  Since its discovery, Berry phase has been demonstrated to play an important
role in many quantum systems. In gapped Bernal bilayer graphene, the Berry
phase can be continuously tuned from zero to 2pi, which offers a unique
opportunity to explore the tunable Berry phase on the physical phenomena. Here,
we report experimental observation of Berry phases-induced valley splitting and
crossing in moveable bilayer graphene p-n junction resonators. In our
experiment, the bilayer graphene resonators are generated by combining the
electric field of scanning tunneling microscope tip with the gap of bilayer
graphene. A perpendicular magnetic field changes the Berry phase of the
confined bound states in the resonators from zero to 2pi continuously and leads
to the Berry phase difference for the two inequivalent valleys in the bilayer
graphene. As a consequence, we observe giant valley splitting and unusual
valley crossing of the lowest bound states. Our results indicate that the
bilayer graphene resonators can be used to manipulate the valley degree of
freedom in valleytronics.
