Physicists at the Massachusetts Institute of Technology (MIT) have discovered that three-layer ‘magic-angle’ graphene may be a rare, magnetic-resistant superconductor.


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 may 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 exhibits superconductivity in surprisingly high magnetic fields of up to 10 tesla, which is three times higher than what the material would be expected to withstand if it were a conventional superconductor.

The results strongly suggest that the magical three-layer graphene, which was initially discovered by the same group, is a very rare type of superconductor, known as “spin triplet”, 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 biological tissues and image them. 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 operate under higher magnetic fields to produce clearer, deeper images of the human body.

New evidence for triple spin superconductivity in triple-layered graphene could also help scientists design stronger superconductors for practical quantum computing.

“The value of this experiment is what it teaches us about basic superconductivity, and how materials can behave, so that with those lessons learned we can try to design principles for other materials that are easier to manufacture, and maybe that will give you better superconductivity,” says Pablo Jarillo Herrero, Professor Physicists Cecil and Ida Green at the Massachusetts Institute of Technology.

His co-authors on the paper include postdoctoral researcher Yuan Kao and graduate student Jeong Min Park at MIT, and Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan.

strange transformation

Superconducting materials are defined by their highly efficient ability to conduct electricity without energy loss. When exposed to an electric current, the electrons in the superconductor pair up in “cooper pairs” which 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, except for 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 the superconductivity in conventional spin superconductors.

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

But there are a handful of strange superconductors that are unaffected by magnetic fields, even very large strengths. These materials superconduct through pairs of electrons having 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, in such a way that they are not separated from each other but continue to superconduct without disturbance, regardless of the strength of the magnetic field.

Jarillo-Herrero’s group was curious about whether the three-layer magic-angle graphene might bear clues to the unusual triple-spin superconductivity. The team has produced groundbreaking work studying moiré structures of graphene — layers of atom-thin carbon lattices that, when stacked at specific angles, can lead to surprising electronic behaviors.

The researchers initially reported such peculiar properties in two angled sheets of graphene, which they called magic bilayer graphene. They soon followed tests of tri-layer graphene, a sandwich-formation of three sheets of graphene that turned out 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 capable of superconducting at field strengths that would destroy the superconductivity in bilayer graphene.

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

miraculous comeback

In their new study, the physicists tested the superconductivity of three-layer graphene under increasingly higher magnetic fields. They manufactured the material by peeling thin layers of carbon from a block of graphite, stacking three layers together, and rotating the middle layer by 1.56 degrees with respect to the outer layers. They attached an electrode to either end of the material to run a current through it and measure any energy lost in the process. Then they turned on a large magnet in the lab, with a field they directed parallel to the material.

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

“In single spin superconductors, if you kill the superconductivity, it never comes back — it’s gone forever,” Kao says. “Here, he reappears. So this definitely indicates that this material is not a single piece.”

They also noted that after “re-entry,” the superconductivity persisted up to 10 Tesla, the maximum field strength that a laboratory 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 retain superconductivity.

The appearance of triple-layer graphene superconductivity, along with its stability in higher-than-expected magnetic fields, rules out the possibility that the material is an ordinary superconductor. Instead, it is likely to be a very rare, probably triplet, species that hosts Cooper pairs that speed through the material, impermeable 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, as well as more powerful quantum computers.

“Regular quantum computing is very fragile,” says Jarillo Herrero. “You look at him and my faggot 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 power to do computing. The primary component to be aware of is the triple spin superconductors, of a certain type. We have no idea if our species is that kind. But even if this is not the case, this may facilitate the placement of three-layer graphene with other materials to engineer this type of superconductivity. It could be a great hack. But it is still too early.”

Reference: “Violation of the Pauli limit and re-entry of superconductivity into ripple graphene” By Yuan Kao, Jeong Min Park, Kenji Watanabe, Takashi Taniguchi, and Pablo Jarillo-Herrero, July 21, 2021, Available here. nature.
DOI: 10.1038 / s41586-021-03685-y

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


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