If you change the angle of how two component sheets of graphene are oriented, you can find a “magic angle” that converts the two layers of graphene into a Mott insulator.
From Wikipedia, “Mott insulators are a class of materials that should conduct electricity under conventional band theories, but are insulators when measured (particularly at low temperatures). This effect is due to electron–electron interactions, which are not considered in conventional band theory.”
A Mott Insulator is a superconductor.
This discovery makes possible the design of new electronic-conducting materials that can operate at higher temperatures.
From Physics World
|‘Magic-angle’ graphene is an unconventional superconductor|
An ultrathin material consisting of two misaligned sheets of graphene can be easily converted from being a Mott insulator to a superconductor. The surprising discovery, details of which were announced at the March meeting of the American Physical Society (APS), could lead to the development of materials with a range of engineered electronic properties.
Graphene is a sheet of carbon just one atom thick that has a wealth of unique and potentially useful electronic and mechanical properties. Graphene atoms are arranged in a hexagonal lattice and two or more atomic layers can be stacked upon each other to create bilayer and thicker stacks of carbon.
In the new work, which is published in two papers in Nature, Pablo Jarillo-Herrero and colleagues at the Massachusetts Institute of Technology, Harvard University and Japan’s National Institute of Materials Science have shown that a graphene bilayer behaves as a Mott insulator when the two component sheets are oriented at a “magic angle”. In this form of matter, electrical conductivity is supressed by strong interactions between electrons. It can then be transformed into a superconductor by tweaking the electron density of the material
The researchers made their material by taking two sheets of graphene and rotating them out of alignment by a small angle of about 1°. Having the sheets at this magic angle creates a moiré lattice of atoms with a unit cell that is much larger than that of a single sheet of graphene. Electrons are localized at lattice sites and, under certain conditions, electrons can tunnel from one lattice site to another.
……“Physicists now have an exciting new platform to probe the unusual properties of high-temperature superconductors, and possibly to design new materials that operate at even higher temperatures,” said Jarillo-Herrero at a news conference at the APS March meeting. “Usually you have to grow different classes of material to explore each different phase. We can explore all of the physics in one device electrically. It couldn’t get any simpler.”