Scientists at the Massachusetts Institute of Technology (MIT) and Harvard University have discovered that graphene can pull another rabbit from its hat: it can be both an insulator and a superconductor, two opposite electrical extremes.
A graphene sheet. Source: UnderstandingNano.com
Graphene is a material composed of carbon atoms arranged in a hexagonal shape, with only three of the four carbon valence electrons making bonds with three other electrons from a neighboring atom. A sheet of graphene, as shown in the figure, is only one atom thick and it is considered one molecule.
The latest news about graphene comes on top of its many other well-known properties. Graphene is a better conductor than copper, is the thinnest material, is hundreds of times stronger than steel, has a very high tensile strength and is extremely light and flexible. In a paper published today in Nature, scientists report that this wonder material, when properly manipulated, can become an insulator, where electrons won’t flow, and can also be a superconductor, a material with no electrical resistance.
Graphene superlattice without and with rotation. Source: MIT
The researchers, led by Pablo Jarillo-Herrero, an associate professor of physics at MIT, discovered that stacking two sheets of graphene so they are not precisely on top of each other, rather they are rotated so that the top sheet is out of alignment with respect to the lower layer, the unit cell (the smallest repeating unit of the material’s 2D lattice) becomes enlarged. At an angle of rotation of about 1.1 degrees or less – what the scientists call the “magic angle” – the two sheets exhibit nonconducting behavior, similar to special type of materials known as Mott insulators. They also discovered that if a voltage is applied, at a certain level, current was produced flowing with almost no opposition, similar to a superconductor material.
(a) Twisted graphene layers. (b) Small rotation angle of the lattice. Source: Nature
“We can now use graphene as a new platform for investigating unconventional superconductivity,” Jarillo-Herrero says. “One can also imagine making a superconducting transistor out of graphene, which you can switch on and off, from superconducting to insulating. That opens many possibilities for quantum devices.”
This research was supported in part by the Gordon and Betty Moore Foundation and the National Science Foundation.
