Scientists see spins in a 2D magnet


All magnets contain spinning quasiparticles known as magnons, from the modest keepsakes that hang on your refrigerator to the memory-giving discs in your computer to the potent models employed in research facilities. Spin waves are produced when the spin of one magnon affects the spin of its neighbour, which affects the spin of its neighbour, and so on. Spin waves may be able to carry information more effectively than electricity, and they may act as "quantum interconnects" that "glue" together quantum bits to create powerful computers.

Magnons are extremely powerful, yet it might be challenging to find them without large pieces of laboratory equipment. According to Columbia professor Xiaoyang Zhu, such setups are good for carrying out tests but not for creating gadgets like magnetic devices and so-called spintronics. The correct substance, however, can make it much easier to see magnons. This substance is chromium sulphide bromide (CrSBr), a magnetic semiconductor that can be peeled into atom-thin, 2D layers and was created in professor Xavier Roy's group in the Department of Chemistry.

Zhu and colleagues from Columbia, the University of Washington, New York University, and Oak Ridge National Laboratory demonstrate in a recent article published in Nature that magnons in CrSBr can pair up with an exciton, a different quasiparticle that emits light and gives researchers a way to "see" the spinning quasiparticle.

They noticed oscillations from the excitons in the almost visible near-infrared spectrum as they agitated the magnons with light. For the first time, Zhu claimed, "we can observe magnons with a straightforward optical effect."

According to lead author Youn Jun (Eunice) Bae, a postdoc in Zhu's lab, the findings could be interpreted as quantum transduction, or the transfer of one "quanta" of energy to another. Since excitons have an energy that is four orders of magnitude more than that of magnons, Bae said, "we can now easily see minute changes in the magnons." The ability to transduce information from spin-based quantum bits, which typically need to be placed within millimetres of one another, to light, a form of energy that can transmit information up to hundreds of miles via optical fibres, may one day allow researchers to build quantum information networks.

The coherence time, which measures the maximum duration of oscillations, was likewise exceptional, according to Zhu, lasting far longer than the experiment's five-nanosecond limit. Even when the CrSBr devices were composed of only two atom-thin layers, the phenomena could extend over seven micrometres and continue, increasing the prospect of creating spintronic devices at the nanoscale. These gadgets might replace today's technology in the future as more effective substitutes. No particles are actually moving in a spin wave, in contrast to how electrons in an electrical current encounter resistance as they move.

The material was developed at the DOE-sponsored Energy Frontier Research Center, with support from Columbia's Materials Research Science and Engineering Center (MRSEC), which is financed by NSF (EFRC). The next step for the researchers is to investigate the quantum information potential of additional material candidates as well as CrSBr. We are investigating the quantum characteristics of numerous 2D materials that can be stacked like papers to produce a variety of novel physical phenomena in the MRSEC and EFRC, according to Zhu.

For instance, other types of magnetic semiconductors with somewhat different characteristics than CrSBr might emit light in a wider spectrum of colours if magnon-exciton coupling is present in them. We're putting together the tools to build new devices with programmable features, Zhu added.

Reference: nature

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