  Graphene hybrid planar structures consisting of two regions with different
quantum Hall (QH) states exhibit unusual transport properties1-5, originating
from chiral edge states equilibration at the interface of the two different
regions6. Here we present a sub-nanometre-resolved scanning tunnelling
microscopy (STM) and spectroscopy (STS) study of a monolayer-bilayer graphene
planar junction in the QH regime. The atomically well-defined interface of such
a junction allows us to spatially resolve the interface electronic properties.
Around the interface, we detect Landau quantization of massless Dirac fermions,
as expected in graphene monolayer, below the charge neutrality point Nc of the
junction, whereas unexpectedly, only Landau quantization of massive Dirac
fermions, as expected in graphene bilayer, is observed above the Nc. The
observed unconventional interface Landau quantization arises from the fact that
the quantum conductance across the interface is solely determined by the
minimum filling factors (number of edge modes) in the graphene monolayer and
bilayer regions of the junction6,7.
