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A twisted graphene model reveals a moiré pattern, the key to its extraordinary properties.
© 2018 BY YUAN CAO
By Charlie Wood
In 2018, a group of researchers from the Massachusetts Institute of Technology (MIT) came up with a dazzling magic trick of materials science. They stacked two microscopic graphene papers – carbon sheets the thickness of an atom – and twisted one lightly. The application of an electric field transformed the chimney from conductor to insulator and then, suddenly, into a superconductor: a material that conducts electricity without friction. Dozens of laboratories have launched into the newly born field of “twistronics”, hoping to create new electronic devices without the hassle of fusing chemically different materials together.
Two groups, including MIT’s pioneering group, are now delivering on that promise by turning twisted graphene into working devices, including superconducting switches like those used in many quantum computers. The studies mark a crucial step for the material, which is already maturing into a basic scientific instrument capable of capturing and controlling single electrons and photons. Now, it is showing promise as a foundation for new electronic devices, says Cory Dean, a condensed matter physicist at Columbia University whose lab was one of the first to confirm the material’s superconducting properties after the 2018 announcement. “L ‘the idea that this platform can be used as a universal material is not fantasy, ”he says. “This is becoming a fact.”
The secret behind the chameleon-like nature of twisted graphene lies in the so-called “magic corner”. When the researchers rotate the sheets by 1.1 ° of precision, the twist creates a large-scale “moiré” pattern, the atomic-scale equivalent of the darker bands seen when two grids are juxtaposed. By bringing together thousands of atoms, moiré allows them to act in unison, like superatoms. That collective behavior allows a modest number of electrons, guided to the right place by an electric field, to radically change the behavior of the material, from insulator to conductor to superconductor. Interactions with supercells also force electrons to slow down and feel each other’s presence, which makes it easier for them to mate, a requirement for superconductivity.
Now, the researchers have shown that they can dial in desired properties in small regions of the sheet by slapping a pattern of metal ‘gates’ that subject different areas to varying electric fields. Both groups built devices known as Josephson junctions, in which two superconductors flank a thin layer of non-superconducting material, creating a valve for superconducting flow control. “Once the world is shown to be open,” says Klaus Ensslin, physicist at ETH Zurich and co-author of one of the studies, published on October 30 on the arXiv preprint server. Conventional Josephson junctions serve as the workhorse of superconducting electronics, found in magnetic devices for monitoring electrical activity in the brain and ultra-sensitive magnetometers.
The MIT team went further, electrically transforming their Josephson junctions into other submicroscopic gadgets, “just as proof of concept, to show how versatile it is,” says lab head Pablo Jarillo-Herrero, whose group has published his results on arXiv on November 4th. By tuning the carbon into a conductor-insulator-superconducting configuration, they were able to measure how tightly the electron pairs were yoked together, a first clue to the nature of its superconductivity and how it compares to other materials. The team also built a transistor capable of controlling the movement of individual electrons; the researchers studied such single-electron switches as a way to shrink circuits and decrease their thirst for energy.
Magic angle graphene devices are unlikely to challenge silicon consumer electronics anytime soon. Graphene itself is easy to make – sheets of it can be removed from graphite blocks with nothing more than Scotch tape. But the devices need to be cooled to near absolute zero before they can superconduct. And keeping the twist precise is inconvenient, as the sheets tend to wrinkle, disrupting the magic angle. Reliable creation of evenly twisted sheets of even 1 micron or two is still a challenge, and researchers still don’t see a clear path to mass production. “If you wanted to make a truly complex device,” says Jarillo-Herrero, “you would need to create hundreds of thousands of [graphene substrates] and that technology doesn’t exist. “
However, many researchers are excited about the promise of exploring electronic devices without worrying about the constraints of chemistry. Materials scientists typically have to find substances with the right atomic properties and fuse them together. And when the mixture is done, the different elements may not fit together the way you want.
In magic angle graphene, on the other hand, all atoms are carbon, eliminating the disordered boundaries between different materials. And scientists can change the electronic behavior of any patch with the push of a button. These benefits ensure unprecedented control over the material, says Ensslin. “Now you can play like on a piano.”
That control could simplify quantum computers. Those developed by Google and IBM are based on Josephson junctions with properties that are fixed during manufacture. To make picky qubits work, the junctions must be manipulated jointly in cumbersome ways. With twisted graphene, however, the qubits could come from smaller and easier to control single junctions.
Kin Chung Fong, a Harvard University physicist and member of Raytheon’s quantum computing team BBN Technologies, is excited about another potential use of the material. In April, he and his colleagues proposed a twisted graphene device that could detect a single photon of light in the far infrared. This could be useful for astronomers probing the faint light of the early universe; their current sensors can detect lone photons only in the visible or nearly visible parts of the spectrum.
The twistronics field remains in its infancy, and the complicated process of twisting microscopic grains of graphene into the magical position still requires sleight of hand, or at least skilled laboratory work. But regardless of whether twisted graphene finds its way into industrial electronics, it is already profoundly changing the world of materials science, says Eva Andrei, a physics of condensed matter at Rutgers University, New Brunswick, whose lab was one of the first to notice the peculiarity of graphene twisted properties.
“It really is a new era,” he says. “It’s a whole new way to create chemistry-free materials.”
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