MIT physicists have discovered that “magic angle” three-layer graphene may be a superconductor that is resistant to rare earth magnets.

MIT physicists have observed a rare type of superconducting sign in a material called “magic angle” twisted three-layer graphene.Credits: Courtesy of Pablo Jarillo-Herrero, Yuan Cao, Jeong Min Park, etc.

New discoveries can help inform the design of more powerful MRI machines or robust quantum computers.

MIT physicists have observed a rare type of superconducting sign in a material called magic angle twisted three-layer graphene.In the research that appears in NatureResearchers report that the material exhibits superconductivity in a surprisingly high magnetic field of up to 10 Tesla. This is three times what the material would expect to withstand in the case of traditional superconductors.

This result strongly suggests that the magic-angled three-layer graphene first discovered by the same group is a very rare type of superconductor known as the “spin triplet” that is unaffected by high magnetic fields. doing. Such exotic superconductors have the potential to significantly improve techniques such as magnetic resonance imaging, which uses superconducting wires to resonate and image living tissue under magnetic fields. MRI equipment is currently limited to a magnetic field of 1-3 Tesla. If they can be constructed with spin triplet superconductors, MRI can operate under higher magnetic fields to produce clearer, deeper images of the human body.

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

“The value of this experiment is that it teaches us about basic superconductivity, the behavior of materials, so we can use these lessons to make other things that are easier and perhaps better to manufacture. You can design the principles of the material, “superconductivity,” said Pablo Jarillo-Herrero, professor of physics at Ceciland Ida Green at MIT.

Co-authors of the treatise include a postdoctoral fellow, Jung Min-pak, a graduate student at MIT, and Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science.

Strange shift

Superconducting materials are defined by their super-efficient ability to conduct electricity without losing energy. When exposed to electric current, the electrons in the superconductor combine in “Cooper pairs” and pass through the material without resistance, much like an express train passenger.

In the majority of superconductors, these passenger pairs have opposite spins, with one electron rotating up and the other electron rotating down. This is a configuration known as the “spin singlet”. These pairs happily speed up through superconductors, except under high magnetic fields, which can shift the energy of each electron in opposite directions and pull them apart. Thus, and through the mechanism, high magnetic fields can upset the superconductivity of conventional spin singlet superconductors.

“That’s the ultimate reason why superconductivity disappears in a magnetic field of sufficient magnitude,” says Park.

However, there are some exotic superconductors that do not pass magnetic fields up to very high intensities. These materials are superconducting through electron pairs with the same spin. This is a property known as the “spin triplet”. When exposed to a high magnetic field, the energies of both Cooper pair electrons shift in the same direction and continue to superconduct without being separated, regardless of the strength of the magnetic field.

Jarillo-Herrero’s group was curious that the magic-angled three-layer graphene might have signs of this more unusual spin triplet superconductivity. The team has produced pioneering work in the study of graphene moiré structures. This is a thin carbon lattice layer of atoms that, when stacked at a particular angle, can cause surprising electronic behavior.

Researchers initially reported these strange properties on two angled graphene sheets and named them Magic Angle’s two-layer graphene. They immediately tested 3-layer graphene. This is a sandwich configuration of three graphene sheets, which is even stronger than two-layer graphene and maintains superconductivity even at high temperatures. When researchers applied a moderate magnetic field, they found that 3-layer graphene could superconduct at an electric field strength that would destroy the superconductivity of 2-layer graphene.

“I thought this was very strange,” says Jarillo-Herrero.

Super comeback

In their new study, physicists tested the superconductivity of three-layer graphene under ever-increasing magnetic fields. They made the material by stripping a thin layer of carbon from a block of graphite, stacking three layers, and rotating the central layer 1.56 degrees with respect to the outer layer. They attached electrodes to both ends of the material, passing an electric current and measuring the energy lost in the process. Then they turned on a large magnet in the laboratory and directed the magnetic field parallel to the material.

It was observed that when they increased the magnetic field around the three-layer graphene, it remained strong until the point before the superconductivity disappeared, but then mysteriously reappeared at higher magnetic field strengths. This is extremely rare and is known to not occur with conventional spin singlet superconductors.

“In a spin singlet superconductor, if you kill the superconductivity, it will never come back. It will disappear forever,” says Kao. “Here it reappears. Therefore, this definitely indicates that this material is not a spin singlet.”

They also observed that superconductivity lasted up to 10 Tesla after “re-entry”. This is the maximum electric field strength that a laboratory magnet can produce. This is about three times what a superconductor should withstand when it is a conventional spin singlet, according to Pauli’s limit, a theory that predicts the maximum magnetic field in which a material can hold superconductivity.

The reproduction of the superconductivity of three-layer graphene, coupled with its persistence in higher magnetic fields than expected, eliminates the possibility that the material is a common superconductor. Instead, it is probably a very rare type, probably a spin triplet, hosting a Cooper pair that passes through the material at high speeds unaffected by high magnetic fields. The team plans to drill down the material to ensure accurate spin conditions. This helps inform the design of more powerful MRI machines and more robust quantum computers.

“Normal quantum computing is very fragile,” says Jarillo-Herrero. “When you see it, you get tired of it and it disappears. About 20 years ago, theorists proposed some kind of topological superconductivity. [enable] A quantum computer that is in charge of calculations and is extremely robust. It will give infinite more power to do computing. An important factor in achieving this is a particular type of spin triplet superconductor. I don’t know if our type is that type. However, even if this is not the case, this may make it easier to place three-layer graphene along with other materials to design such superconductivity. That could be a big step forward. But it’s still super fast. “

Reference: “Pauli Exclusion Principle Violation and Reentry Superconductivity in Moire Graphene”, Yuan Cao, Jeong Min Park, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero, July 21, 2021 Nature..
DOI: 10.1038 / s41586-021-03685-y

This study was supported by the US Department of Energy, the National Science Foundation, the Gordon and Betty Moore Foundation, Fundacion Ramon Areces, and the CIFAR Quantum Materials Program. MIT physicists have discovered that “magic angle” three-layer graphene may be a superconductor that is resistant to rare earth magnets.

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