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==== Strong magnetic fields ====
In magnetic fields above 10 [[Tesla (unit)|Tesla]]s or so additional plateaus of the Hall conductivity at ''σ''<sub>''xy''</sub> = ''νe''<sup>2</sup>/''h'' with ''ν'' = 0, ±1, ±4 are observed.<ref name="nu-0-1-4">{{cite journal |last=Zhang |first=Y. |last2=Jiang |first2=Z. |last3=Small |first3=J. P. |last4=Purewal |first4=M. S. |last5=Tan |first5=Y.-W. |last6=Fazlollahi |first6=M. |last7=Chudow |first7=J. D. |last8=Jaszczak |first8=J. A. |last9=Stormer |first9=H. L. |last10=Kim |first10=P. |title=Landau-Level Splitting in Graphene in High Magnetic Fields |doi=10.1103/PhysRevLett.96.136806 |journal=Physical Review Letters |volume=96 |page=136806 |year=2006 |bibcode=2006PhRvL..96m6806Z |issue=13 |arxiv=cond-mat/0602649 }}</ref> A plateau at ''ν'' = 3<ref name="nu-3">{{cite journal |last=Du |first=X. |last2=Skachko |first2=Ivan |last3=Duerr |first3=Fabian |last4=Luican |first4=Adina |last5=Andrei |first5=Eva Y. |title=Fractional quantum Hall effect and insulating phase of Dirac electrons in graphene |doi=10.1038/nature08522 |journal=Nature |volume=462 |pages=192–195 |year=2009 |issue=7270 |pmid=19829294 |arxiv=0910.2532 |bibcode=2009Natur.462..192D }}</ref> and the fractional quantum Hall effect at ''ν'' = {{frac|1|3}} were also reported.<ref name="nu-3"/><ref name="nu-one3rd">{{cite journal |last=Bolotin |first=K. |last2=Ghahari |first2=Fereshte |last3=Shulman |first3=Michael D. |last4=Stormer |first4=Horst L. |last5=Kim |first5=Philip |title=Observation of the fractional quantum Hall effect in graphene |doi=10.1038/nature08582 |journal=Nature |volume=462 |pages=196–199 |year=2009 |issue=7270 |pmid=19881489 |arxiv=0910.2763 |bibcode=2009Natur.462..196B }}</ref>
These observations with ''ν'' = 0, ±1, ±3, ±4 indicate that the four-fold degeneracy (two valley and two spin degrees of freedom) of the Landau energy levels is partially or completely lifted.
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