Magnetic semiconductor preserves 2D quantum properties in 3D material

The researchers published their findings today (Feb. 19) in Nature Materials.

"Although the functionalities displayed by two dimensional (2D) materials are vast and their potential is revolutionary, maintaining their superior properties beyond the 2D limit remains a formidable challenge," said first author Yinming Shao, assistant professor of physics at Penn State, explaining that such materials are typically crystals that are only one atom thick and can be applied in a variety of fashions, including for flexible electronics, energy storage and quantum technologies. "Realization, understanding and control of nanoscale confinement are, thus, crucial for both exploration of quantum physics and future quantum technologies."

The team examined quasiparticles known as excitons, which have unique optical properties and can carry energy without an electrical charge, in a semiconductor material. Semiconductors -- which are ubiquitous across computers, phones and other electronics -- conduct electricity under certain conditions and inhibit it under others. Excitons are produced when light hits a semiconductor, energizing an electron to jump to the next energy level. The resulting excited electron and the hole it left are jointly referred to as an exciton. Excitons occur homogenously across typical 3D semiconductors, like silicon.

"But the binding energy for the excitons in bulk materials like silicon is usually small, meaning it's not very stable and it's not easy to observe," Shao said, explaining that excitons are most stable and exhibit superior properties only in 2D monolayers.

The conventional method for preparing 2D materials was developed in 2004 and led to the discovery of graphene, the single layer of carbon that is highly conductive and stronger than steel. The process is simple, but labor intensive, as each layer must be exfoliated from a bulk crystal by applying a piece of sticky tape and peeling it off.

In this thin, 2D state, excitons can carry energy without charge, as well as emit light when its electron and hole recombine, which Shao said is useful for advanced optical applications. To preserve those properties in materials large enough for such applications, however, researchers would need to produce a huge number of layers.

To do this without peeling and stacking each layer by hand, the researchers turned to another aspect of physics: magnetism. Specifically, they focused on chromium sulfide bromide (CrSBr), a layered magnetic semiconductor that co-author Xavier Roy, professor of chemistry at Columbia University, has researched extensively and further developed since 2020.

At room temperature, CrSBr acts as a normal semiconductor just like silicon. Cooling CrSBr down, to around -223 degrees Fahrenheit, brings it to a ground state, or the state of lowest energy. This transforms it into an antiferromagnetic system, in which the magnetic moments -- usually referred to as "spin" -- of the system's particles align in a regular, repeating pattern. Specifically for CrSBr, this antiferromagnetic ordering ensures that each layer alternates its magnetic alignment, effectively canceling out a magnetic moment and rendering the material insensitive to external magnetic forces. As a result, excitons tend to stay in the layer with the same spin, rather than hooping to the neighboring layers with the opposite spins. Like cars on alternating one-way streets, these established boundaries keep excitons confined to the layer with which they share the same spin directions.

"This is an effective approach to create a single layer of atomic material without exfoliating it out, while still preserving a sharp interface," Shao said. "This means we could achieve the same behavior of confined excitons demonstrated in 2D materials in a bulk material."

Using optical spectroscopy techniques, theoretical modeling and calculation, the researchers determined that this magnetic confinement held firm no matter how many layers were in the system and no matter which layer they confined, including surface layers.

"We did a lot of work to check that this actually holds, and it does," Shao said.

Shao's team's finding was corroborated by another research group out of Germany -- Florian Dirnberger and Alexey Chernikov from TUD Dresden University of Technology -- who were investigating the same quirk of magnetic semiconductors. The two groups decided to compare notes, Shao said, and found that they all had come to the same conclusion.

"Our data lines up really well, which is remarkable because we used two different crystal materials in different labs," Shao said. "Our results are in agreement with each other and align well with theoretical predictions, so we wrote this joint paper."

The aligned result came from harnessing the behaviors of magnetism, Van der Waals interactions and excitons, according to Shao, to achieve quantum confinement with potential applications for advancing optical systems and quantum technologies.

"The marriage of these different aspects of physics was a crucial aspect of this discovery," Shao said.

Shao completed his doctorate and a postdoctoral fellowship at Columbia University. Other contributors are Siyuan Qiu, Evan J. Telford, Brian S.Y. Kim, Francesco L. Ruta, Andrew J. Mills, Daniel G. Chica, Avalon H. Dismukes, Michael E. Ziebel, Yiping Wang, Jeongheon Choe, Youn Jue Bae, Xiaoyang Zhu, Xavier Roy and D. N. Basov, Columbia University; Florian Dirnberger, Sophia Terres and Alexey Chernikov, TUD Dresden University of Technology, Germany; Swagata Acharya and Rupert Huber, National Renewable Energy Laboratory, United States; Dimitar Pashov, King's College London, United Kingdom; Mikhail I. Katsnelson, Radboud University, Netherlands; Kseniia Mosina and Mark van Schilfgaarde, University of Chemistry and Technology Prague, Czech Republic; and Zdenek Sofer, University of Regensburg, Germany. Dirnberger is also affiliated with the Technical University of Munich. Kim is also affiliated with the University of Arizona. Mills is also affiliated with the Flatiron Institute. A full list of the authors and their affiliations may be found in the paper.

The U.S. Department of Energy, the European Research Council, the U.S National Science Foundation, the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter and the Emmy Noether Program supported this work.