Credit: © 2009 NPG

The fractional quantum Hall effect manifests as a series of plateaus in the conduction of a material as charge density and magnetic field are varied. It is important because it results from a distinct phase of matter characterized by the collective behaviour of strongly interacting electrons. Although there has been considerable theoretical work suggesting that the effect should exist in graphene, it has so far not been observed.

Now, independent teams at Rutgers University1 and Columbia University2 have observed the fractional quantum Hall effect in suspended graphene devices. The observations rely on keeping the graphene surface pristine, so that the collective behaviour of the electrons is not disrupted by scattering. The effect is shown to survive to higher temperatures than are typical for other semiconductors. This is due to graphene's low dielectric constant, which reduces dielectric screening.

The suspended graphene also showed an insulating state at low carrier density whose properties can be tuned with magnetic field. While such a state has been previously observed in more disordered graphene samples, its origin is the subject of a debate, which these new results may help to settle.