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The quantum Hall effect, already used in gallium arsenide to achieve a primary standard of electrical resistance, is about to make a major contribution to redefining the international system of units (SI) from fundamental constants, notably for redefining the kilogram from the Planck constant. The use of the quantum Hall effect in metrology requires complete experimental mastery including, first, the reproducibility of the measurement of resistance with a nearly one billionth accuracy in relative value (10-9). In gallium arsenide, this objective is very restrictive: the standard must be cooled to 1.5 K, immersed in a 10 T magnetic field, and measured with currents not exceeding tens of mA, which confines these measurements to some National Institutes of Metrology and prevents its transfer to industrial users.
In graphene, a 2D crystal of carbon atoms, the quantum Hall effect is predicted to be extremely robust thanks to the quasi-relativistic properties of the electrons that propagate there. This, in theory, paves the way for the achievement of resistance standards operating under less restrictive and less costly measurement conditions. However, the achievement of this objective has so far remained out of reach, because obtaining homogeneous graphene sheets over several centimeters with load carriers of good mobility and low concentration is a challenge.
It was not until 2015 that the promises of graphene for application in metrology were fully realized, thanks to the work of a consortium of five French laboratories [1]. The graphene used for this demonstration was produced at CRHEA-CNRS using an original propane/hydrogen vapor deposition technique on silicon carbide, developed in partnership with NOVASiC. The uniformity and good electronic properties of the material were highlighted at L2C-CNRS/University of Montpellier and CINAM-CNRS/Université d'Aix Marseille; LPN-CNRS has produced large quantum Hall effect devices (100 mm x 420 mm) with metal contacts of very low strength (<1W), et enfin, le LNE a réalisé des mesures métrologiques de haute précision qui ont montré, dans le meilleur dispositif, une quantification de la résistance de Hall parfaite à 1x10-9près, à des champs magnétiques aussi faibles que 3,5 T, des températures aussi élevées que 10 K et des courants de mesure aussi élevés que 0,5 mA, ce qui constitue de nouveaux records.
By comparing this graphene device to a gallium arsenide device, the universality of the quantum Hall effect has also been verified by the LNE with unprecedented accuracy (8.2×10-11), supporting the direct relationship of quantum Hall resistance to Planck's constant and electron charge, crucial for the establishment of the new SI.
These new findings [1] follow preliminary work [2] that shed light on the physics of the quantum Hall effect in CVD graphene on SiC. All this work demonstrates the maturity of graphene technology for a very demanding application, contributes to the future overhaul of the international unit system and paves the way for a significant improvement in electrical measurements in general.
Illustration
Figure: a) Schematic view of the graphene stallion. Contact resistance is indicated. The electron density is 1.8×1011 cm-2 and mobility is 9400 cm2V-1s-1. b) Longitudinal resistance (red) and transverse (blue) of the graphene standard, depending on the density of magnetic flux, at a temperature of 1.3 K. The transverse resistance of a gallium arsenide semiconductor stallion is shown in black (dotted) for comparison. The dotted outlines surround the magnetic field intervals where the transverse resistance in graphene and in gallium arsenide is consistent with h/2e2 to 1×10-9.
Reference:
[1] R. Ribeiro-Palau, F. Lafont, J. Brun-Picard, D. Kazazis, A. Michon, F. Cheynis, O. Couturaud, C. Consejo, B. Jouault, W. Poirier, F. Schopfer, published in Nature Nanotechnology on September 7, 2015, https://dx.doi.org/10.1038/nnano.2015.192
[2] F. Lafont, R. Ribeiro-Palau, D. Kazazis, A. Michon, O. Couturaud, C. Consejo, T. Chassagne, M. Zielinski, M. Portail, B. Jouault, F. Schopfer and W. Poirier, Nature Communications 6, 6806 (2015).
Find out more: www.lne.fr
