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Why does the n=0 Landau level in graphene have half the degeneracy of the other levels?

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I've looked through several papers that talk about the anomalous integer quantum Hall effect of graphene (such as http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.95.146801), and they all state that the Hall effect is anomalous for graphene because the n=0 Landau level has half the degeneracy of the n>1 levels, with little to no explanation and without citing any (helpful) sources.

One of the papers (http://journals.aps.org/prb/pdf/10.1103/PhysRevB.75.165411) states that "the Dirac fermions have acquired an effective gap in a form of a “relativistic mass” due to the Coulomb interaction. Such a gap reduces the degeneracy of only the zeroth LL." Could someone explain this in more depth, specifically why the Coulomb interaction only breaks the degeneracy of the zeroth LL?

This post imported from StackExchange Physics at 2014-05-04 11:25 (UCT), posted by SE-user Izzhov

asked Apr 29, 2014 in Theoretical Physics by Izzhov (20 points) [ no revision ]

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