The intrinsic resistance peak (ridge) structures were recently found to appear in the carrier density dependence plot of the resistance of the AB-stacked multilayer graphene with even numbers of layers. The ridges are due to topological changes in the Fermi surface. Here, these structures were studied in AB-stacked multilayer graphene with odd numbers of layers (5 and 7 layers) by performing experiments using encapsulated high-quality graphene samples equipped with top and bottom gate electrodes. The intrinsic resistance peaks that appeared on maps plotted with respect to the carrier density and perpendicular electric field showed particular patterns depending on graphene's crystallographic structure, and were qualitatively different from those of graphene with even numbers of layers. Numerical calculations of the dispersion relation and semi-classical resistivity using information based on the Landau level structure determined by the magnetoresistance oscillations, revealed that the difference stemmed from the even-odd layer-number effect on the electronic band structure.(Corresponding Author yagi@hiroshima-u.ac.jp)

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Landau levels of AB-stacked four-to six-layer graphene were studied experimentally in the presence of a perpendicular electric field. The Landau levels at a low magnetic field showed a characteristic structure that originated from the mini-Dirac cones created by the perpendicular electric fields. Six-fold or twelve-fold degenerate Landau levels arising from the mini-Dirac cones were observed in the vicinity of the charge neutrality point as a radial structure in a plot of their Landau fan diagrams. The structure of fourlayer graphene had approximate electron-hole symmetry near the charge neutrality point, while the five-and six-layer graphene showed asymmetry. Numerical calculations of the dispersion relation and Landau level spectra indicated that trigonal warping played an essential role in forming the experimentally observed Landau level structure in low magnetic fields. In high magnetic fields, trigonal warping becomes less significant and the Landau level spectra become simpler, approaching those that were calculated without considering trigonal warping. The appearance of mini-Dirac cones in a perpendicular electric field would be a common feature of AB-stacked multilayer graphene.

Most studies on moiré superlattices formed from a stack of h-BN (two-dimensional hexagonal boron nitride) and graphene have focused on single layer graphene; graphene with multiple layers is less understood. Here, we show that a moiré superlattice of multilayer graphene shows features arising from the anisotropic Fermi surface affected by the superlattice structure. The moiré superlattice of a h-BN/AB-stacked tetralayer graphene heterostructures exhibited resistivity peaks showing a complicated dependence on the perpendicular electric field. The peaks were not due to secondary Dirac cones forming, but rather opening of the energy gap due to folding of the anisotropic Fermi surface. In addition, superlattice peaks resulted from mixing of light- and heavy-mass bilayer-like bands via the superlattice potential. The gaps did not open on the boundary of the superlattice Brillouin zone, but rather opened inside it, which reflected the anisotropy of the Fermi surface of multilayer graphene.

This report describes a Pt-Ru-C electrode prepared by a co-sputtering technique for use as the anode catalyst in direct methanol fuel cells. The 50-100-nm thick electrocatalyst layer was prepared on an Au foil substrate. From the X-ray diffractometer (XRD) measurements, it was determined that (1) Pt and Ru are compatible and (2) Pt and C are incompatible in the Pt-Ru-C electrodes. Based on the results of the background cyclic voltammograms (CVs), the addition of C to the Pt and Pt-Ru causes an increase in the electrochemical surface area. The CVs support the XRD data that Pt and Ru form an alloy. For the methanol oxidation, the Pt 72 Ru 23 C 5 electrode showed the highest current density and negative onset potential. It was also determined that the Pt-Ru-C containing 5-25 at.% C causes a negative shift in the onset potential for the methanol oxidation.