  The electronic dispersion of a graphene bilayer is highly dependent on
rotational mismatch between layers and can be further manipulated by electrical
gating. This allows for an unprecedented control over electronic properties and
opens up the possibility of flexible band structure engineering. Here we
present novel magnetotransport data in a twisted bilayer, crossing the
energetic border between decoupled monolayers and coupled bilayer. In addition
a transition in Berry phase between pi and 2pi is observed at intermediate
magnetic fields. Analysis of Fermi velocities and gate induced charge carrier
densities suggests an important role of strong layer asymmetry for the observed
phenomena.
