Physicists Create Tunable Superconductivity in Twisted Graphene 'Nanosandwich' 1 February 2021, by Jennifer Chu

Physicists create tunable superconductivity in twisted graphene 'nanosandwich' 1 February 2021, by Jennifer Chu

The researchers can also tune the structure's superconductivity by applying and varying the strength of an external electric field. By tuning the trilayer structure, the researchers were able to produce ultra-strongly coupled superconductivity, an exotic type of electrical behavior that has rarely been seen in any other material.

"It wasn't clear if magic-angle bilayer graphene was an exceptional thing, but now we know it's not alone; it has a cousin in the trilayer case," Jarillo- Herrero says. "The discovery of this hypertunable superconductor extends the twistronics field into entirely new directions, with potential applications in Credit: CC0 Public Domain quantum information and sensing technologies."

His co-authors are lead author Jeong Min Park and Yuan Cao at MIT, and Kenji Watanabe and Takashi When two sheets of graphene are stacked atop Taniguchi of the National Institute of Materials each other at just the right angle, the layered Science in Japan. structure morphs into an unconventional superconductor, allowing electric currents to pass A new super family through without resistance or wasted energy. Shortly after Jarillo-Herrero and his colleagues This "magic-angle" transformation in bilayer discovered that superconductivity could be graphene was observed for the first time in 2018 in generated in twisted bilayer graphene, theorists the group of Pablo Jarillo-Herrero, the Cecil and proposed that the same phenomenon might be Ida Green Professor of Physics at MIT. Since then, seen in three or more layers of graphene. scientists have searched for other materials that can be similarly twisted into superconductivity, in A sheet of graphene is an atom-thin layer of the emerging field of "twistronics." For the most graphite, made entirely of carbon atoms arranged part, no other twisted material has exhibited in a honeycomb lattice, like the thinnest, sturdiest superconductivity other than the original twisted chicken wire. The theorists proposed that if three bilayer graphene, until now. sheets of graphene were stacked like a sandwich, with the middle layer rotated by 1.56 degrees with In a paper appearing in Nature, Jarillo-Herrero and respect to the outer layers, the twisted configuration his group report observing superconductivity in a would create a kind of symmetry that would sandwich of three graphene sheets, the middle encourage electrons in the material to pair up and layer of which is twisted at a new angle with flow without resistance—the hallmark of respect to the outer layers. This new trilayer superconductivity. configuration exhibits superconductivity that is more robust than its bilayer counterpart. "We thought, why not, let's give it a try and test this idea," Jarillo-Herrero says.

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Park and Cao engineered trilayer graphene graphene. They also discovered a second way to structures by carefully slicing a single gossamer control the material's superconductivity that has not sheet of graphene into three sections and stacking been possible in bilayer graphene and other twisted each section on top of each other at the precise structures. By using an additional electrode, the angles predicted by the theorists. researchers could apply an electric field to change the distribution of electrons between the structure's They made several trilayer structures, each three layers, without changing the structure's measuring a few micrometers across (about 1/100 overall electron density. the the diameter of a human hair), and three atoms tall. "These two independent knobs now give us a lot of information about the conditions where "Our structure is a nanosandwich," Jarillo-Herrero superconductivity appears, which can provide says. insight into the key physics critical to the formation of such an unusual superconducting state," Park The team then attached electrodes to either end of says. the structures, and ran an electric current through while measuring the amount of energy lost or Using both methods to tune the trilayer structure, dissipated in the material. the team observed superconductivity under a range of conditions, including at a relatively high critical "We saw no energy dissipated, meaning it was a temperature of 3 kelvins, even when the material superconductor," Jarillo-Herrero says. "We have to had a low density of electrons. In comparison, give credit to the theorists—they got the angle right."aluminum, which is being explored as a superconductor for quantum computing, has a He adds that the exact cause of the structure's much higher density of electrons and only becomes superconductivity—whether due to its symmetry, as superconductive at about 1 kelvin. the theorists proposed, or not—remains to be seen, and is something that the researchers plan to test "We found magic-angle trilayer graphene can be in future experiments. the strongest coupled superconductor, meaning it superconducts at a relatively high temperature, "For the moment we have a correlation, not a given how few electrons it can have," Jarillo- causation," he says. "Now at least we have a path Herrero says. "It gives the biggest bang for your to possibly explore a large family of new buck." superconductors based on this symmetry idea." The researchers plan to fabricate twisted graphene "The biggest bang" structures with more than three layers to see whether such configurations, with higher electron In exploring their new trilayer structure, the team densities, can exhibit superconductivity at higher found they could control its superconductivity in two temperatures, even approaching room temperature. ways. With their previous bilayer design, the researchers could tune its superconductivity by "If we could make these structures as they are now, applying an external gate voltage to change the at industrial scale, we could make superconducting number of electrons flowing through the material. bits for quantum computation, or cryogenic As they dialed the gate voltage up and down, they superconductive electronics, photodetectors, etc. measured the critical temperature at which the We haven't figured out how to make billions of material stopped dissipating energy and became these at a time," Jarillo-Herrrero says. superconductive. In this way, the team was able to tune bilayer graphene's superconductivity on and "Our main goal is to figure out the fundamental off, similar to a transistor. nature of what underlies strongly coupled superconductivity," Park says. "Trilayer graphene is The team used the same method to tune trilayer not only the strongest-coupled superconductor ever

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found, but also the most tunable. With that tunability we can really explore superconductivity, everywhere in the phase space."

More information: Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene, Nature (2021). DOI: 10.1038/s41586-021-03192-0 , www.nature.com/articles/s41586-021-03192-0

Provided by Massachusetts Institute of Technology APA citation: Physicists create tunable superconductivity in twisted graphene 'nanosandwich' (2021, February 1) retrieved 26 September 2021 from https://phys.org/news/2021-02-physicists-tunable- superconductivity-graphene-nanosandwich.html

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