Physicists have discovered new ways to make strange metals

Sketch of the electron interaction inside a quantum material that produces a strange metal. Image credit: Paul Neves

Physicists at the Massachusetts Institute of Technology and their colleagues have accidentally discovered a new way to make a state of matter known as a strange metal by tinkering with a quantum material characterized by atoms arranged in the shape of a sheriff's star. Strange metals are of interest due to their unusual physical properties, as they have been found in high-temperature superconductors, which are key to a variety of applications.

This work introduces an entirely new approach to the creation and study of strange metals with electrons that behave differently than those found in traditional metals such as copper. "This is a potentially new way to design these unusual materials," said Joseph G. Checkelsky, the study's principal investigator and associate professor of physics.

Linda Ye, Ph.D., 21, of the Massachusetts Institute of Technology, was the first author of a paper on the work published earlier this year in Nature Physics. "A new way to make strange metals will help us develop a unified theory behind its actions. To date, this has been very challenging and could lead to a better understanding of other materials, including high-temperature superconductors," said Ye, who is now an assistant professor at Caltech.

The Nature Physics paper was accompanied by a news and opinion piece titled "A Strange Metal Gets a Strange Metal."

In 2018, Checkelsky and many of the same colleagues reported on a class of quantum materials known as kagome metals. The members of the Kagome metal family are made up of atomic layers arranged in a repeating cell lattice that resembles the Star of David or the sheriff's coat of arms. This pattern is also common in Japanese culture, especially as a basket weaving pattern.

"We were interested in the Kagome lattice because the theory suggests that it should provide all sorts of interesting characteristics for the electrons sitting on it," Linda Ye said. In fact, in their 2018 paper, Ye, Checkelsky, and colleagues including Riccardo Comin and Liang Fu (also in MIT's physics department) reported that their new Kagome metal produced Dirac fermions, nearly massless particles similar to light-carrying photons.

"In that case, Dirac fermions were more or less expected in the calculations," Ye said. But the strange metal found in the current work is completely unexpected, "and it really takes us to a new regime," she said.

After discovering Dirac fermions, the researchers wanted to see if they could find "a more interesting feature called a flat band" in Kagome's lattice," Ye said. It is a phenomenon in which electrons are essentially stationary, although each electron still rotates around its own axis.

Keeping electrons still allows them to really talk to each other. That's when all the really interesting things in condensed matter physics happen.

Look for flat bands

More specifically, the team is looking for a flat zone at the Fermi level, which can be seen as the surface of the ocean. They discovered it and began to explore the electrical properties of the system under high voltage and magnetic fields.

They found that the electrons in the flat band strongly interacted with other electrons in the system. Ye said that the results can again be compared to the ocean. Undisturbed electrons in a flat band can be considered a calm sea. Once they begin to interact with the people around them, the calm surface of the sea turns into a raging storm, with electrons working in two different ways. The result: a strange metal.

"We knew the flat belt would bring something interesting, but we didn't know exactly what it would give us. What we found was a strange metal," Ye said.

She noted that this work showed that the Kagome lattice was "a very important design principle for the new electronic state." As a result, she now aims to extend this work to other lattices.

This discovery is the result of years of research. Ye himself began exploring the Kagome system around 2015. "It's a long project," she said. "It's very rewarding to build it up step by step and discover a lot of interesting things along the way. ”

Ye 和 Checkelsky 的麻省理工学院合著者是麻省理工学院物理学博士后助理 Shiang Fang; Mingu Kang,麻省理工学院博士,23 岁,现就职于康奈尔大学; 访问本科生Yonghun Lee; Caolan John 和 Paul M. Neves,麻省理工学院物理学研究生;S.Y. Frank Zhao,麻省理工学院物理学博士后助理; 里卡多·科明(Riccardo Comin),1947届职业发展物理学副教授。

其他作者是维也纳理工大学和莱布尼茨固态与材料研究所的约瑟夫·考夫曼(Josef Kaufmann); 劳伦斯伯克利国家实验室的 Jonathan Denlinger、Chris Jozwiak、Aaron Bostwick 和 Eli Rotenberg; 哈佛大学的 Efthimios Kaxiras 和 David C. Bell; 以及莱布尼茨固态与材料研究所的奥列格·詹森(Oleg Janson)。

更多信息：Linda Ye 等人，戈薇金属中的跳跃挫折诱导的平带和奇怪的金属性，Nature Physics （2024）。 DOI： 10.1038/s41567-023-02360-5

William R. Meier, "一种奇怪的金获取奇异metal的方法", Nature Physics (2024). DOI： 10.1038/s41567-024-02416-0

期刊信息： Nature Physics