Physicists at the Massachusetts Institute of Technology have noticed signs of a rare type of superconductivity in a material called the “magic angle” of bent three-layer graphene. Credits: Courtesy of Pablo Jarillo-Herrero, Yuan Cao, Jeong Min Park, et al
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The new findings could 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 twisted magic three-layer graphene. In a study appearing in natureThe researchers report that the material exhibits superconductivity in extremely high magnetic fields of up to 10 Tesla, which is three times higher than the material would be expected to survive if it were a conventional superconductor.
The results strongly suggest that the magic three-layer graphene, originally discovered by the same group, is a very rare type of superconductor, known as a “spin triplet”, resistant 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 tissue and image it. Current MRI machines are limited to magnetic fields from 1 to 3 Tesla. If they could be constructed using a three-turn superconductor, MRIs could operate under higher magnetic fields to produce clearer, deeper images of the human body.
New evidence for triple-turn superconductivity in triple-layered graphene could also help scientists design more powerful superconductors for practical quantum computing.
“The value of this experiment is what it teaches you about basic superconductivity, and how materials can behave, so with this lesson we can try to design principles for other materials that are easier to make, and maybe that will give you better superconductivity,” said Pablo. . Jarillo-Herrero, Cecil and Ida Green Professor of Physics at the Massachusetts Institute of Technology.
Co-authors of the paper include Yuan Kao and graduate students Jeong Min Park at the Massachusetts Institute of Technology, Kenji Watanabe and Takashi Taniguchi from the National Institute of Materials Science in Japan.
strange transformation
Superconducting materials are defined by their highly efficient ability to conduct electricity without losing energy. When exposed to an electric current, the electrons in superconductors are paired in “copper pairs” which then travel through the material unimpeded, like passengers on a fast train.
In most superconductors, this pair of passengers has opposite spins, with one electron spinning up and the other down — a configuration known as a “single spin”. The pair is accelerated by superconductors, except for the high magnetic field, which can shift the energy of each electron in the opposite direction, separating the pair from each other. In this way, and through the mechanism, the high magnetic field can interfere with the superconductivity in conventional spin superconductors.
“This is the main reason why superconductivity dissipates in sufficiently large magnetic fields,” Park said.
But there are some strange superconductors that are not affected by magnetic fields, despite their enormous strength. These materials are superconducting by means of pairs of electrons having the same spin – a property known as “triple spin”. When exposed to a high magnetic field, the energies of the two electrons in the Cooper pair shift in the same direction, so that they do not separate from each other but continue to be superconductors without interference, regardless of the strength of the magnetic field.
The Jarillo-Herrero group wanted to know if triple-layered magic angle graphene could provide clues to the unusual three-turn superconductivity. The team has produced groundbreaking work studying the structure of graphene moiré – atomic thin layers of a carbon lattice that, when stacked at certain angles, can cause surprising electronic behavior.
The researchers initially reported such strange properties in two angular graphene sheets, which they called magic bilayer graphene. They immediately followed tests of tri-layer graphene, a sandwich formation of three graphene sheets that was found to be stronger than its two-layer counterpart, while maintaining its superconductivity at higher temperatures. When the researchers applied a simple magnetic field, they noticed that the three-layer graphene was capable of superconducting at a field strength that would destroy the superconductivity in bilayer graphene.
“We thought this was a very strange thing,” said Jarilo Herrero.
magic return
In their new study, the physicists tested the superconductivity of three-layer graphene under increasingly higher magnetic fields. They made the material by peeling a thin layer of carbon from a graphite block, stacking the three layers together, and rotating the middle layer by 1.56 degrees with respect to the outer layer. They attached electrodes to both ends of the material to flow current through it and measured the energy lost in the process. Then they turned on a large magnet in the lab, with the field they directed parallel to the material.
When they increased the magnetic field around the three-layer graphene, they noticed that the superconductivity persisted quite strongly before disappearing, but then reappears interestingly at higher field strengths – a very unusual awakening not known to occur in conventional superconductors.
“In a single-turn superconductor, if you kill the superconductivity, it never comes back — it’s gone forever,” Kao said. “Here, he reappeared. So this clearly shows that this material is not piecemeal.”
They also note that after “re-entry”, the superconductivity lasts up to 10 Tesla, the maximum field strength a laboratory magnet can produce. This is about three times higher than what a superconductor would have to withstand if it were a conventional single spin, according to the Pauli limit, a theory that predicts the maximum magnetic field over which a material can maintain superconductivity.
The emergence of superconductivity of three-layer graphene, combined with its higher-than-expected stability in magnetic fields, rules out the possibility that the material is an ordinary superconductor. Instead, it’s more likely that this extremely rare species, possibly a triplet, harbored a pair of Coopers traveling through the material, impermeable to high magnetic fields. The team plans to drill through the material to confirm the precise spin state, which could help design more powerful MRIs, as well as more powerful quantum computers.
“Ordinary quantum computing is very fragile,” said Jarillo Herrero. “You look at it and it disappears faggot. About 20 years ago, theorists proposed a type of topological superconductivity that, if achieved in any material, could be [enable] A quantum computer in which the state responsible for computing is very strong. This will give more and more unlimited power to perform calculations. The key element to look out for is the triple spin superconductor, of a certain kind. We don’t know if our species is like that. But even if this were not the case, it could facilitate the placement of three-layer graphene with other materials to engineer this type of superconductivity. This can be a great hack. But it’s still too early.”
Reference: “Breach of the Pauli boundary and re-entry of superconductivity into ripple graphene” By Yuan Kao, Park Jeong Min, Kenji Watanabe, Takashi Taniguchi, and Pablo Jarillo-Herrero, July 21, 2021, 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 Arises Foundation, and the Sevare Quantum Materials Program.
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