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Electron-hole asymmetric integer and fractional quantum Hall effect in bilayer graphene

Science  04 Jul 2014:
Vol. 345, Issue 6192, pp. 55-57
DOI: 10.1126/science.1250270

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Breaking down graphene degeneracy

Bilayer graphene has two layers of hexagonally arranged carbon atoms stacked on top of each other in a staggered configuration. This spatial arrangement results in degenerate electronic states: distinct states that have the same energy. Interaction between electrons can cause the states to separate in energy, and so can external fields (see the Perspective by LeRoy and Yankowitz). Kou et al., Lee et al., and Maher et al. used three distinct experimental setups that clarify different parameter regimes of bilayer graphene.

Science, this issue p. 55, p. 58, p. 61; see also p. 31

Abstract

The nature of fractional quantum Hall (FQH) states is determined by the interplay between the Coulomb interaction and the symmetries of the system. The distinct combination of spin, valley, and orbital degeneracies in bilayer graphene is predicted to produce an unusual and tunable sequence of FQH states. Here, we present local electronic compressibility measurements of the FQH effect in the lowest Landau level of bilayer graphene. We observe incompressible FQH states at filling factors ν = 2p + 2/3, with hints of additional states appearing at ν = 2p + 3/5, where p = 2, 1, 0, and 1. This sequence breaks particle-hole symmetry and obeys a νν + 2 symmetry, which highlights the importance of the orbital degeneracy for many-body states in bilayer graphene.

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