  Interactions among electrons can give rise to striking collective phenomena
when the kinetic energy of charge carriers is suppressed. One example is the
fractional quantum Hall effect, in which correlations between electrons moving
in two dimensions under the influence of a strong magnetic field generate
excitations with fractional charge. Graphene provides a platform to study
unique many-body effects due to its massless chiral charge carriers and the
fourfold degeneracy that arises from their spin and valley degrees of freedom.
Here we report local electronic compressibility measurements of a suspended
graphene flake performed using a scanning single-electron transistor. Between
Landau level filling v = 0 and 1, we observe incompressible fractional quantum
Hall states that follow the standard composite fermion sequence v = p/(2p \pm
1) for all integer p \leq 4. In contrast, incompressible behavior occurs only
at v = 4/3, 8/5, 10/7 and 14/9 between v = 1 and 2. These fractions correspond
to a subset of the standard composite fermion sequence involving only even
numerators, suggesting a robust underlying symmetry. We extract the energy gaps
associated with each fractional quantum Hall state as a function of magnetic
field. The states at v = 1/3, 2/3, 4/3 and 8/5 are the strongest at low field,
and persist below 1.5 T. The unusual sequence of incompressible states provides
insight into the interplay between electronic correlations and SU(4) symmetry
in graphene.
