A model of twisted graphene reveals a moiré pattern—key to its striking properties.
© 2018 BY YUAN CAO
In 2018, a group of researchers at the Massachusetts Institute of Technology (MIT) pulled off a dazzling materials science magic trick. They stacked two microscopic cards of graphene—sheets of carbon one atom thick—and twisted one ever so slightly. Applying an electric field transformed the stack from a conductor to an insulator and then, suddenly, into a superconductor: a material that frictionlessly conducts electricity. Dozens of labs leapt into the newly born field of “twistronics,” hoping to conjure up novel electronic devices without the hassles of fusing together chemically different materials.
Two groups—including the pioneering MIT 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 science tool able to capture and control individual electrons and photons. Now, it’s showing promise as the basis of 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. “The idea that this platform can be used as a universal material is not fantasy,” he says. “It’s becoming fact.”
The secret behind twisted graphene’s chameleonlike nature lies with the so-called “magic angle.” When researchers rotate the sheets by precisely 1.1°, the twist creates a large-scale “moiré” pattern—the atom-scale equivalent of the darker bands seen when two grids are juxtaposed. By bringing thousands of atoms together, the moiré allows them to act in unison, like superatoms. That collective behavior enables a modest number of electrons, shepherded to the right place by an electric field, to radically change the material’s behavior, from insulator to conductor to superconductor. Interactions with the supercells also force electrons to slow down and feel each other’s presence, which makes it easier for them to pair off, a requirement for superconductivity.
Now, researchers have shown they can dial desired properties into small regions of the sheet by slapping on a pattern of metallic “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 nonsuperconducting material, creating a valve for controlling the flow of superconductivity. “Once you have demonstrated that then the world is open,” says Klaus Ensslin, a physicist at ETH Zurich, and a co-author on one of the studies, posted to the preprint server arXiv on 30 October. Conventional Josephson junctions serve as the workhorse of superconducting electronics, found in magnetic devices for monitoring electrical activity in the brain, and ultrasensitive magnetometers.
The MIT group went further, electrically transforming their Josephson junctions into other submicroscopic gadgets, “just as proof of concept, to show how versatile this is,” says lab leader Pablo Jarillo-Herrero, whose group posted its results to arXiv on 4 November. By tuning the carbon into a conductor-insulator-superconductor configuration, they were able to measure how tightly the electron pairs were yoked together—an early clue to the nature of its superconductivity and how it compares with other materials. The team also built a transistor that can control the movement of single electrons; researchers have studied such single-electron switches as a way to shrink circuits and diminish their thirst for energy.
Magic angle graphene devices are unlikely to challenge consumer silicon electronics anytime soon. Graphene itself is easy to make: Sheets of it can be stripped off blocks of graphite with nothing more than Scotch tape. But the devices must be chilled nearly to absolute zero before they can superconduct. And maintaining the precise twist is awkward, as the sheets tend to wrinkle, disrupting the magic angle. Reliably creating smoothly twisted sheets even just 1 micron or two across is still a challenge, and researchers don’t yet see a clear path toward mass production. “If you wanted to do a real complex device,” Jarillo-Herrero says, “you’d need to create hundreds of thousands of [graphene substrates] and that technology doesn’t exist.”
Nevertheless, many researchers are excited by 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 concoction is finished, the different elements may not mesh in the desired way.
In magic angle graphene, in contrast, all the atoms are carbon, eliminating messy boundaries between different materials. And scientists can change the electronic behavior of any given patch at the press of a button. These advantages grant unprecedented control over the material, Ensslin says. “Now, you can play like on a piano.”
That control could simplify quantum computers. Those being developed by Google and IBM rely on Josephson junctions with properties that are fixed during fabrication. To operate the finicky qubits, the junctions must be manipulated jointly in cumbersome ways. With twisted graphene, however, qubits could come from single junctions that are smaller and easier to control.
Kin Chung Fong, a Harvard University physicist and member of Raytheon BBN Technologies’s quantum computing team, is enthusiastic about another potential use for the material. In April, he and his colleagues proposed a twisted graphene device that could detect a single photon of far infrared light. That could be useful for astronomers probing the faint light of the early universe; their current sensors can spot lone photons only in the visible or nearly visible parts of the spectrum.
The field of twistronics remains in its infancy, and the fussy process of twisting microscopic specks of graphene to the magic position still requires sleight of hand, or at least deft lab work. But regardless of whether twisted graphene finds its way into industrial electronics, it’s already profoundly changing the world of materials science, says Eva Andrei, a condensed matter physicist at Rutgers University, New Brunswick, whose lab was one of the earliest to notice twisted graphene’s peculiar properties.
“It’s a really new era,” she says. “It’s a totally new way of making materials without chemistry.”