Unconventional Sequence of Fractional Quantum Hall States in Suspended Graphene

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Science  07 Sep 2012:
Vol. 337, Issue 6099, pp. 1196-1199
DOI: 10.1126/science.1224784

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Skipping the Odds

When confined to a plane and placed in a magnetic field at low temperatures, electrons are separated by energy into the so-called Landau levels; adding an extra electron after a Landau level that has been filled is costly. In some systems, electron-electron interactions cause the appearance of sublevels, in a phenomenon known as the fractional quantum Hall effect (FQHE). This effect has been observed in graphene, but the number of levels that had been resolved was limited. Feldman et al. (p. 1196) directly measured the change in the chemical potential caused by varying electron density, which is controlled by gate voltage. Once the FQH states were identified, the Landau levels with odd-numerator fractional fillings were found to be missing between filling factors 1 and 2, because of the broken and preserved symmetries of graphene. These observations help to explain how the FQHE in graphene is different from that observed in conventional semiconductors, and the technique will also allow local measurements to be made; hence, monitoring of spatial variations in sample behavior is possible.