Physicists create adjustable superconductivity in twisted graphene ‘nano sandwich’

When two sheets of graphene are stacked together at just the right angle, the layered structure turns into an unconventional superconductor, allowing electrical currents to pass without resistance or wasted energy.

This “magic angle” transformation in bilayer graphene was first observed in 2018 in the group of Pablo Jarillo-Herrero, de Cecil and Ida Green Professor of Physics at MIT. Since then, scientists have been looking for other materials that can be similarly turned into superconductivity, in the emerging field of ‘twistronics’. For the most part, no other twisted material has shown superconductivity to date other than the original twisted bilayer graphene.

In an article that appears in NatureJarillo-Herrero and his group report observing superconductivity in a sandwich of three graphene plates, the middle layer of which has been rotated at a new angle from the outer layers. This new three-layer configuration exhibits superconductivity that is more robust than its two-layer counterpart.

The researchers can also fine-tune the superconductivity of the structure by applying and varying the strength of an external electric field. By tuning the three-layer structure, the researchers were able to produce ultra-tightly coupled superconductivity, an exotic type of electrical behavior rarely observed in any other material.

“It wasn’t clear if magic angle bilayer graphene was an exceptional thing, but now we know it’s not the only one; it has a cousin in the three-layer housing,” said Jarillo-Herrero. “The discovery of this hypertunable superconductor expands the twistronics field in completely new directions, with potential applications in quantum information and sensor technologies.”

Its co-authors are lead authors Jeong Min Park and Yuan Cao at MIT, and Kenji Watanabe and Takashi Taniguchi from the National Institute of Materials Science in Japan.

A new super family

Shortly after Jarillo-Herrero and colleagues discovered that superconductivity could be generated in twisted bilayer graphene, theorists proposed that the same phenomenon could be observed in three or more layers of graphene.

A layer of graphene is an atom-thin layer of graphite, made entirely of carbon atoms arranged in a honeycomb grid, like the thinnest, toughest chicken wire. The theorists proposed that if three sheets of graphene were stacked as a sandwich, with the middle layer rotated 1.56 degrees from the outer layers, the twisted configuration would create a kind of symmetry that would encourage electrons in the material to pair and flow without resistance – the hallmark of superconductivity.

“We thought, let’s give it a try and test this idea,” says Jarillo-Herrero.

Park and Cao designed three-layer graphene structures by carefully cutting a single gossamer sheet of graphene into three sections and stacking each section at the precise angles predicted by the theorists.

They made several three-layered structures, each a few micrometers in diameter (about 1/100 the diameter of a human hair) and three atoms long.

“Our structure is a nano sandwich,” says Jarillo-Herrero.

The team then attached electrodes to both ends of the structures and ran an electrical current while measuring the amount of energy lost or dissipated in the material.

“We didn’t see any energy lost, which means it was a superconductor,” says Jarillo-Herrero. “We have to give credit to the theorists – they have the right angle.”

He adds that the exact cause of the structure’s superconductivity – whether or not because of its symmetry, as the theorists have suggested, or not – remains to be seen, and that’s something the researchers want to test in future experiments.

“Right now we have a link, not a causal link,” he says. “At least now we have a path to potentially explore a large family of new superconductors based on this symmetry idea.”

“The Biggest Bang”

Exploring their new three-layer structure, the team found that they could control superconductivity in two ways. Their previous bilayer design allowed the researchers to fine-tune the superconductivity by applying an external gate voltage to change the number of electrons flowing through the material. As they turned the gate voltage up and down, they measured the critical temperature at which the material stopped dissipating energy and became superconducting. In this way, the team was able to switch the superconductivity of double-layer graphene on and off, similar to a transistor.

The team used the same method to match three-layer graphene. They also discovered a second way to control the superconductivity of the material that was not possible in bilayer graphene and other twisted structures. By using an extra electrode, the researchers were able to apply an electric field to change the distribution of electrons between the three layers of the structure, without changing the overall electron density of the structure.

“These two independent buttons now give us a lot of information about the conditions under which superconductivity occurs, which can provide insight into the key physics critical to the formation of such an unusual superconducting state,” said Park.

Using both methods to tune the three-layer structure, the team observed superconductivity under a range of conditions, including at a relatively high critical temperature of 3 Kelvin, even if the material had a low electron density. In comparison, aluminum, which is being investigated as a superconductor for quantum computers, has a much higher electron density and only becomes superconducting at about 1 kelvin.

“We found that magic-angle three-layer graphene can be the strongest coupled superconductor, meaning it is superconducting at a relatively high temperature, given the number of electrons it can hold,” said Jarillo-Herrero. “It gives the greatest bang for your buck.”

The researchers plan to fabricate twisted graphene structures with more than three layers to see if such configurations, with higher electron densities, can show superconductivity at higher temperatures, even at room temperature.

“If we could make these structures as they are today, on an industrial scale, we could make superconducting bits for quantum computation, or cryogenic superconducting electronics, photo detectors, etc. We haven’t figured out how to make billions of them at a time,” Jarillo- Herrrero says.

“Our main goal is to understand the fundamental nature of what underlies highly coupled superconductivity,” said Park. “Three-layer graphene is not only the most highly coupled superconductor ever found, but also the most tunable. With that tunability, we can really investigate superconductivity anywhere in phase space.”