Made of a single layer of carbon atoms linked in a hexagonal honeycomb pattern, graphene’s structure is simple and seemingly delicate. Since its discovery in 2004, scientists have found that graphene is in fact exceptionally strong. And although graphene is not a metal, it conducts electricity at ultrahigh speeds, better than most metals.
In 2018, MIT scientists 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. 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.
Now the MIT team reports their latest advancements in graphene twistronics, in two papers published in the journal Nature.
In the first study, the researchers, 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.
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.
The team detected these variations at an ultrahigh angular resolution of 0.002 degree. That’s equivalent to being able to see the angle of an apple against the horizon from a mile away.
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.
In the second study, the team reports creating a new twisted graphene structure with not two, but four layers of graphene. 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. This suggests that researchers may be able to more easily and controllably study the exotic properties of magic-angle graphene in four-layer systems.
These two studies are aiming to better understand the puzzling physical behavior of magic-angle twistronics devices. It could help designers engineer high-temperature superconductors and quantum computing devices.
News Source: MIT
