Scientists “See” Spinning Quasiparticles in a 2D Magnet

Magnons are spinning quasiparticles found in every magnet. All magnets, whether the basic keepsakes you hang on your refrigerator and the discs that give your computer memory storage and the potent models used in research facilities, are subject to this. Spin waves are produced when one magnon spins in a way that influences its neighbor, who in turn affects their neighbor's spin, and so on. Magnons can act as "quantum interconnects" that "glue" quantum bits together into powerful computers, allowing spin waves to potentially carry information more effectively than electricity.

Despite the great potential of magnons, it is frequently challenging to find them without large pieces of laboratory apparatus. Such configurations, according to Columbia researcher Xiaoyang Zhu, are suitable for carrying out tests but not for creating devices like magnonic devices and so-called spintronics. The proper material, chromium sulfide bromide (CrSBr), a magnetic semiconductor that can be peeled into atom-thin, 2D layers, was created in the lab of Xavier Roy, a professor in the Department of Chemistry, and makes it much easier to view magnons.

A recent study by Zhu and associates from Columbia, the University of Washington, New York University, and Oak Ridge National Laboratory demonstrates that magnons in CrSBr can pair up with an exciton, a different quasiparticle that emits light, providing the researchers with a way to "see" the spinning quasiparticle.

They detected oscillations from the excitons in the near-infrared range, which is almost visible to the human eye, as they agitated the magnons with light. Zhu stated that "for the first time, we can view magnons with a straightforward optical effect."

Youn Jun (Eunice) Bae, the lead author and a postdoc in Zhu's lab, suggested that the findings could be interpreted as quantum transduction, or the transfer of one "quanta" of energy to another. Considering that excitons have an energy that is four orders of magnitude more than that of magnons, Bae said that because of how strongly they pair with one another, it is now possible to see even the smallest changes in the magnons. Researchers may one day be able to construct quantum information networks using this transduction that can take information from spin-based quantum bits, which typically need to be placed close together (within millimeters), and convert it to light, an energy that can transmit information up to hundreds of miles via optical fibers.

According to Zhu, the coherence time—the maximum duration of the oscillations—was also exceptional, exceeding the experiment's five-nanosecond time limit by a significant amount. The prospect of creating nano-scale spintronic devices has increased because the phenomenon could spread over seven micrometers and last even when the CrSBr devices were only composed of two atom-thin layers. These gadgets might someday replace today's technology as more effective options. No particles are truly moving in a spin wave, unlike the traveling electrons in an electrical current that hit resistance.

The researchers will then investigate additional material candidates and the quantum information potential of CrSBr. The quantum properties of various 2D materials that can be stacked like papers to produce a variety of novel physical phenomena are being investigated in the MRSEC and EFRC, according to Zhu.

For instance, if additional varieties of magnetic semiconductors with marginally different characteristics from CrSBr exhibit magnon-exciton coupling, they may emit light in a wider spectrum of colors. To build new devices with programmable features, "We're putting the toolbox together," Zhu added.

The work was funded by the DOE-funded Energy Frontier Research Center, with funding from Columbia's Materials Research Science and Engineering Center (MRSEC), which is housed under the National Science Foundation (EFRC).

By COLUMBIA UNIVERSITY