Controlling how electrons interact in semiconductors is crucial for the development of electronic and optical devices. Physicists at the University of Regensburg have now discovered a surprisingly efficient new mechanism that can be used to vary the interaction between electrons and even limit their movement to one dimension: magnetic order.
Electronics and optoelectronics of the future are more dependent than ever on the ultimate compact components. Atomically thin layers of so-called transition metal dichalcogenides, which can be removed from volume crystals using adhesive tape, have therefore been the focus of solid-state research for several years. Since electrons in these ultra-thin layers can only avoid each other in two dimensions, they interact particularly strongly there. This is particularly evident in the optical properties. When light hits these materials, electrons can be energetically excited, leaving a positively charged vacancy, a so-called hole, at their original position.
Due to their opposite charge, the electron can orbit the hole and thus form a bound state similar to an atom, a so-called exciton. These particles give the material completely new optical properties, which can be tailored in a targeted manner through structural adjustments, such as stacking different atomically thin layers – tempting prospects for the development of wafer-thin solar cells and LEDs. Nevertheless, practical applications based on these materials have been difficult to implement so far, as the current methods of producing atomic layers are not compatible with industrial mass production.
An international team of physicists from Regensburg, Ann Arbor, Prague and Dresden has now discovered a new mechanism in the extraordinary material chromium sulfide bromide (CrSBr) that does not rely on structural adaptations in their search for alternative ways to constrain and control electrons. The material CrSBr has a layered structure in which the spins of the electrons – a quantum mechanical property that generates a magnetic moment – are all aligned along one direction within an atomically thin layer. The spin direction of the neighboring layers depends on the temperature surrounding them. If the spins of two adjacent layers are aligned antiparallel, this could restrict the electron movement to a single atomic layer and thus form a "magnetic cage".
To test this assumption, Prof. Rupert Huber's team in Regensburg used ultrashort laser pulses that are only a few femtoseconds short – a hundred trillion times faster than the blink of an eye – to excite excitons in the material. A second ultrashort light pulse, chosen in the mid-infrared region of the electromagnetic spectrum, scanned the atom-like energy levels of the excitons by exciting specific transitions between different orbitals.
Using this method, which can be imagined as a slow-motion camera, the researchers investigated how the excitons behave. This gave them insight into their binding energy, motion, and lifespan, and even managed to control the bond strength of the excitons in a CrSBr crystal. The necessary high-quality crystals were grown by the Prague team of Prof. Zdeněk Sofer. By systematically varying the temperature, the research team observed a sudden change in the energy structure of the excitons, which is directly related to the magnetic order of the material.
A complex quantum theory, developed by Prof. Mackillo Kira's group at the University of Michigan, analyzed this energy shift at the microscopic level. They found that the dimensionality of the excitons is dictated by the magnetic order. As expected, the antiparallel spin orientation at low temperatures causes electrons and holes to be trapped within a single layer of the material. In combination with CrSBr's special crystal structure, this "magnetic cage" further restricts the movement of the excitons within the plane. As a result, excitons are essentially limited to a single dimension, which leads to high binding energies even in crystals with hundreds of layers. However, as the temperature increases, the spin orientation is lost, breaking up the magnetic cage. The excitons can again move freely along all spatial dimensions and spread across several layers, which drastically reduces their binding energy but at the same time extends their lifespan.
The antiparallel spin orientation in adjacent layers of the magnetic crystal CrSBr limits the strongly bound excitons to one dimension, with the binding energies inside and on the surface of the crystal being very different.
"It was fascinating to see how we were able to change the behavior of these excitons abruptly by cooling the material. To ensure that this behavior is clearly due to the magnetic phase transition, we applied an external magnetic field in another experiment. This allowed us to actually control the temperature at which the magnetic cage opens," explains Marlene Liebich, the first author of the study. "The magnetic order represents a new adjusting screw for tailoring excitons and their interactions. This could significantly change future electronics and information technologies," adds Dr. Niloufar Nilforoushan, an author of the study.
A second publication together with colleagues from Dresden, New York and Prague, which was published at the same time in the journal Nature Materials, complements these findings in an excellent way. In this study, the magnetic restriction of excitons was demonstrated using a different measurement method that examined the light reflected from the sample surface. Dr. Florian Dirnberger, an author of both publications, is enthusiastic: "Surprisingly, magnetic confinement is so effective that you can distinguish excitons in different atomically thin layers of the material." In fact, the team found that excitons on the surface have significantly different properties than those inside the material.
These results open up exciting possibilities for future spintronicdevices and targeted control of phase transitions – a unique perspective for information processing technologies. The newly observed surface excitons could also contribute significantly to these advances, as their different properties are of great importance, especially for sensor applications.
Original publications:
M. Liebich, M. Florian, N. Nilforoushan, F. Mooshammer, A. D. Koulouklidis, L. Wittmann, K. Mosina, Z. Sofer, F. Dirnberger, M. Kira and R. Huber, "Controlling Coulomb correlations and fine structure of quasi-one-dimensional excitons by magnetic order",
Nature Materials (2025), https://doi.org/10.1038/s41563-025-02120-1 (external link, opens in a new window)(external link, opens in a new window)
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, "Magnetically confined surface and bulk excitons in a layered antiferromagnet",
Nature Materials (2025), https://doi.org/10.1038/s41563-025-02129-6 (external link, opens in a new window)