MIT’s pioneering advances in topology

State-of-the-art X-ray and neutron spectroscopies reveal that the existence of topological singularities in the topological material crystal stabilizes magnetism above the classical transition temperature. Credit: Ella Maru Studio

MIT researchers show how topology helps create magnetism at high temperatures.

Researchers who have worked for years to understand the electron arrangement or topology and magnetism in some semimetals have been frustrated by the fact that the materials only show magnetic properties when they are cooled above a few degrees. Absolute zero.

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

Open Access Research, recently published in the journal Natural communicationIt shows evidence for the first time that topography can stabilize magnetic order, even 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 full of records, which represent magnetic moments in matter,” says Drucker, who served as first author of the paper. „For a magnet to work, the records must all point in the same direction or have a certain shape to them. But at high temperatures, the magnetic moments are all in different directions, and the magnet breaks apart, like the records are in a river.

„But what’s important about this study is that it actually changes water,” he continues. „What we’ve shown is that if you change the properties of the water rather than the logs, you can change how the logs interact with each other, resulting in magnetism.”

The role of topology in enhanced magnetism

In essence, Li says, the topological structures known as Weyl nodes found in CeAlGe—an exotic semimetal composed of cerium, aluminum, and germanium—can significantly increase the operating temperature for magnetic devices, opening wide doors. Range of applications.

While they are already being used to make 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 to maintain magnetism at significantly higher temperatures, the study opens the door to even more possibilities, Nguyen says.

„There’s a lot of potential that people have demonstrated — in this material and other topological materials,” he says. „This shows a general way in which the working temperature for these materials can be significantly improved,” says Sirivipoon.

That „very surprising and counterintuitive” result could have significant implications for future work on topological materials, says Linda Yeh, assistant professor of physics in Caltech’s Department of Physics, Mathematics and Astronomy.

„The finding by Drucker and collaborators is intriguing and important,” says Yeh, who was not involved in the research. „Their work suggests that not only do electronic topological nodes play a role in stabilizing stable magnetic orders, but they may also play a much broader role in the generation of magnetic fluctuations. A natural implication from this is that the effects of topological states on materials may extend much further than previously believed.”

Princeton University Physics professor Andrey Pernevik agrees, calling the findings „puzzling and remarkable.”

„Wells nodes are known to be topologically conserved, but the influence of this conservation on the thermodynamic properties of a phase is not well understood,” says Andriy Pernevik, who was not involved in the work. „The MIT team’s paper shows that, above the ordering temperature, the Weyl fermions appearing in this system are governed by a nested wave vector … suggesting that the shielding of the Weyl nodes somehow affects the magnetic fluctuations!”

Unraveling the magnetic mystery

The surprising results challenge long-held understandings of magnetism and topology, Li says, and are the result of careful experimentation and the team’s desire to explore areas that would otherwise be overlooked.

„The assumption was that there was nothing new above the magnetic transition temperature,” explains Li. „We used five different experimental approaches and were able to build this comprehensive story in a consistent way and put this puzzle together.”

To prove magnetism at high temperatures, the researchers combined cerium, aluminum, and germanium in a furnace and began forming millimeter-sized crystals of the material.

Those samples were then subjected to battery tests, including thermal and electrical conductivity tests, each of which revealed clues to the material’s unusual magnetic behavior.

„But we also undertook some more exotic methods to test this material,” Drucker says. „We hit the material with a beam of X-rays, calibrated to the same energy level as the cerium in the material, and then measured how the beam scatters.

„Those tests have to be done at a much larger facility, at the Department of Energy’s National Laboratory,” he continues. „Ultimately, we had to do similar experiments at three different national laboratories to show that this hidden order exists, and we found strong evidence.”

Part of the challenge, says Nguyen, is that conducting such experiments on topological materials is usually very difficult and usually provides only indirect evidence.

„In this case, what we did was do a lot of experiments using different studies, and by putting them all together, it gives us a much more comprehensive story,” he says. „In this case, it was five or six different traces and a large list of instruments and measurements that came into play in this study.”

Implications and future directions

Going forward, Li says, the team plans to investigate whether the relationship between topography and magnetism can be demonstrated in other materials.

„We believe this principle is general,” he says. „So we think it could be in many materials, which is exciting because it expands our understanding of what topology can do. We know it can play a role in increasing conductivity, and now we’ve shown it can play a role in magnetism as well.”

Additional future work will also address potential applications for topological materials, including their use in thermoelectric devices that convert heat to electricity, Li says. Although such devices are already used to power small devices such as watches, they are not yet efficient enough to power cell phones or other large devices.

„We have studied many good thermoelectric materials, all of which are topological materials,” says Li. „If they can show this performance with magnetism … they open up very good thermoelectric properties. For example, it will enable them to run at high temperatures. Right now, many people only run at very low temperatures to collect waste heat. A natural consequence of this is the ability to operate at high temperatures.” .”

Developing a better understanding of topological materials

Ultimately, Drucker says, the research indicates that while topological semimetals have been studied for years, relatively little is understood about their properties.

„Our work highlights that when you look at these different scales, when you use different experiments to study some of these materials, these really important thermoelectric and electrical and magnetic properties start to emerge,” Drucker says. „So I think this gives a hint not only to how we can use these things for different applications, but also to other fundamental studies to follow up on how we can better understand these effects of temperature fluctuations.”

Reference: „Topology Stabilized Fluctuations in a Magnetic Nodal Semimetal” by Nathan C. Drucker, Thanh Nguyen, Fei Han, Pum Sirivipoon, Ji Luo, Nina Andrejevic, Jiming Zhu, Gregory Petnick, Quinn D. Nguyen, Lando K. Nguyen, Tongtong Liu, Travis J. Williams, Matthew B. Stone, Alexander I. Kolesnikov, Songxue Chi, Jaime Fernandez-Baca, Christie S. Nelson, Ahmet Alatas, Tom Hogan, Alexander A. Puretzky, Shengxi Huang, and Mingta Li. , August 25, Natural communication.
DOI: 10.1038/s41467-023-40765-1

This work was supported by grants from the US Department of Energy, Office of Science, Basic Energy Sciences; National Science Foundation (NSF) Designing materials to revolutionize and engineer our future plans; and an NSF Convergence Accelerator Award.