In the quantum anomalous Hall effect, quantized Hall resistance and vanishing longitudinal resistivity are predicted to result from the presence of dissipationless, chiral edge states and an insulating two-dimensional bulk, without requiring an external magnetic field. Here, we explore the potential of this effect in magnetic topological insulator thin films for metrological applications. Using a cryogenic current comparator system, we measure quantization of the Hall resistance to within one part per million and, at lower current bias, longitudinal resistivity under 10 mΩ at zero magnetic field. Increasing the current density past a critical value leads to a breakdown of the quantized, low-dissipation state, which we attribute to electron heating in bulk current flow. We further investigate the prebreakdown regime by measuring transport dependence on temperature, current, and geometry, and find evidence for bulk dissipation, including thermal activation and possible variable-range hopping.

Quantized magnetotransport is observed in 5.6 × 5.6 mm2 epitaxial graphene devices, grown using highly constrained sublimation on the Si-face of SiC(0001) at high temperature (1900 °C). The precise quantized Hall resistance of Rxy=h2e2 is maintained up to record level of critical current Ixx = 0.72 mA at T = 3.1 K and 9 T in a device where Raman microscopy reveals low and homogeneous strain. Adsorption-induced molecular doping in a second device reduced the carrier concentration close to the Dirac point (n ≈ 1010 cm−2), where mobility of 18760 cm2/V is measured over an area of 10 mm2. Atomic force, confocal optical, and Raman microscopies are used to characterize the large-scale devices, and reveal improved SiC terrace topography and the structure of the graphene layer. Our results show that the structural uniformity of epitaxial graphene produced by face-to-graphite processing contributes to millimeter-scale transport homogeneity, and will prove useful for scientific and commercial applications.

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A well-controlled technique for high-temperature epitaxial growth on 6H-SiC(0001) substrates is shown to allow the development of monolayer graphene that exhibits promise for precise metrological applications. Face-to-face and face-to-graphite annealing in a graphite-lined furnace at 1200 • C-2000 • C with a 101-kPa Ar background gas lowers the rates of SiC decomposition and Si sublimation/diffusion and thus provides a means to control the rate of graphene layer development. We studied a wide range of growth temperatures and times and describe the resulting sample surface morphology changes and graphene layer structures. The experimental results are compared to a kinetic model based on two diffusion processes: Si vapor diffusion in the Ar-filled gap and atomic diffusion through graphitic surface layers.

We measure the proton gyromagnetic ratio in H 2 0 by the low field method, $,(low). The result $,(low) = 2.67 513 376 loss-' 7 & (0.11 ppm), leads to a value of the fine structure constant of a-' = 137.0 359 840 (0.037 ppm) and a value for the quantized Hall resistance in SI units of RH = 25 812.80460 Q (0.037 ppm). To achieve this result, we measured the dimensions of a 2.1-m solenoid with an accuracy of 0.04 pm, and then measured the NMR frequency of a water sample in the field of the solenoid.