'Magic-angle' graphene discovered to have a very rare form of superconductivity

'Magic angle' twisted trilayer graphene doesn't only have an impressively exotic name, it might be a particularly rare type of superconductor, according to new research – one that could be useful everywhere from medical equipment to quantum computers.

Scientists are finding that stacking single-atom layers of graphene on top of each other at slightly different angles can create new materials with exciting properties, which led to the recent discovery of magic-angle twisted trilayer graphene.

Now, a new study from the same team shows that this material could be a "spin-triplet" superconductor – one that isn't affected by high magnetic fields – which potentially makes it even more useful.

"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," says physicist Pablo Jarillo-Herrero, from the Massachusetts Institute of Technology (MIT).

Superconducting materials are much sought-after: They can conduct electricity without losing heat, which could revolutionize our power grids, our portable gadgets, and much more – if we can ever get them working at normal temperatures.

Usually, electrons in superconductors couple up in what are called Cooper pairs – each with opposite spins (one up and one down), traveling through the material like linked passengers in an express train.

That's the conventional spin-singlet configuration, anyway. Rare types of superconductor are spin-triplet, however, which means the electrons have the same spin. Crucially, this means that a high magnetic field doesn't derail our imaginary express train, because the energy of both electrons shifts in the same direction.

Through a series of experiments, the team was able to show that magic-angle twisted trilayer graphene continued to behave like a superconductor at magnetic fields in excess of 10 Tesla – three times higher than would be expected from a spin-singlet material.

What's more, the superconductivity disappeared and then came back as the strength of the magnetic field was ramped up.

"In spin-singlet superconductors, if you kill superconductivity, it never comes back – it's gone for good," says physicist Yuan Cao from MIT. "Here, it reappeared again. So this definitely says this material is not spin-singlet."

The case isn't closed yet – more research is going to be required to check the spin states of electrons in this special type of graphene. What we can say though is that the early results are very promising, and very exciting for scientists.

One area where spin-triplet superconductors could be helpful is in MRI scans: If these machines could operate under higher magnetic fields, they could produce much more detailed pictures. For now though, ultra-low temperatures in the lab are still required for the material to act as a superconductor.

The material and its rare properties also show promise for future research into quantum computing. A key issue for realizing the promise of practical, accessible quantum computers is improving their stability – something that spin-triplet superconductors of a certain type could help with.

"We have no idea if our type is of that type," says 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."

The research has been published in Nature.

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