a colleague at the University of Regensburg who is part of this project, he recalls. “We examined the material at the same time in Dresden and New York using different sample preparation and different measuring equipment. Yet we got the same results, which speaks to their high reproducibility. That’s something I’m really happy about!”

Surface glow
Excitons are created when photons strike a semiconductor. These quasiparticles absorb light, store its energy, and can pass through the material layer. When they dissolve, they release the stored energy as light.

“Excitons play a critical role in the optical behavior of nanomaterials,” emphasizes Dr. Florian Dirnberger, who was involved in the recently published discovery as a project leader in Dresden, and now heads an Independent Emmy Noether Research Group at the Technical University of Munich.

“Excitons have been known for many decades,” Dirnberger continues, “but only in the last four years have materials physicists really started to explore their potential when excitons are deliberately created in magnetic crystals. In addition to storing and transporting energy, they can carry and release information through light.” And he adds: “Although research into these exotic quasiparticles is still in its infancy, they could eventually lay the foundation for new technologies combining photonics and magnetism. Our findings mark an important contribution to this field.”

International success
This research is the result of international collaboration involving scientists from the USA, Germany, the UK, the Netherlands, and the Czech Republic. By combining advanced material synthesis, highly sensitive spectroscopy, and complex many-body theory, the team explored the structure of luminous quasiparticles in novel semiconductor magnets. These findings are significant not only for deepening our understanding of magnetic materials but also for driving future technological innovations in this emerging field.

Cluster of Excellence ct.qmat
The Cluster of Excellence ct.qmat – Complexity and Topology in Quantum Matter – has been jointly run by Julius-Maximilians-Universität (JMU) Würzburg and Technische Universität (TU) Dresden since 2019. Over 300 scientists from more than thirty countries and four continents study topological quantum materials that reveal surprising phenomena under extreme conditions such as ultra-low temperatures, high pressure, or strong magnetic fields. ct.qmat is funded through the German Excellence Strategy of the Federal and State Governments and is the only Cluster of Excellence in Germany to be based in two different federal states.

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Wissenschaftliche Ansprechpartner:

Prof. Alexey Chernikov, Institut für Angewandte Physik, Technische Universität Dresden, Tel.: +49 351 463 336439, alexey.chernikov@tu-dresden.de

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Originalpublikation:

Magnetically confined surface and bulk excitons in a layered antiferromagnet, Y. Shao, F. Dirnberger, S. Qiu, S. Acharya, S. Terres, E. J. Telford, D. Pashov, B. S. Y. Kim, F. L. Ruta, D. G. Chica, A. H. Dismukes, M. E. Ziebel, Y. Wang, J. Choe, Y. J. Bae, A. J. Millis, M. I. Katsnelson, K. Mosina, Z. Sofer, R. Huber, X. Zhu, X. Roy, M. van Schilfgaarde, A. Chernikov, and D. N. Basov, Nat Mater. 24, 391–398 (2025).

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Luminous Magnets

think-design | Jochen Thamm

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