Mini-Dirac cones in AB-stacked graphene in perpendicular electric fields

AB-stacked multilayer graphene with number of layers shows semi-metallic band structure arising from the overlap of bilayer-like bands. By conducting low temperature magnetotransport experiments using encapsulated graphene samples with a top and a bottom gate electrode, we observed that mini-Dirac cones are formed in a perpendicular electric field. In zero perpendicular electric fields, Landau level near the charge neutrality point showed structures which are characteristic of semi-metallic band structures. The structure showed significant variations in perpendicular electric fields. The variations of the Landau level structure were interpreted as a formation of levels arising from mini-Dirac cones from numerical calculations of dispersion relations and the Landau levels. The mini-Dirac cones originate from a trigonal warping in the band structure. The perpendicular electric field opens energy gaps between bilayer-like band, and the trigonal warping closes the gaps locally, thereby mini-Dirac cones form.