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Making more magnetism possible with topology

来源机构: 麻省理工学院    发布时间:2023-10-10点击量:1

Researchers who have been working for years to understand electron arrangement, or topology, and magnetism in certain semimetals have been frustrated by the fact that the materials only display magnetic properties if they are cooled to just a few degrees above absolute zero.

A new MIT study led by Mingda Li, associate professor of nuclear science and engineering, and co-authored by Nathan Drucker, a graduate research assistant in MIT’s Quantum Measurement Group and PhD student in applied physics at Harvard University, along with Thanh Nguyen and Phum Siriviboon, MIT graduate students working in the Quantum Measurement Group, is challenging that conventional wisdom.

The open-access research, published in Nature Communications, for the first time shows evidence that topology can stabilize magnetic ordering, even well above the magnetic transition temperature — the point at which magnetism normally breaks down.

“The analogy I like to use to describe why this works is to imagine a river filled with logs, which represent the magnetic moments in the material,” says Drucker, who served as the first author of the paper. “For magnetism to work, you need all those logs pointing in the same direction, or to have a certain pattern to them. But at high temperatures, the magnetic moments are all oriented in different directions, like the logs would be in a river, and magnetism breaks down.

“But what’s important in this study is that it’s actually the water that’s changing,” he continues. “What we showed is that, if you change the properties of the water itself, rather than the logs, you can change how the logs interact with each other, which results in magnetism."

A surprising connection between topology and magnetism

In essence, Li says, the paper reveals how topological structures known as Weyl nodes found in CeAlGe — an exotic semi-metal composed of cerium, aluminum and germanium — can significantly increase the working temperature for magnetic devices, opening the door to a wide range of applications.

While they are already being used to build sensors, gyroscopes, and more, topological materials have been eyed for a wide range of additional applications, from microelectronics to thermoelectric and catalytic devices. By demonstrating a method for maintaining magnetism at significantly higher temperatures, the study opens the door to even more possibilities, Nguyen says.

“There are so many opportunities people have demonstrated — in this material and other topological materials,” he says. “What this shows is a general way that can significantly improve the working temperature for these materials,” adds Siriviboon.

That “quite surprising and counterintuitive” result will have substantial impact on future work on topological materials, adds Linda Ye, assistant professor of physics in Caltech’s Division of Physics, Mathematics and Astronomy.

“The discovery by Drucker and collaborators is intriguing and important,” says Ye, who was not involved in the research. “Their work suggests that electronic topological nodes not only play a role in stabilizing static magnetic orders, but more broadly they can be at play in the generation of magnetic fluctuations. A natural implication from this is that influences from topological Weyl states on materials can extend far beyond what was previously believed.”

Princeton University professor of physics Andrei Bernevig agrees, called the findings “puzzling and remarkable.”

“Weyls nodes are known to be topologically protected, but the influence of this protection on the thermodynamic properties of a phase is not well understood,” says Andrei Bernevig, who was not involved in the work. “The paper by the MIT group shows that short-range order, above the ordering temperature, is governed by a nesting wave vector between the Weyl fermions that appear in this system … possibly suggesting that the protection of the Weyl nodes somehow influences magnetic fluctuations!”

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