New research has demonstrated that a triple stack of graphene sheets twisted at a very specific angle could demonstrate superconductivity that survives exposure to intense magnetic fields. The study was published in the journal Nature.
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Superconductors - substances capable of conducting electricity without resistance - are poised to form the foundation of future technological and electronic advances, particularly in quantum computing.
While traditional conductors gradually lose resistance as they get colder - allowing progressively more electrons to flow - superconductors have a ‘critical temperature’ at which resistance is lost completely, allowing the free flow of electrons.
The fact that most materials capable of becoming superconductors only do so at very low temperatures has made close-to-room temperature superconductors the ‘holy grail’ of the materials science field.
This near room-temperature superconducting behavior is something that can be seen in graphene — single layers of carbon atoms in a hexagonal arrangement. When these atom thin sheets of graphene are double stacked, and a little twist is applied, they begin to act as a superconductor — even at close to room temperatures.
High temperatures are not the only thing that ‘turn-off’ superconductivity in a material. Exposure to a high magnetic field can also knock a superconductor into a regular conductive state. This has posed a challenge to developers of magnetic resonance imaging (MRI) devices, machines that rely on both superconductivity and intense magnetic fields.
The Twist Between Superconductivity and Graphene Relationship
Physicists from the Massachusetts Institute of Technology have found that not only is bilayer graphene a superconductor with a higher critical temperature, adding a third layer and applying a very specific angle — 54.7356° also known as the ‘magic angle’ — seems to allow superconductivity to be retained even in strong magnetic fields¹.
The team, led by Pablo Jarillo-Herrero, a Physics professor at MIT, discovered that when a trilayer of graphene is twisted in this way it seems to exhibit superconductivity in magnetic fields with a magnetic flux density as high as 10 Tesla. This is three times greater than the material could endure if it were a standard superconductor.
What the researchers believe they are seeing is a rare form of superconductivity called spin-triplet superconductivity.
“The value of this experiment is what it teaches us about fundamental superconductivity, about how materials can behave,” says Jarillo-Herrero. “So 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.”
What Makes a Conductor ‘Super’?
One of the most striking demonstrations of how superconductors work can be seen by placing an ordinary magnet over the top of such a material while it is cooled with liquid nitrogen. The magnet ‘levitates’ in place above the superconductor during this experiment. Whereas a normal conductor produces currents in a magnet moving past it via electromagnetic induction, superconductors ‘push’ the magnetic fields out by inducing surface currents. Instead of allowing the magnetic field to pass through it — with this passage measured by magnetic flux — the superconductor acts as a faux-magnet with the opposite polarity, repelling the ‘real’ magnet — a phenomenon called the Meissner effect.
The key to explaining superconductivity lies in understanding how electrons behave in materials at extremely low temperatures. Thermal energy randomly vibrates atoms in a material, and the higher the temperature, the faster the atoms vibrate.
At high temperatures, electrons — which all have the same negative charge — repel each other and act as free particles. Yet, there is still a tiny attraction between electrons in solids and liquids, and at low temperatures, electrons group together into what is known as Cooper pairs.
In Cooper pairs — named after American physicist Leon Cooper who first described this pairing up phenomenon in the mid-1950s — the electrons have an opposite spin. This is a quantum mechanical quantity that describes how a particle will behave when exposed to a magnetic field. One electron possesses spin ‘up’ and the other has spin ‘down.’ This state is described as a spin-singlet.
Cooper pairs travel unimpeded through a superconductor until they are exposed to a strong magnetic field. The electrons are then pulled in opposite directions, ripping the Cooper pairing apart.
Magnetic fields, therefore, destroy superconductivity. This is at least the case for spin-singlet superconductors. For exotic superconductors such as spin-triplet superconductors, the situation can be quite different.
More ‘Super’ Superconductors
In some exotic superconductors, electrons pair up with the same spin rather than opposite spins — or so-called spin-triplet pairs.
Spin describes how a particle behaves in a magnetic field. Particles of opposite spin move in opposite directions. However, if these electrons have the same spin, the Cooper pairing is not destroyed. Superconductivity is then preserved, even in extremely strong magnetic fields.
What Jarillo-Herrero and his team — already known for their pioneering work with the electronic properties of twisted graphene — wanted to discover was whether magic-angle trilayer graphene may display signs of spin-triplet superconductivity.
The researchers previously observed signs of this phenomenon in magic-angle bilayer graphene, but their new study showed that the effect is much stronger when an extra layer is added, with superconductivity retained at higher temperatures.
Surprisingly, trilayer graphene retained superconductivity in strong magnetic fields that would have wiped it out in its bilayer counterpart. To test this, the researchers exposed the magic-angle trilayer graphene to magnetic fields of increasing strengths. They found that superconductivity disappeared at a specific strength, but the graphene regained superconductivity at high field strengths.
This behavior is not seen in conventional spin-singlet superconductors.
The reintroduced superconductivity lasted in the magic-angle trilayer graphene up to a magnetic flux of 10 Tesla, but this was the maximum flux the team’s magnet could achieve. This means that this resurrected superconductivity could actually persist in even stronger fields.
The conclusion reached by the team; magic-angle trilayer graphene is not a run-of-the-mill superconductor.
“In spin-singlet superconductors, if you kill superconductivity, it never comes back — it’s gone for good,” says MIT postdoctoral researcher Yuan Cao. “Here, it reappeared again. So this definitely says this material is not spin-singlet.”
The question is: what exactly is the spin-state demonstrated by the material? This is something the team will now attempt to further investigate. Even with this question yet unanswered, we can still predict the kinds of applications that would benefit from this boosted resistance to magnetic fields.
Applications of Magic-Angle Trilayer Graphene Superconductors
The fact that this type of superconductor can resist high magnetic fields makes it incredibly useful across a range of applications; in particular, magnetic resonance imaging (MRI), which uses superconducting wires under intense magnetic fields to image biological tissues.
The functioning MRI devices are currently limited to their ability to resist a magnetic flux of no more than 3 Tesla, so if magic-angle graphene trilayer does display spin-triplet superconductivity, it could be used in such machines to boost their resistance to magnetic flux. The net result of this should be MRIs that can produce sharper and deeper images of human tissues.
Magic-angle trilayer graphene could be used in quantum computers to provide more resistant superconductors and much more powerful machines.
“Regular quantum computing is super fragile. You look at it and, poof, it disappears,” says Jarillo-Herrero. “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.”
This results in a quantum computer with computing power that far exceeds anything currently available. However, the team does not yet know if the exotic superconductivity they have found in the magic-angle trilayer graphene is the right type to facilitate this computing boost.
“The key ingredient to realizing that would be spin-triplet superconductors, of a certain type. We have no idea if our type is of that type,” concludes Jarillo-Herrero. “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.”
References and Further Reading
¹ Jarillo-Herrero. P., Cao. Y., Park. J. M., et al,  Pauli-limit violation and re-entrant superconductivity in moiré graphene. Nature. https://doi.org/10.1038/s41586-021-03685-y