  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
four-layer 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.
