MIT Physicists Find ‘Magic Angle’ Trilayer Graphene May Be Rare, Magnet Resistant Superconductor

New findings could help inform the design of more powerful MRI machines or robust quantum computers.

Physicists at MIT have observed signs of a rare type of superconductivity in a material called three-layer magically twisted graphene. In a study published in Nature, the researchers report that the material exhibits superconductivity at surprisingly high magnetic fields of up to 10 Tesla, which is three times higher than what the material would have to endure if it were a conventional superconductor.

The results strongly imply that the magic-angled three-layer graphene, which was originally discovered by the same group, is a very rare type of superconductor, known as a “spin-triplet,” which is impermeable to high magnetic fields. Such exotic superconductors could dramatically improve technologies such as magnetic resonance imaging, which uses superconducting wires under a magnetic field to resonate with and image biological tissue. MRI machines are currently limited to magnetic fields of 1 to 3 Tesla. If they could be built with spin-triplet superconductors, MRIs could work under higher magnetic fields to produce sharper, deeper images of the human body.

New evidence for spin-triplet superconductivity in three-layer graphene could also help scientists design more powerful superconductors for practical quantum computing.

“The value of this experience is what it teaches us about fundamental superconductivity, about how materials can behave, so that with these lessons learned, we can try to design principles for other materials that would be easier to manufacture, which could possibly give you better superconductivity, ”says Pablo Jarillo-Herrero, Cecil and Ida Green professor of physics at MIT.

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

Strange change

Superconducting materials are defined by their super efficient ability to conduct electricity without wasting energy. When exposed to an electric current, electrons in a superconductor couple into “Cooper pairs” which then pass through the material without resistance, like passengers on an express train.

In the vast majority of superconductors, these passenger pairs have opposite spins, with one electron spinning up and the other down – a configuration known as a “spin-singlet”. 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 derail superconductivity in conventional single spin superconductors.

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

But there are a handful of exotic superconductors that are impervious to magnetic fields, up to very large forces. These materials are superconducting through pairs of electrons having the same spin, a property known as a “spin-triplet”. When exposed to high magnetic fields, the energy of the two electrons of a Cooper pair moves in the same direction, so that they are not separated but continue to be superconducting without being disturbed, regardless of the strength of the magnetic field.

Jarillo-Herrero’s group were curious whether the magic-angled three-layer graphene might contain signs of this more unusual spin-triplet superconductivity. The team produced pioneering work in the study of moire structures of graphene – layers of thin atomic carbon networks that, when stacked at specific angles, can give rise to surprising electronic behaviors.

The researchers initially reported such curious properties in two tilted graphene sheets, which they dubbed Magic Angle Bilayer Graphene. They quickly followed up with tests of three-layer graphene, a sandwich configuration of three sheets of graphene that was found to be even stronger than its bilayer counterpart, retaining 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 the bilayer graphene.

“We thought it was something very strange,” said Jarillo-Herrero.

A great comeback

In their new study, the physicists tested the superconductivity of three-layer graphene under increasingly high magnetic fields. They made the material by peeling thin layers of carbon from a block of graphite, stacking three layers together, and rotating the middle one 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 that they oriented parallel to the material.

As they increased the magnetic field around the three-layer graphene, they observed that the superconductivity remained strong up to a point before disappearing, but then reappeared oddly 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 for good,” Cao explains. “Here he reappeared. So that clearly says that this material is not a spin-singlet.

They also observed that after “reentry”, superconductivity persisted up to 10 Tesla, the maximum field strength that the laboratory magnet could produce. This is about three times higher than what the superconductor would have to withstand if it were a conventional spin singlet, according to the Pauli limit, a theory that predicts the maximum magnetic field at which a material can retain superconductivity. .

The reappearance of the superconductivity of three-layer graphene, coupled with its persistence at higher than expected magnetic fields, rules out the possibility that the material is an ordinary superconductor. Instead, it’s probably a very rare type, possibly a spin-triplet, harboring Cooper pairs that quickly traverse the material, impervious to high magnetic fields. The team plans to dig deeper into the material to confirm its exact spin state, which could help inform the design of more powerful MRI machines, as well as more robust quantum computers.

“Classical quantum computing is extremely fragile,” Jarillo-Herrero says. “You look at him and, poof, he disappears. About 20 years ago, theorists proposed a type of topological superconductivity which, if made in any material, could [enable] a quantum computer where the states responsible for the computation are very robust. It would give infinitely more power to do computing. The key ingredient to achieving this would be triplet spin superconductors, of some type. We have no idea if our guy is that guy. But even if this is not the case, it could facilitate the association of three-layer graphene with other materials to design this type of superconductivity. It could be a major breakthrough. But it is still very early.

Reference: “Pauli-limit violation and re-entrant superconductivity in moiré graphene” by Yuan Cao, 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 Fundacion Ramon Areces, and the CIFAR Quantum Materials Program.