- #76

- 525

- 7

*Observation of the Fractional Quantum Hall Effect in Graphene*

http://arxiv.org/abs/0910.2763

Long story short, although the integer quantum Hall effect in graphene has been observed, this is the first observation of the fractional effect. Looks like they found states corresponding to a filling fraction v=0.30, v=0.46 and v=0.68. On theoretical grounds these would probably correspond to v=1/3, v=1/2 and v=2/3 respectively. The v=1/3 and v=2/3 are probably correct, the v=1/2 might be false. TWhen electrons are confined in two dimensions and subjected to strong magnetic fields, the Coulomb interactions between them become dominant and can lead to novel states of matter such as fractional quantum Hall liquids. In these liquids electrons linked to magnetic flux quanta form complex composite quasipartices, which are manifested in the quantization of the Hall conductivity as rational fractions of the conductance quantum. The recent experimental discovery of an anomalous integer quantum Hall effect in graphene has opened up a new avenue in the study of correlated 2D electronic systems, in which the interacting electron wavefunctions are those of massless chiral fermions. However, due to the prevailing disorder, graphene has thus far exhibited only weak signatures of correlated electron phenomena, despite concerted experimental efforts and intense theoretical interest. Here, we report the observation of the fractional quantum Hall effect in ultraclean suspended graphene, supporting the existence of strongly correlated electron states in the presence of a magnetic field. In addition, at low carrier density graphene becomes an insulator with an energy gap tunable by magnetic field. These newly discovered quantum states offer the opportunity to study a new state of matter of strongly correlated Dirac fermions in the presence of large magnetic fields.

Some background info: the quantum Hall effect is a topological phase where the bulk of the system develops a mobility gap, thus turning it into an insulator. At the sime time the edges develop massless modes, thus allowing for conductance along the edge. The conductivity is quantized in units of v*e^2/h -- v being an integer or a fractional number. The integer effect is triggered by disorder in the system, the fractional effect requires a dominating Coulomb force.