Trilayer graphene may be magnet-proof superconductor for quantum computers

July 21, 2021 // By Nick Flaherty
Trilayer graphene may be magnet-proof superconductor for quantum computers
Researchers at MIT have found that trilayer graphene remains superconducting in magnetic fields up to 10T, opening up new designs of MRI scanners and quantum computers

Researchers in the US have observed properties in twisted trilayer graphene that could provide a new way to build MRI scanners and quantum computers.

The MIT physicists saw a rare type of superconductivity in the material in magnetic fields of 10 Tesla, three times higher than a conventional superconductor. The project is backed by the Gordon and Betty Moore Foundation set up by one of the founders of Intel, a Spanish research foundation and a Canadian quantum materials research programme.

The discovery could be used to boost the magnetic field of MRI scanners from the current 1 to 3 Tesla for higher resolution, more accurate imaging. The ability to stack layers of graphene one atom thick with different orientation is opening up a wide range of new properties, such as collecting power from neutrinos.

“The value of this experiment is what it teaches us about fundamental superconductivity, about how materials can behave, so that with those lessons learned, we can try to design principles for other materials which would be easier to manufacture, that could perhaps give you better superconductivity,” said Pablo Jarillo-Herrero, Professor of Physics at MIT.

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

When exposed to an electric current, electrons in a superconductor couple up in Cooper pairs that then travel through the material without resistance. In most superconductors, these pairs have opposite spins, or spin-singlets. However, high magnetic fields can pull these singlets apart.

When the pairs of electronics have the same spin, the electrons move in the same direction in the field and retain the superconductivity in higher field, and can also be switched on and off.

The researchers fabricated a sandwich of three layers by peeling away atom-thin layers of carbon from a block of graphite, stacking three layers together, and rotating the middle one 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 and measure any energy lost in the process. Then they turned on a large magnet in the lab, with a field which they oriented parallel to the material.

As they increased the magnetic field around trilayer graphene, they observed that superconductivity held strong up to a point before disappearing, but then curiously reappeared at higher field strengths — a property that is highly unusual and not known to occur in conventional spin-singlet superconductors.

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“In spin-singlet superconductors, if you kill superconductivity, it never comes back — it’s gone for good,” said researcher Yuan Cao. “Here, it reappeared again. So this definitely says this material is not spin-singlet.”

They also observed that superconductivity persisted up to 10 Tesla, the maximum field strength that the lab’s magnet could produce. This is about three times higher than what the superconductor should withstand if it were a conventional spin-singlet, according to Pauli’s limit.

The team plans to drill down on the material to confirm its exact spin state, which could help to inform the design of more powerful MRI machines, and also more robust quantum computers.

“Regular quantum computing is super fragile,” said Jarillo-Herrero. “You look at it and, poof, it disappears. About 20 years ago, theorists proposed a type of topological superconductivity that, if realized in any material, could [enable] a quantum computer where states responsible for computation are very robust. That would give infinite more power to do computing. The key ingredient to realize that would be spin-triplet superconductors, of a certain type. We have no idea if our type is of that type. But even if it’s not, this could make it easier to put trilayer graphene with other materials to engineer that kind of superconductivity. That could be a major breakthrough. But it’s still super early.”

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

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