Physicists at MIT have discovered that ‘magic angle’ three-layer graphene could be a rare, magnetically-resistant superconductor.

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Physicists at the Massachusetts Institute of Technology have noticed signs of a rare type of superconductivity in a material called the “magic angle” of twisted three-layer graphene. Credit: Courtesy of Pablo Jarillo-Herrero, Yuan Cao, Jeong Min Park, et al

The new findings could help design more powerful MRI machines or powerful quantum computers.

Physicists at the Massachusetts Institute of Technology have noticed signs of a rare type of superconductivity in a material called magic-angle twisted three-layer graphene. In a study appearing in natureThe researchers report that the material shows superconductivity in surprisingly high magnetic fields of up to 10 Tesla, which is three times higher than what the material could be expected to withstand if it were a conventional superconductor.

The results strongly suggest that the magical three-layer graphene, initially discovered by the same group, is a very rare type of superconductor known as a “spin triplet,” which is impervious to high magnetic fields. Such exotic superconductors could greatly improve techniques such as magnetic resonance imaging, which uses superconducting wires under a magnetic field to resonate with and image biological tissues. MRI machines are currently limited to magnetic fields from 1 to 3 Tesla. If they could be built using triple-spin superconductors, the MRI could work under higher magnetic fields to produce clearer, deeper images of the human body.

New evidence for triple-spin superconductivity in three-layer graphene could also help scientists design stronger superconductors for practical quantum computers.

“The value of this experience is what it teaches us about basic superconductivity and how materials can behave, so with these lessons we can try to design principles for other materials that are easier to fabricate, and maybe that will give you better superconductivity. says Pablo Jarillo-Herrero, Cecil and Ida Green professor of physics at the Massachusetts Institute of Technology.

Co-authors on the paper include Yuan Kao and graduate student Jeong Min Park at the Massachusetts Institute of Technology, Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan.

strange transformation

Superconducting materials are characterized by their highly efficient ability to conduct electricity without energy loss. When exposed to an electric current, the electrons in a superconductor form “cooper pairs” that then travel through the material without resistance, like passengers on a fast train.

In the vast majority of superconductors, these passenger pairs have an opposite spin, with one electron spinning up and the other down — a configuration known as a “spin singular.” These pairs are accelerated by a superconductor, with the exception of high magnetic fields, which can shift the energy of each electron in opposite directions, separating the pair from each other. In this way, and through mechanisms, high magnetic fields can disrupt superconductivity in conventional spin superconductors.

“This is the ultimate reason why superconductivity disappears in a large enough magnetic field,” says Park.

But there are some strange superconductors that are unaffected by magnetic fields, even very strong ones. These materials superconduct through electron pairs of the same spin – a property known as “triple spin”. When exposed to high magnetic fields, the energy of both electrons in the Cooper pair shifts in the same direction, such that they are not separated from each other but remain superconducting undisturbed, regardless of the strength of the magnetic field.

Jarillo-Herrero’s group wondered if three-layer magic-angle graphene might hold clues to unusual triple-spin superconductivity. The team has done groundbreaking work studying graphene moiré structures — layers of atomically thin carbon lattices that, when stacked at specific angles, can lead to surprising electronic behavior.

The researchers initially reported such peculiar properties in two slanted sheets of graphene, which they called magic bilayer graphene. They soon followed tests of three-layer graphene, a sandwich formation of three sheets of graphene that proved to be stronger than its two-layer counterpart while retaining its superconductivity at higher temperatures. When the researchers applied a modest magnetic field, they noticed that the three-layer graphene was superconducting at field strengths that would destroy the superconductivity in double-layer graphene.

“We thought this was a very strange case,” says Jarilo Herrero.

miraculous comeback

In their new study, the physicists tested the superconductivity of three-layer graphene under increasingly higher magnetic fields. They made the material by peeling thin layers of carbon from a block of graphite, stacking three layers on top of each other and rotating the middle layer 1.56 degrees relative to the outer layers. They attached an electrode to both ends of the material to run a current through it and measure the energy lost in the process. They then turned on a large magnet in the lab, with a field that they aimed parallel to the material.

When they increased the magnetic field around the three-layer graphene, they noticed that the superconductivity held up quite strongly before disappearing, but then intriguingly reappeared at higher field strengths — a very unusual resurgence not known to occur in conventional superconductors.

“In superconductors with one spin, if you kill the superconductivity, it never comes back — it’s gone forever,” says Kao. “Here he comes out again. This certainly indicates that this material does not consist of one piece.”

They also noted that after “re-entry,” the superconductivity persisted to 10 Tesla, the maximum field strength a lab magnet could produce. This is about three times higher than what a superconductor would have to withstand if it were a conventional spin single, according to the Pauli limit, a theory that predicts the maximum magnetic field in which a material can maintain superconductivity.

The appearance of graphene’s three-layer superconductivity, combined with its stability in higher-than-expected magnetic fields, rules out the possibility that the material is an ordinary superconductor. Instead, it’s probably a very rare, potentially triangular species harboring Cooper pairs hurtling through the material, impervious to high magnetic fields. The team plans to drill into the material to confirm its precise spin state, which could help design more powerful MRIs and more powerful quantum computers.

“Regular quantum computing is very fragile,” Jarillo Herrero says. “You look at him and my fag disappears. About 20 years ago, theorists proposed a type of topological superconductivity that, if achieved in any material, could: [enable] A quantum computer where the states responsible for the computation are very powerful. This would give more infinite computing power. The most important element to look out for are the triple spin superconductors, of some type. We have no idea if our species is of that kind. But even if not, it could facilitate the placement of three-layer graphene with other materials to develop this kind of superconductivity. It could be a great hack. But it is still too early.”

Reference: “Violation of the Pauli Limit and Return of Superconductivity in Ripple Graphene” by Yuan Kao, Jeong Min Park, Kenji Watanabe, Takashi Taniguchi, and Pablo Jarillo-Herrero, Available July 21, 2021, Available here. nature.
DOI: 10.1038/s41586-021-03685-y

This research was supported by the United States Department of Energy, the National Science Foundation, the Gordon and Betty Moore Foundation, the Ramon Arises Foundation and the Sevare Quantum Materials Program.

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