Twisted stacks of 2-D carbon act like a weird type of superconductor

Stacks of graphene, carefully twisted, gain a superpower: They become superconductors. Now scientists have new evidence that this “magic-angle” graphene is a member of a truly strange class of superconductor.

Like all superconductors, the materials, known as unconventional superconductors, transmit electricity without resistance when cooled. But these odd superconductors require less cooling than most. And there’s no accepted theory that explains how they do it.

Clues could now come from 2-D sheets of carbon called graphene, stacked atop one another and twisted just-so. A triple layer of twisted graphene has a key hallmark of many unconventional superconductors, MIT physicist Pablo Jarillo-Herrero and colleagues report November 6 in Science.

That raises hopes that scientists can analyze the material to better understand unconventional superconductors and design new ones suitable for technological applications, perhaps even one that works at room temperature.

In 2018, Jarillo-Herrero and colleagues discovered that two sheets of graphene, twisted at a certain “magic angle” relative to one another, could conduct electricity without resistance. At the time, signs already pointed toward unconventional superconductivity. But “there was no proof,” Jarillo-Herrero says. Now, “the evidence keeps getting stronger and stronger and stronger that it is an unconventional superconductor.”

In conventional superconductors, electrons pair up in a manner that eases their journey through the material. These pairs, known as Cooper pairs, are forged by the electrons’ interactions with the atoms that make up the crystal lattice of the material. In the 1980s, scientists discovered superconductors that didn’t fit this explanation, such as copper-based materials called cuprates. Cooper pairs also form in these unconventional superconductors, but it’s not fully understood what causes the electrons to buddy up.

With the help of magic-angle graphene, scientists hope to make progress on a theory that can account for unconventional superconductors’ idiosyncrasies. That’s because magic-angle graphene is simpler and perhaps easier to understand than previously studied types of unconventional superconductors.

“This is a chemically pristine system. It’s just carbon,” says physicist Ali Yazdani of Princeton University, who works on magic-angle graphene but was not involved with the new work. “We’ve been looking, always, for materials that are simpler, that show this exotic superconductivity, so as to study it more carefully.”

Much scrutiny has focused on what’s called the superconducting gap. Splitting a superconductor’s electron pairs apart takes a certain amount of energy. The superconducting gap is the amount of energy needed to free an electron, and it’s a key identifying characteristic of superconductors.

In standard superconductors, that energy gap is usually a consistent size for any electron coursing through it. But in unconventional superconductors, the gap can depend on the momentum of the electrons. Electrons traveling in certain directions will not experience a gap at all. These gapless momenta are called nodes, and they’re a feature of many unconventional superconductors.

To look for this effect in magic-angle graphene, the scientists made a sandwich of materials. An insulator called hexagonal boron nitride was surrounded by two magic-angle graphene stacks, each consisting of three layers of graphene. The researchers measured how electrons from the magic-angle graphene could leap across the insulator through a process called quantum tunneling. That indicated how much energy was needed to break apart the superconductor’s electron pairs, revealing the energy gap of the material.

The gap behaved as expected for an unconventional superconductor, one with nodes in its energy gap. Simultaneously, the researchers showed that current flowed through the material without resistance. Adjusting the magnetic field and temperature also produced results that aligned with expectations for a superconductor with nodes.

In unconventional superconductors, the nodes mean that electrons traveling in certain directions don’t participate in the Cooper pairs. “And this experiment shows very convincingly that that’s what happens in these magic-angle twisted graphene systems,” says physicist Allan MacDonald of the University of Texas at Austin, who was not involved with the research.

Earlier studies also showed hints of this behavior. The growing consensus is adding to the excitement. “When the chorus comes together, it’s very good,” Yazdani says. “When everybody’s singing the same hymn with many different experiments, that’s when we have progress.”