Graphene’s merits in electronic devices and as a light bulb coating are still being debated. But new results suggest the atom-thick carbon sheet has one clear advantage: precise but practical calibrations of electrical resistance.
This might seem like a minor use for the world’s most celebrated wonder material, but it’s one that sits at the very base of the electrical engineering pyramid. A more practical calibration could help national standards laboratories and industries that depend on those standards. It may also help disseminate the International System of Units (SI), which could be overhauled as early as 2018.
The most exacting metrology laboratories calibrate their electrical units based on quantum mechanical phenomena. The ohm, the SI unit of electrical resistance, is calibrated by taking advantage of the quantum Hall effect. The Hall effect occurs when a magnetic field is applied perpendicular to the flow of current. The resulting force on electrons causes them to migrate to the side, which in turn raises a voltage perpendicular to the flow of current.
In the quantum version of the Hall effect, which occurs in a thin layer of material, the voltage and resulting resistance are “quantized” and so take on discrete integer values. Conveniently, the quantum Hall resistance is expected to be completely independent of the kind of device that’s built. Instead it depends only on two unvarying constants of nature: the fundamental charge of the electron and a quantum mechanical measure dubbed the Planck constant.
Unfortunately, the physical conditions required to take advantage of the quantum resistance standard are exacting. State-of-the-art measurements, which are taken using a device made of thin layers of gallium arsenide and aluminum gallium arsenide, can require a 10-Tesla magnetic field (and so a massive superconducting magnet) and temperatures within a few degrees of absolute zero.
Researchers have long suspected that the unique behavior of electrons in graphene, namely the big spacing between electron energy levels when the material is exposed to a magnetic field, could be exploited to produce precise measurements of resistance under less extreme physical conditions.
Several recent results support that idea. In August, Jan-Theodoor Janssen at the UK’s National Physical Laboratory and colleagues reported a way to build a small system for a graphene resistance standard that can operate at a higher temperature and lower magnetic field. This week, a team based at France’s National Metrology and Testing Laboratory and various departments at the National Center for Scientific Research showed that graphene can indeed be used to calibrate resistance with great accuracy, rivalling that of gallium devices, and that it can do so over an even wider range of operational conditions.
