Fractional quantum Hall effect and insulating phase of Dirac electrons in graphene

The fractional quantum Hall effect is a quintessential manifestation of the collective behaviour associated with strongly interacting charge carriers confined to two dimensions and subject to a strong magnetic field. It is predicted that the charge carriers present in graphene — an atomic layer of carbon that can be seen as the 'perfect' two-dimensional system — are subject to strong interactions. Nevertheless, the phenomenon had eluded experimental observation until now: in this issue two groups report fractional quantum Hall effect in suspended sheets of graphene, probed in a two-terminal measurement setup. The researchers also observe a magnetic-field-induced insulating state at low carrier density, which competes with the quantum Hall effect and limits its observation to the highest-quality samples only. These results pave the way for the study of the rich collective behaviour of Dirac fermions in graphene. The fractional quantum Hall effect (FQHE) is the quintessential collective quantum behaviour of charge carriers confined to two dimensions but it has not yet been observed in graphene, a material distinguished by the charge carriers' two-dimensional and relativistic character. Here, and in an accompanying paper, the FQHE is observed in graphene through the use of devices containing suspended graphene sheets; the results of these two papers open a door to the further elucidation of the complex physical properties of graphene. 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 field1,2. 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 analogue3. 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 existence4,5,6,7,8,9. 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 measurements10. 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.