  Monolayer graphene and bilayer graphene have strikingly different properties.
One such difference is the shape of the Fermi surface. Although anisotropic
band structures can be detected in optical measurements, they have so far been
difficult to detect in transport experiments on twodimensional materials. Here
we describe a ballistic transport experiment using high-quality graphene that
revealed Fermi surface anisotropy in the magnetoresistance. The shape of the
Fermi surface is closely related with the cyclotron orbit in real space.
Electron trajectories in samples with triangular lattices of holes depend on
the anisotropy of the Fermi surface. We found that this results in the
magnetoresistance which are dependent on crystallographic orientation of the
antidot lattice, which indicates the anisotropic Fermi surface of bilayer
graphene which is a trigonally-warped circle in shape. While in monolayer,
shape of magnetoresistance was approximately independent of the orientation of
antidot lattice, which indicates that the Fermi surface is a circle in shape.
The ballistic transport experiment is a new method of detecting anisotropic
electronic band structures in two-dimensional electron systems.
