Graphene as a superconductor? Double layers could be the key

The familiar is often overlooked. A recent paper published in the journal Science Advances describes a newly-discovered property of double layers of graphene that may make it possible to turn the material into a superconductor. The system has been studied before, because it is also a semiconductor with a band gap, but never with instruments sensitive enough to recognize the necessary properties for superconductivity. "It is an overseen property of a well-studied system," said first author Dr. Dmitry Marchenko.

There are many different allotropes, or forms, of pure carbon, depending on how the atoms formed bonds. Some familiar allotropes include diamonds, graphite, and carbon nanotubes. Graphene is a 2-dimensional form of carbon, in which the atoms are arranged in a hexagonal crystal lattice. Graphite does conduct electricity well in normal conditions — but it is not a superconductor.

In a normal conducting material, the moving electrons will collide with the ions in the material’s lattice. These collisions will take some of the electrons’ energy and convert it to heat, which is the source of electrical resistance. All superconductors, however, have zero resistivity; the electrons can flow without interference. This means that superconductors can theoretically maintain a current indefinitely without degradation, as well as support extremely high currents without suffering significant losses to heat.

Researchers at MIT have found superconductivity in graphene bilayers before, but their system required the two flat lattices to be twisted precisely 1.1 degrees relative to each other. Other successful attempts at turning graphene into a superconductor have necessitated adding other metals, which limits the material's capabilities at higher temperatures. While an exciting development, there is limited potential for real-world applications, as the necessary conditions are challenging to manufacture in bulk.

Scientists at the Helmholtz-Zentrum Berlin, however, recently published a paper that describes the properties of bilayer graphene in which the lattices lie exactly on top of each other. This system is suitable for larger-scale production; the material is produced by heating a silicon carbide crystal until the silicon atoms evaporate from the surface, leaving first a single layer of graphene, then a second.

The researchers found a flat area in the band structure, which is a prerequisite for superconductivity. Band theory approximates the quantum mechanical state of a solid, describing the ranges of possible energies for electrons in the material. In a flat area, the energy level does not change with position. Angle-resolved photoemission spectroscopy (ARPES) is a direct method of studying the electronic structure of the surface of solids. The researchers in this study used an ARPES instrument with resolution high enough to find the flat area next to the band gap.

Superconductors are already being used in scientific and medical equipment, such as MRI machines, which need to generate large magnetic fields. Superconducting electromagnets are incredibly strong; the lack of resistance in the wire coils means that they can create strong magnetic fields by conducting much higher currents than their non-superconducting counterparts, without losing any energy to heat.

There are many potential future applications of superconductors, including electric power transmission. Superconducting transmission lines would eliminate power transmission losses, which the U.S. Energy Information Administration estimates to be around five percent. This could increase the feasibility of renewable energy, which is often generated far from the populations that use it.

In its existing form, graphene is an excellent conductor of electricity. Exploiting the property discovered by these researchers could produce the ultimate, purest conductor with uses in everything from solar power to water purification.