IMAGE: © 2018 BY YUAN CAO
In 2018, a group of researchers from the Massachusetts Institute of Technology (MIT) came up with a dazzling magic trick for materials science. They arranged two microscopic graphene maps — carbon sheets one atom thick — and twisted one slightly. The application of an electric field transforms the stack from a conductor to an insulator and then suddenly into a superconductor: a material that rubs without electricity. Dozens of labs have jumped into the newborn field of twistronics, hoping to create new electronic devices without having to merge chemically different materials.
Two groups ̵
The secret of the chameleon nature of twisted graphene lies in the so-called “magic angle”. When the researchers rotate the sheets by exactly 1.1 °, the twist creates a scale moire, the atomic scale equivalent to the darker bands observed when comparing two lattices. Gathering thousands of atoms together, the moire allows them to act in unison, like superatoms. This collective behavior allows the modest number of electrons passed in the right place by an electric field to radically change the behavior of the material, from an insulator through a conductor to a superconductor. The interaction with the supercells also forces the electrons to slow down and sense the presence of the other, which facilitates their pairing, a requirement for superconductivity.
Researchers have now shown that they can gather the desired properties in small areas of the leaf by slamming metal “gates” that subject different areas to different electric fields. Both groups have built devices known as Josephson junctions, in which two superconductors surround a thin layer of non-conductive material, creating a valve to control the flow of superconductivity. “Once you demonstrate that the world is open,” said Klaus Enslin, a physicist at ETH Zurich and co-author of one of the studies published on the arXiv prepress server on October 30. Josephson’s conventional junctions serve as the workhorse of superconducting electronics found in magnetic devices to monitor electrical activity in the brain and ultrasensitive magnetometers.
The Massachusetts Institute of Technology group has gone further, electrically transforming its Josephson junctions into other submicroscopic devices, “just as proof of concept to show how flexible it is,” said Pablo Jarilo-Herrero, head of the lab. your results in arXiv on November 4th. By adjusting the carbon in a wire-insulator-superconductor configuration, they were able to measure how tightly the electron pairs are stored – an early key to the nature of its superconductivity and how it compares to other materials. The team has 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 reduce their thirst for energy.
Magical graphene angle devices are unlikely to challenge consumer silicon electronics any time soon. Graphene itself is easy to make: The sheets of graphene can be removed from graphite blocks with nothing more than scotch tape. But the devices must be cooled to almost zero before they can superconduct. And maintaining precise twisting is inconvenient, as the sheets tend to wrinkle, breaking the magic angle. Reliable creation of smoothly twisted sheets, even just 1 micron or two, is still a challenge and researchers still do not see a clear path to mass production. “If you want to make a really complex device,” says Jarilo-Herrero, “you’re going to have to create hundreds of thousands [graphene substrates] and that technology does not exist. “
However, many researchers are excited about the promise to study electronic devices without worrying about the limitations of chemistry. Usually, material scientists have to find substances with the right atomic properties and merge them. And when the mixing is finished, the different elements may not connect in the desired way.
In the magic corner, graphene, in contrast, is all carbon atoms, eliminating the scattered boundaries between different materials. And scientists can change the electronic behavior of a patch at the touch of a button. These benefits provide unprecedented control over the material, says Ensslin. “Now you can play like a piano.”
This control can simplify quantum computers. Developed by Google and IBM, Josephson relies on junctions with properties that are fixed during production. In order to control the fine qubits, the intersections must be manipulated jointly in cumbersome ways. With twisted graphene, however, qubits can come from single junctions that are smaller and easier to manage.
Kin Chung Fong, a physicist at Harvard University and a member of Raytheon BBN Technologies’ quantum computing team, is enthusiastic about other potential uses of the material. In April, he and his colleagues proposed a twisted graphene device that could detect a photon of far-infrared light. This could be useful for astronomers studying the dim light of the early universe; their current sensors can detect solitary photons only in the visible or near-visible parts of the spectrum.
The field of twistronics remains in its infancy, and the futile process of rotating microscopic spots from graphene to a magical position still requires dexterity or at least skillful laboratory work. But whether or not twisted graphene is in industrial electronics, it is already profoundly changing the world of materials science, says Eva Andrew, a condensed matter physicist at Rutgers University in New Brunswick, whose laboratory was one of the earliest to notice the peculiarity of twisted Graphene Properties.
“It’s really a new era,” she says. “It’s a whole new way of making materials without chemicals.”