Physicists create adjustable superconductivity in twisted graphene ‘nanosandwich’

When two sheets of graphene are stacked on top of each other just at the right angle, the layered structure changes into an unconventional superconductor, which transmits electric currents without resistance or wasted energy.

This ‘magic angle’ transformation into two-layer graphene was first observed in 2018 in the group of Pablo Jarillo-Herrero, the Cecil and Ida Green professor of physics at MIT. Since then, scientists have been looking for other materials that can be similarly twisted into superconductivity, in the emerging field of ‘twistronics’. So far, no other twisted material has superconducted except the original twisted two-layer graphene.

In a paper appears in Nature, Jarillo-Herrero and his group report observation of superconductivity in a sandwich of three graphene sheets, the middle layer of which has been rotated in a new angle with respect to the outer layers. This new three-layer configuration features superconductivity that is more robust than its two-layer counterpart.

The researchers can also adjust the superconductivity of the structure by applying and varying the strength of an external electric field. By adjusting the three-layer structure, the researchers were able to produce superconducting superconductivity, an exotic type of electrical behavior rarely seen in any other material.

“It was not clear whether magic corner-layer double-layer graphene was an exceptional thing, but now we know that it is not alone; it has a cousin in the three-layer case,” says Jarillo-Herrero. “The discovery of this hypertunuable superconductor expands the twistronics field in entirely new directions, with potential applications in quantum information and sensing technologies.”

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

A new superfamily

Shortly after Jarillo-Herrero and his colleagues discovered that superconductivity can be generated in twisted two-layer graphene, theorists suggested that the same phenomenon occurs in three or more layers of graphene.

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

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

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

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

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

The team attached electrodes to both sides of the structures and traversed an electric current while measuring the amount of energy lost in the material.

“We did not see any energy decompose, which means it was a superconductor,” says Jarillo-Herrero. “We have to give credit to the theorists – they got it right.”

He adds that the exact cause of the superconductivity of the structure, whether due to symmetry, as suggested by the theorists, or not, has yet to be seen, and this is something the researchers plan to test in future experiments .

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

“The biggest bang”

In the investigation into their new three-layer structure, the team found that they could control its superconductivity in two ways. With their previous two-layer design, the researchers were able to set its superconductivity by applying an external gate voltage to change the number of electrons flowing through the material. When 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 turn on and off two-layer graphene superconductivity, similar to a transistor.

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

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

Using both methods to adjust the three-layer structure, the team observed superconductivity under a range of conditions, including at a relatively high critical temperature of 3 kelvin, even when the material has a low density of electrons. By comparison, aluminum, which is being investigated as a superconductor for quantum computing, has a much higher density of electrons and only becomes superconducting with about 1 kelvin.

“We found that the triangular graphene of the magic angle can be the strongest coupled superconductor, which means it is superconducting at a relatively high temperature, given how few electrons it can have,” says Jarillo-Herrero. “It gives the biggest blow to your money.”

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

“If we could make these structures as they are now, on an industrial scale, we could make superconducting pieces for quantum computing, or cryogenic superconducting electronics, photodetectors, etc. We have not yet figured out how to make billions of these at once, ‘Says Jarillo-Herrrero.

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