Physicists create tunable superconductivity in twisted graphene ‘nanosandwich’

When two sheets of graphene are stacked on top of each other at the right angle, the layered structure turns into an unconventional superconductor, allowing electric currents to flow without resistance or wasted energy.

This “magic angle” transformation in bilayer graphene was first observed in 2018 in the group of Pablo Jarillo-Herrero, Cecil and Ida Green professor of physics at MIT. Since then, scientists have looked for other materials that can be similarly transformed into superconductivity, in the emerging field of “twistronics”. For the most part, no other twisted material has exhibited superconductivity other than the original twisted bilayer graphene, so far.

In an article published in Nature, Jarillo-Herrero and his group report observing superconductivity in a sandwich of three sheets of graphene, the middle layer of which is twisted at a new angle relative to the outer layers. This new three-layer configuration exhibits more robust superconductivity than its bilayer counterpart.

Researchers can also tune the superconductivity of the structure by applying and varying the strength of an external electric field. By fitting the three-layer structure, the researchers were able to produce ultra-strongly coupled superconductivity, a type of exotic electrical behavior rarely seen in any other material.

“It wasn’t clear if the magical angled bilayer graphene was an exceptional thing, but now we know it’s not alone; it has a cousin in the case of the three layers, ”explains Jarillo-Herrero. “The discovery of this hypertunable superconductor expands the field of twistronics in entirely new directions, with potential applications in quantum sensing and information technology.”

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

A new super family

Shortly after Jarillo-Herrero and his 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 graphene sheet is a thin layer of graphite, made up entirely of carbon atoms arranged in a honeycomb lattice, like the thinnest and strongest mesh. Theorists proposed that if three sheets of graphene were stacked like a sandwich, with the middle layer turned 1.56 degrees from the outer layers, the twisted configuration would create a kind of symmetry that would encourage the electrons in the material to pair up. and flow without resistance – the hallmark of superconductivity.

“We thought, why not, let’s try and test this idea,” Jarillo-Herrero says.

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

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

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

The team then attached electrodes to either end of the structures and passed an electric current through while measuring the amount of energy lost or dissipated in the material.

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

He adds that the exact cause of the structure’s superconductivity – whether due to its symmetry, as theorists have proposed or not – remains to be seen, and this is something the researchers plan to test in further. future experiences.

“Right now we have correlation, not causation,” he says. “Now at least we have a way to possibly explore a large family of new superconductors based on this idea of ​​symmetry.”

“The biggest bang”

By exploring their new three-layer structure, the team found that they could control its superconductivity in two ways. With their previous bilayer design, researchers could tune its superconductivity by applying an external gate voltage to change the number of electrons flowing through the material. By increasing and decreasing the gate voltage, they measured the critical temperature at which the material stopped dissipating energy and became superconducting. In this way, the team was able to tune the superconductivity of bilayer graphene on and off, like a transistor.

The team used the same method to tune the 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 additional electrode, the researchers could 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 may provide insight into the key physics essential to the formation of such an unusual superconducting state,” Park explains.

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

“We have found that magic-angle trilayer graphene may be the most powerful coupled superconductor, which means that it is superconducting at a relatively high temperature, considering how few electrons it can have,” Jarillo explains. -Herrero. “It gives you the most 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 exhibit superconductivity at higher temperatures, even when approaching room temperature.

“If we could make these structures as they are now, on an industrial scale, we could make superconducting bits for quantum computing, or cryogenic superconducting electronics, photodetectors, etc. We didn’t get it. how to make billions of them at a time, “Jarillo-Herrrero says.

“Our main goal is to understand the fundamental nature of what underlies strongly coupled superconductivity,” says Park. “Tri-layer graphene is not only the most powerful coupled superconductor ever found, but also the most tunable. With this tunability, we can truly explore superconductivity, anywhere in phase space.”