  In graphene, which is an atomic layer of crystalline carbon, two of the
distinguishing properties of the material are the charge carriers
two-dimensional and relativistic character. The first experimental evidence of
the two-dimensional nature of graphene came from the observation of a sequence
of plateaus in measurements of its transport properties in the presence of an
applied magnetic field. These are signatures of the so-called integer quantum
Hall effect. However, as a consequence of the relativistic character of the
charge carriers, the integer quantum Hall effect observed in graphene is
qualitatively different from its semiconductor analogue. As a third
distinguishing feature of graphene, it has been conjectured that interactions
and correlations should be important in this material, but surprisingly,
evidence of collective behaviour in graphene is lacking. In particular, the
quintessential collective quantum behaviour in two dimensions, the fractional
quantum Hall effect (FQHE), has so far resisted observation in graphene despite
intense efforts and theoretical predictions of its existence. Here we report
the observation of the FQHE in graphene. Our observations are made possible by
using suspended graphene devices probed by two-terminal charge transport
measurements. This allows us to isolate the sample from substrate-induced
perturbations that usually obscure the effects of interactions in this system
and to avoid effects of finite geometry. At low carrier density, we find a
field-induced transition to an insulator that competes with the FQHE, allowing
its observation only in the highest quality samples. We believe that these
results will open the door to the physics of FQHE and other collective
behaviour in graphene.
