  The Integer Quantum Hall Effect (IQHE) is a distinctive phase of
two-dimensional electronic systems subjected to a perpendicular magnetic field.
Thus far, the IQHE has been observed in semiconductor heterostructures and in
mono- and bi-layer graphene. Here we report on the IQHE in a new system:
trilayer graphene. Experimental data are compared with self-consistent Hartree
calculations of the Landau levels for the gated trilayer. The plateau structure
in the Hall resistivity determines the stacking order (ABA versus ABC). We find
that the IQHE in ABC trilayer graphene is similar to that in the monolayer,
except for the absence of a plateau at filling factor v=2. At very low filling
factor, the Hall resistance vanishes due to the presence of mixed electron and
hole carriers induced by disorder.
