  There are known two distinct types of the integer quantum Hall effect. One is
the conventional quantum Hall effect, characteristic of two-dimensional
semiconductor systems, and the other is its relativistic counterpart recently
observed in graphene, where charge carriers mimic Dirac fermions characterized
by Berry's phase pi, which results in a shifted positions of Hall plateaus.
Here we report a third type of the integer quantum Hall effect. Charge carriers
in bilayer graphene have a parabolic energy spectrum but are chiral and exhibit
Berry's phase 2pi affecting their quantum dynamics. The Landau quantization of
these fermions results in plateaus in Hall conductivity at standard integer
positions but the last (zero-level) plateau is missing. The zero-level anomaly
is accompanied by metallic conductivity in the limit of low concentrations and
high magnetic fields, in stark contrast to the conventional, insulating
behavior in this regime. The revealed chiral fermions have no known analogues
and present an intriguing case for quantum-mechanical studies.
