Paradox Reveals Quantum Geometry Wizardry in Superconductivity’s “Magic Angle”

Physicists at Ohio State University have learned more about graphene’s potential to be a superconductor of electricity.

Scientists identify quantum geometry as crucial to the process.

Scientists have produced new evidence of how the graphwhen twisted to a precise angle, can become a superconductor that moves electricity without loss of energy.

In a study published on February 15, 2023 in the journal Naturethe team led by physicists at Ohio State University reported their discovery of the key role that quantum geometry plays in allowing this twisted graphene to become a superconductor.

Graphene is a single layer of carbon atoms, lead, found in a pencil.

In 2018, researchers at the Massachusetts Institute of Technology discovered that under the right conditions, graphene could become a superconductor if one piece of graphene was placed on top of another piece and the layers were twisted to a specific angle – 1.08 degrees – creating the twist bilayer graphene.

Ever since, researchers have been studying this twisted bilayer graphene and trying to figure out how this “magic angle” works, said Marc Bockrath, professor of physics at Ohio State and co-author of Nature paper.

“The conventional theory of superconductivity doesn’t work in this situation,” Bockrath said. “We did a series of experiments to understand the origin of why this material is a superconductor.”

In a conventional metal, high speed electrons are responsible for the conductivity.

But twisted bilayer graphene has a type of electronic structure known as a “flat band,” where the electrons move very slowly—in fact, at a speed approaching zero if the angle is exactly at the magic one.

Under the conventional theory of superconductivity, slow-moving electrons should not be able to conduct electricity, said study co-author Jeanie Lau, also a professor of physics at Ohio State.

With great precision, Haidong Tian, ​​first author of the paper and a student in Lau’s research group, was able to obtain a device so close to the magic angle that the electrons were almost stopped by usual physical standards of condensed matter. The sample nevertheless showed superconductivity.

“It’s a paradox: How can electrons that move so slowly even conduct electricity, let alone superconduct? It is very remarkable,” said Lau.

In their experiments, the research team demonstrated the slow speeds of electrons and provided more precise measurements of electron motion than had previously been available.

And they also found the first clues to what makes this graphene material so special.

“We can’t use the speed of the electrons to explain how the twisted bilayer graphene works,” Bockrath said. “Instead, we had to use quantum geometry.”

As with all things quantum, quantum geometry is complex and not intuitive. But the results of this study are connected to the fact that an electron is not only a particle, but also a wave – and thus has wave functions.

“The geometry of the quantum wave functions in flat bands, together with the interaction between electrons, leads to the flow of electrical current without dissipation in bilayer graphene,” said co-author Mohit Randeria, professor of physics at Ohio State.

“We found that conventional equations could explain maybe 10% of the superconducting signal we found. Our experimental measurements suggest that quantum geometry is 90% of what makes this a superconductor,” said Lau.

The superconducting effects of this material can only be found in experiments at extremely low temperatures. The ultimate goal is to be able to understand the factors that lead to high-temperature superconductivity, which will potentially be useful in real-world applications such as electrical transmission and communications, Bockrath said.

“It would have a huge impact on the community,” he said. “It’s a long way off, but this research certainly moves us forward in understanding how that could happen.”

Reference: “Proof of Dirac flatband superconductivity enabled by quantum geometry” by Haidong Tian, ​​Xueshi Gao, Yuxin Zhang, Shi Che, Tianyi Xu, Patrick Cheung, Kenji Watanabe, Takashi Taniguchi, Mohit Randeria, Fan Zhang, Chun Ning Lau and Marc W. Bockrath, 15 February 2023, Nature.
DOI: 10.1038/s41586-022-05576-2

Bockrath and Lau’s experimental groups, including graduate students Tian, ​​Xueshi Gao, Yuxin Zhang, and Shi Che, collaborated with theorists Randeria at Ohio State and Tianyi Xu, Patrick Cheung, and F. Zhang at the University of Texas at Dallas, and with researchers from the National Institute for Materials Science in Japan.

The study was supported by the Department of Energy Office of Science, the Ohio State Center for Emergent Materials, the National Science Foundation MRSEC and the Army Research Office.

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