Graphene hybrid planar structures consisting of two regions with differentquantum Hall (QH) states exhibit unusual transport properties1-5, originatingfrom chiral edge states equilibration at the interface of the two differentregions6. Here we present a sub-nanometre-resolved scanning tunnellingmicroscopy (STM) and spectroscopy (STS) study of a monolayer-bilayer grapheneplanar junction in the QH regime. The atomically well-defined interface of sucha 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 thejunction, whereas unexpectedly, only Landau quantization of massive Diracfermions, as expected in graphene bilayer, is observed above the Nc. Theobserved unconventional interface Landau quantization arises from the fact thatthe quantum conductance across the interface is solely determined by theminimum filling factors (number of edge modes) in the graphene monolayer andbilayer regions of the junction6,7.
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