SCIENTIFIC EDUCATIONAL CENTER science idea

Physicists at the Massachusetts Institute of Technology have observed signs of a rare type of superconductivity in a material called three-layer graphene, twisted at a magical angle. The researchers report that this material demonstrates superconductivity in surprisingly high magnetic fields of up to 10 Tesla, which is three times higher than what the material is predicted to withstand if it were a conventional superconductor.
The results strongly suggest that the three-layer graphene with a magic angle, which was originally discovered by the same group, is a very rare type of superconductor known as a "spin-triplet", which is immune to strong magnetic fields.
Such exotic superconductors can significantly improve technologies such as magnetic resonance imaging, in which superconducting wires are used in a magnetic field to resonance with biological tissue and obtain an image. MRI machines are currently limited to magnetic fields from 1 to 3 Tesla. If they could be constructed from spin-triplet superconductors, MRI could work in stronger magnetic fields to get clearer and deeper images of the human body.
New evidence of spin-triplet superconductivity in three-layer graphene may also help scientists develop stronger superconductors for practical quantum computing.
Strange shift
Superconducting materials are distinguished by their super-efficient ability to conduct electricity without loss of energy. Under the influence of an electric current, electrons in a superconductor combine into "Cooper pairs", which then pass through the material without resistance.
In the vast majority of superconductors, these pairs have opposite spins: one electron rotates up and the other rotates down – a configuration known as a "spin singlet".
These pairs successfully pass through the superconductor, except for strong magnetic fields that can shift the energy of each electron in opposite directions, breaking the pair. Thus, strong magnetic fields can disrupt superconductivity in conventional spin-singlet superconductors.
"This is the main reason why superconductivity disappears in a sufficiently strong magnetic field," the scientists say.
But there are several exotic superconductors that are impervious to magnetic fields, up to a very large force. These materials have superconductivity through pairs of electrons with the same spin-a property known as a"spin-triplet". When exposed to strong magnetic fields, the energy of both electrons in the Cooper pair shifts in one direction, so that they do not stretch, but continue to remain in the superconducting state without disturbances, regardless of the magnetic field strength.
Scientists were curious whether a three-layer graphene with a magic angle could have signs of this unusual spin-triplet superconductivity. The group conducted groundbreaking work on the study of Moire structures of graphene-layers of carbon lattices up to an atom thick, which, when folded at certain angles, can cause amazing electronic behavior.
Initially, the researchers reported such curious properties of two inclined sheets of graphene, which they called a two-layer graphene at a magical angle. Soon, they continued testing three-layer graphene, a sandwich configuration of three graphene sheets, which turned out to be even stronger than its two-layer counterpart, maintaining superconductivity at higher temperatures.
When the researchers applied a moderate magnetic field, they noticed that three-layer graphene is capable of superconductivity at a field strength that will destroy the superconductivity in two-layer graphene.
Super Return
In their new study, physicists have tested the superconductivity of three-layer graphene in increasingly high magnetic fields. They made the material by separating thin layers of carbon from a block of graphite, adding three layers together and rotating the average by 1.56 degrees relative to the outer layers. They attached an electrode to the end of the material to pass current and measure the energy loss in the process. Then they turned on a large magnet in the laboratory, directing the field parallel to the material.
By increasing the magnetic field around the three-layer graphene, they observed that the superconductivity remained strong until a certain moment before disappearing, but then, oddly enough, reappeared at higher values of the field – a return that is very unusual and is not known how it happens in ordinary spin-singlet superconductors.
"In spin-singlet superconductors, if you kill the superconductivity, it will never come back – it's gone forever," physicists say. "Here she appeared again. So it definitely says that this material is not a spin singlet."
They also noticed that after the" re-entry", the superconductivity was maintained up to 10 Tesla, the maximum field strength that a laboratory magnet could create. This is about three times higher than what a superconductor would have to withstand if it were an ordinary spin singlet, according to the Pauli limit, a theory that predicts the maximum magnetic field at which a material can maintain superconductivity.
The resumption of the superconductivity of three-layer graphene, combined with its resistance to higher magnetic fields than expected, eliminates the possibility that this material is an ordinary superconductor.
Instead, it is probably a very rare type, perhaps a spin-triplet containing Cooper pairs that move through the material, immune to strong magnetic fields. The team of scientists plans to deploy the material to confirm its exact state of rotation, which can help in the development of more powerful MRI devices, as well as more reliable quantum computers.
"Conventional quantum computing is very fragile," the researchers say. "You look at it, and suddenly it disappears. About 20 years ago, theorists proposed a type of topological superconductivity that, if implemented in any material, could help create a quantum computer in which the states responsible for calculations are very stable.
-This would give infinitely more possibilities for calculations. The key ingredient for implementing this will be spin-triplet superconductors of a certain type. We have no idea if our type belongs to this type. But even if this is not the case, it may make it easier to combine three-layer graphene with other materials to create such superconductivity. This could be a big breakthrough."
The study was published in the journal Nature
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