Twisted Graphene Could Power New Generation of Superconducting Electronics | Science

By Charlie WoodNovember 19, 2020 at 12:05 p.m.

In 2018, a group of researchers from the Massachusetts Institute of Technology (MIT) performed a dazzling materials science magic trick. They stacked two microscopic graphene cards – sheets of carbon an atom thick – and twisted one very slightly. The application of an electric field transformed the stack from a conductor to an insulator and then, suddenly, into a superconductor: a material that conducts electricity without friction. Dozens of laboratories have jumped into the newly born field of “twistronics”, hoping to create new electronic devices without the hassle of melting chemically different materials.

Two groups – including the pioneering group at MIT – are now keeping that promise by turning twisted graphene into working devices, including superconducting switches like those used in many quantum computers. The studies mark a crucial milestone for the material, which is already emerging as a basic scientific tool capable of capturing and controlling individual electrons and photons. Now it looks promising as a basis 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. “The idea that this platform can be used as a universal material is not a fantasy,” he says. “It becomes a fact.”

The secret to the chameleon nature of twisted graphene lies in the so-called “magic angle”. When the researchers rotate the leaves 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 together thousands of atoms, moiré allows them to act in unison, like superatoms. This collective behavior allows a modest number of electrons, pushed 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 sense the presence of each other, facilitating their coupling, a requirement for superconductivity.

Now, researchers have shown that they can dial in desired properties in small regions of the sheet by knocking on a pattern of metal “doors” that subject different areas to varying electric fields. The two groups built devices known as Josephson junctions, in which two superconductors flank a thin layer of non-superconducting material, creating a valve to control the flow of superconductivity. “Once you’ve demonstrated that the world is open,” says Klaus Ensslin, physicist at ETH Zurich, and co-author of one of the studies, posted on the arXiv preprint server on October 30. Conventional Josephson junctions serve as the workhorse of superconducting electronics, found in magnetic devices to monitor electrical activity in the brain and in ultra-sensitive magnetometers.

The MIT Group went further, electrically transforming their Josephson junctions into other submicroscopic gadgets, “just like a proof of concept, to show just how versatile it is,” says lab chief Pablo Jarillo-Herrero, whose the group published its results on arXiv on November 4. By tuning the carbon into a conductor-insulator-superconductor configuration, they were able to measure how closely the electron pairs were attached together – an early clue to the nature of its superconductivity and how it compares to other materials. The team also built a transistor that can control the movement of single electrons; researchers studied these one-electron switches as a way to narrow circuits and lessen their thirst for energy.

Magic angle graphene devices are unlikely to challenge silicon consumer electronics any time soon. Graphene itself is easy to make: the sheets of it can be removed from graphite blocks with nothing more than scotch tape. But devices must be cooled to almost absolute zero before they can superconductor. And maintaining the precise twist is inconvenient, as the leaves tend to crumple, disrupting the magic angle. Reliably creating twisted sheets in a fluid manner, even at just 1 micron or two in diameter, remains a challenge, and researchers do not yet see a clear path to mass production. “If you wanted to create a really complex device,” Jarillo-Herrero says, “you would need to create hundreds of thousands of [graphene substrates] and this technology does not exist.

Nevertheless, many researchers are excited by the promise of exploring electronic devices without worrying about the constraints of chemistry. Materials scientists usually have to find substances with the right atomic properties and merge them. And when the concoction is complete, the different elements may not mesh in the way you want.

In magic angle graphene, on the other hand, all atoms are carbon, eliminating the messy boundaries between different materials. And scientists can change the electronic behavior of any patch with the push of a button. These advantages give unprecedented control over the material, says Ensslin. “Now you can play like on a piano.”

This control could simplify quantum computers. Those developed by Google and IBM are based on Josephson junctions whose properties are fixed during manufacture. To make the finicky qubits work, the junctions must be jointly manipulated in a heavy manner. With twisted graphene, however, the qubits could come from simple junctions which are smaller and easier to control.

Kin Chung Fong, a Harvard University physicist and a member of the quantum computing team at Raytheon BBN Technologies, is excited about another potential use for the material. In April, he and his colleagues proposed a twisted graphene apparatus capable of detecting a single photon of far infrared light. This could be useful for astronomers probing the dim light of the early universe; their current sensors can only spot single photons in visible or near visible parts of the spectrum.

The field of twistronics is still in its infancy, and the difficult process of twisting microscopic specks of graphene into magical positions still requires sleight of hand, or at least some skillful lab 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 condensed matter physicist at Rutgers University in New Brunswick, whose laboratory was one of the first to notice the peculiarity of twisted graphene. Properties.

“It’s a really new era,” she said. “It’s a whole new way to make materials without chemicals.”