A scanning electron microscope image showing a contact, two-atom thick, free-floating graphene flake. It has a free floating metal bridge hovering over it. Credit: Fabian Geisenhof/Jakob Lenz
The electrical resistance of a material is usually dependent on its fundamental properties and physical dimensions. This resistance can be fixed, but it is possible to change in discrete steps, rather than constantly, under certain circumstances. This quantification of electrical resistance usually occurs in strong magnetic fields or at low temperatures where electrons move in two-dimensional ways. A University of Gttingen research team has demonstrated this effect at low temperatures, in almost total absence of a magnetic force in naturally occurring double layer graphene that is only two atoms thick. Nature published the results.
Two-layer graphene was used by the University of Gttingen and Ludwig Maximilian University of Munich teams. The delicate graphene flakes are made by using standard microfabrication methods. The flake is held in place so it hangs like a bridge and is held at its edges by two metal contacts. Double-layered graphene flakes show an extremely high level of electrical resistance at low temperatures, and virtually undetectable magnetic field. The electrical current flows freely without losing any energy. This is because the magnetism is generated differently to conventional magnets. Instead of aligning the intrinsic magnetic moments electrons, but by moving charged particles within the graphene double-layer itself, it is created in a different way.
Professor Thomas Weitz, University of Gttingen, says that the particles create their own magnetic field which allows them to quantify the electrical resistance.
The contacts with gold are shown in yellow, the graphene double layer red and the metal bridge in blue. Credit: Fabian Geisenhof/Jakob Lenz
This effect is unique because it requires an electric field. However, it can also be controlled with applied magnetic and electric fields. The effect can be turned on or off, and the direction of the charged particles can also be reversed. This allows for high control.
Weitz says that this makes it an interesting candidate for potential applications in the development and use of innovative components for computer parts in the field spintronics. This could have implications for data storage. It is also a benefit that this effect can be demonstrated in a system made up of a simple, naturally occurring material. This contrasts with the more recent 'heterostructures', which require a precise and complex mixture of materials.
"First, however the effect must still be investigated further and methods to stabilize it at higher temperatures are needed. Currently, it occurs at five degrees above absolute zero (the latter being 273 degrees below 0oC).
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More information: Thomas Weitz. Quantum anomalous Hall Octet Driven by Orbital Magnetism in Bilayer Graphene. Nature (2021). Information from Nature Thomas Weitz, Quantum abnormal Hall octet drove by orbital magnetism within bilayer graphene. (2021). DOI: 10.1038/s41586-021-03849-w
