Fit around the tip of a device similar to an STM, a SQUID can measure a sample’s magnetic field flowing through the ring at a microscopic scale. Conventional SQUIDs resemble a small bisected ring, the two halves of which are made of superconducting material and joined together by two junctions. So the group consulted with researchers at the Weizmann Institute for Science, who had developed a scanning technique they call “scanning nano-SQUID,” where SQUID stands for Superconducting Quantum Interference Device. “In principle it could be done, but would take an enormous amount of time.” “Going over an entire micron-scale structure to look at millions of atoms is something that STM is not best suited for,” Jarillo-Herrero says. However, researchers have only been able to scan small patches of magic-angle graphene, spanning at most a few hundred square nanometers, using this approach. Several groups have imaged magic-angle structures, using scanning tunneling microscopy, or STM, a technique that scans a surface at the atomic level.
Since Jarillo-Herrero and his group first discovered magic-angle graphene, others have jumped at the chance to observe and measure its properties. “Once understood, physicists believe these devices could help design and engineer a new generation of high-temperature superconductors, topological devices for quantum information processing, and low-energy technologies.” “These two studies are aiming to better understand the puzzling physical behavior of magic-angle twistronics devices,” says Cao, a graduate student at MIT. This suggests that researchers may be able to more easily and controllably study the exotic properties of magic-angle graphene in four-layer systems. They observed that the new four-layer magic-angle structure is more sensitive to certain electric and magnetic fields compared to its two-layer predecessor. In the second study, the team report creating a new twisted graphene structure with not two, but four layers of graphene. We now have characterized how much twist variation you can have, and what is the degradation effect of having too much.” “And we see that you can have a little bit of variation and still show superconductivity and other exotic physics, but it can’t be too much. “This is the first time an entire device has been mapped out to see what is the twist angle at a given region in the device,” says Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT. They found that structures with a narrower range of angle variations had more pronounced exotic properties, such as insulation and superconductivity, versus structures with a wider range of twist angles. That’s equivalent to being able to see the angle of an apple against the horizon from a mile away. The team detected these variations at an ultrahigh angular resolution of 0.002 degree. The results revealed regions within the structure where the angle between the graphene layers veered slightly away from the average offset of 1.1 degrees. In the first study, the researchers, along with collaborators at the Weizmann Institute of Science, have imaged and mapped an entire twisted graphene structure for the first time, at a resolution fine enough that they are able to see very slight variations in local twist angle across the entire structure. Now the MIT team reports their latest advancements in graphene twistronics, in two papers published this week in the journal Nature. It was a monumental discovery that helped launch a new field known as “twistronics,” the study of electronic behavior in twisted graphene and other materials.
In 2018, MIT scientists led by Pablo Jarillo-Herrero and Yuan Cao discovered that when two sheets of graphene are stacked together at a slightly offset “magic” angle, the new “twisted” graphene structure can become either an insulator, completely blocking electricity from flowing through the material, or paradoxically, a superconductor, able to let electrons fly through without resistance. And although graphene is not a metal, it conducts electricity at ultrahigh speeds, better than most metals. Since its discovery in 2004, scientists have found that graphene is in fact exceptionally strong. Made of a single layer of carbon atoms linked in a hexagonal honeycomb pattern, graphene’s structure is simple and seemingly delicate.
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