  Bilayer graphene contains, compared to graphene monolayer, an additional
graphene sheet and, therefore, extra degrees of freedom, making it a unique
system for complex electronic states to emerge. Here, we show that there are
two types of domain walls, i.e., coupling domain wall and potential domain
wall, in gapped graphene bilayers, which make microscopic electronic properties
of the bilayers varying spatially. The coupling domain wall separates two
graphene bilayer regions with different interlayer coupling strengths and the
potential domain wall is a boundary separating two adjacent regions with
different chemical potentials between two layers. We present a microscopically
study, using scanning tunnelling microscopy and spectroscopy, around the two
types of domain walls. The well-defined domain walls allow us to spatially
resolve the energy gap and Landau levels around them, which show novel
behaviour beyond what is expected from current theoretical models. Our result
indicates that the graphene bilayer may exhibit exotic electronic properties
related to the microscopic symmetry of the two layers.
