D. Kazazis

Quantum Hall resistance standard in graphene devices under relaxed experimental conditions.

The quantum Hall effect provides a universal standard for electrical resistance that is theoretically based on only the Planck constant h and the electron charge e. Currently, this standard is implemented in GaAs/AlGaAs, but graphenes electronic properties have given hope for a more practical device. Here, we demonstrate that the experimental conditions necessary for the operation of devices made of high-quality graphene grown by chemical vapour deposition on silicon carbide can be extended and significantly relaxed compared with those for state-of-the-art GaAs/AlGaAs devices. In particular, the Hall resistance can be accurately quantized to within 1u2009×u200910(-9) over a 10u2005T wide range of magnetic flux density, down to 3.5u2005T, at a temperature of up to 10u2005K or with a current of up to 0.5u2005mA. This experimental simplification highlights the great potential of graphene in the development of user-friendly and versatile quantum standards that are compatible with broader industrial uses beyond those in national metrology institutes. Furthermore, the measured agreement of the quantized Hall resistance in graphene and GaAs/AlGaAs, with an ultimate uncertainty of 8.2u2009×u200910(-11), supports the universality of the quantum Hall effect. This also provides evidence of the relation of the quantized Hall resistance with h and e, which is crucial for the new Système International dunités to be based on fixing such fundamental constants of nature.

Quantum Hall resistance standards from graphene grown by chemical vapour deposition on silicon carbide

Replacing GaAs by graphene to realize more practical quantum Hall resistance standards (QHRS), accurate to within 10−9 in relative value, but operating at lower magnetic fields than 10u2009T, is an ongoing goal in metrology. To date, the required accuracy has been reported, only few times, in graphene grown on SiC by Si sublimation, under higher magnetic fields. Here, we report on a graphene device grown by chemical vapour deposition on SiC, which demonstrates such accuracies of the Hall resistance from 10u2009T up to 19u2009T at 1.4u2009K. This is explained by a quantum Hall effect with low dissipation, resulting from strongly localized bulk states at the magnetic length scale, over a wide magnetic field range. Our results show that graphene-based QHRS can replace their GaAs counterparts by operating in as-convenient cryomagnetic conditions, but over an extended magnetic field range. They rely on a promising hybrid and scalable growth method and a fabrication process achieving low-electron-density devices.

Commensurability resonances in two-dimensional magnetoelectric lateral superlattices

Hybrid lateral superlattices composed of a square array of antidots and a periodic one-dimensional magnetic modulation are prepared in

Magnetic field driven ambipolar quantum Hall effect in epitaxial graphene close to the charge neutrality point

mathrm{Ga[Al]As}

Tuning the transport properties of graphene films grown by CVD on SiC(0001): Effect of in situ hydrogenation and annealing

heterostructures. The two-dimensional electron gases exposed to these superlattices are characterized by magnetotransport experiments in vanishing average perpendicular magnetic fields. Despite the absence of closed orbits, the diagonal magnetoresistivity in the direction perpendicular to the magnetic modulation shows pronounced classical resonances. They are located at magnetic fields where snake trajectories exist which are quasi-commensurate with the antidot lattice. The diagonal magnetoresistivity in the direction of the magnetic modulation increases sharply above a threshold magnetic field and shows no fine structure. The experimental results are interpreted with the help of numerical simulations based on the semiclassical Kubo model.

Linear negative magnetoresistance in two-dimensional Lorentz gases

We have investigated the disorder of epitaxial graphene close to the charge neutrality point (CNP) by various methods: (i) at room temperature, by analyzing the dependence of the resistivity on the Hall coefficient; (ii) by fitting the temperature dependence of the Hall coefficient down to liquid helium temperature; (iii) by fitting themagnetoresistances at low temperature. All methods converge to give a disorder amplitude of (20±10) meV. Because of this relatively low disorder, close to the CNP, at low temperature, the sample resistivity does not exhibit the standard value~h/4e^2 but diverges. Moreover, themagnetoresistance curves have a unique ambipolar behavior,which has been systematically observed for all studied samples.This is a signature of both asymmetry in the density of states and in-plane charge transfer. The microscopic origin of this behavior cannot be unambiguously determined. However, we propose a model in which the SiC substrate steps qualitatively explain the ambipolar behavior.