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Observation of the fractional quantum Hall effect in graphene

2009; Nature Portfolio; Volume: 462; Issue: 7270 Linguagem: Inglês

10.1038/nature08582

1476-4687

Kirill I. Bolotin, Fereshte Ghahari, Michael Shulman, H. L. Störmer, Philip Kim,

Quantum Computing Algorithms and Architecture

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. When electrons are confined in two dimensions and subject to strong magnetic fields, the Coulomb interactions between them can become very strong, leading to the formation of correlated states of matter, such as the fractional quantum Hall liquid1,2. In this strong quantum regime, electrons and magnetic flux quanta bind to form complex composite quasiparticles with fractional electronic charge; these are manifest in transport measurements of the Hall conductivity as rational fractions of the elementary conductance quantum. The experimental discovery of an anomalous integer quantum Hall effect in graphene has enabled the study of a correlated two-dimensional electronic system, in which the interacting electrons behave like massless chiral fermions3,4. However, owing to the prevailing disorder, graphene has so far exhibited only weak signatures of correlated electron phenomena5,6, despite intense experimental and theoretical efforts7,8,9,10,11,12,13,14. Here we report the observation of the fractional quantum Hall effect in ultraclean, suspended graphene. In addition, we show that at low carrier density graphene becomes an insulator with a magnetic-field-tunable energy gap. These newly discovered quantum states offer the opportunity to study correlated Dirac fermions in graphene in the presence of large magnetic fields.