MIT physicists have discovered that a ‘magic angle’ of three-layer 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 could help design more powerful MRIs or powerful quantum computers.

Physicists at the Massachusetts Institute of Technology have noticed signs of a rare type of superconductivity in a material called three-layer graphene that is twisted at a magic angle. In a study published in nature, the 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 withstand if it were a conventional superconductor.

The results strongly suggest that three-layer magic-angle graphene, which was originally discovered by the same group, is a very rare type of superconductor, known as “spin triplet”, impermeable 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 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, MRI machines could operate under higher magnetic fields to produce clearer, deeper images of the human body.

New evidence for spin-tripping superconductivity in three-layer graphene could also help scientists design more robust 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 these lessons we can try to design principles for other materials that are easier to fabricate, which might give you better superconductivity,” says Pablo Jarillo Herrero, Professor Physicists Cecil and Ida Green at the Massachusetts Institute of Technology.

Co-authors on his paper include postdoctoral fellow Yuan Kao and graduate student Jeong Min Park at MIT, as well as Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan.

strange change

Superconducting materials are defined by their ultra-efficient ability to conduct electricity without wasting energy. When exposed to an electric current, the electrons in a superconductor form into “cooper pairs” which then pass 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 happily pass through a superconductor, except under high magnetic fields, which can move the energy of each electron in opposite directions, separating the pair. In this way, and through mechanisms, high magnetic fields can disrupt superconductivity in conventional single-spin superconductors.

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

But there are a few strange superconductors that are unaffected by magnetic fields, of very large forces. These materials are superconducting with pairs of electrons having the same spin – a property known as “triple spin”. When exposed to high magnetic fields, the energy of two Cooper pair electrons move in the same direction, so that they are not separated but continue to have superconductivity without experiencing disturbance, regardless of the strength of the magnetic field.

Jarillo-Herrero’s group was curious to see if graphene with its magical three-corners might contain signs of this more unusual triple-spin superconductivity. The team has done pioneering work studying moiré structures of graphene — layers of thin atomic carbon networks that, when stacked at specific angles, can lead to surprising electronic behaviors.

The researchers initially reported such strange properties in two slanted sheets of graphene, which they called Magic Angle Bilayer Graphene. They soon followed tests of tri-layer graphene, a sandwich-formation of three sheets of graphene found 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 three-layer graphene is capable of superconducting at field strengths that would destroy the superconductivity in bilayer graphene.

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

Fantastic comeback

In their new study, the physicists tested the superconductivity of three layers of graphene under increasingly high magnetic fields. They made the material by exfoliating very thin layers of carbon from a block of graphite, stacking three layers together, and rotating the middle layer 1.56 degrees from the outer layers. They attached an electrode to each end of the material to pass a current and measure any energy lost in the process. Then they ignited a large magnet in the lab, with a field oriented parallel to the material.

As they increased the magnetic field around the three-layer graphene, they noticed that the superconductivity remained fairly strong before disappearing, but then reappeared strangely at higher field strengths — a return that is very unusual and does not occur in conventional single-spin superconductors.

“In single-spin superconductors, if you kill the superconductivity, it never comes back — it’s gone forever,” Kao explains. “Here he appeared again. So this clearly says that this article 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 bottom singular, according to the Pauli limit, a theory that predicts the maximum magnetic field in which a material can maintain 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, possibly triangular, species harboring Cooper pairs that rapidly traverse materials, impervious to high magnetic fields. The team plans to dig deeper into the material to confirm its precise spin state, which could help design more powerful MRIs, as well as more powerful quantum computers.

“Classical 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 which, if made into any material, could [enable] A quantum computer where the states responsible for the computation are very powerful. It will give infinitely more power to do computing. The key to making this happen would be the spin of triplex superconductors, of some sort. We have no idea if our guy is that guy. But even if this were not the case, three-layer graphene could be easy to link with other materials to design 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, nature.
DOI: 10.1038 / s41586-021-03685-y

This research was funded by the US 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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