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Even-denominator fractional quantum Hall states at an isospin transition in monolayer graphene

Nature Physics volume 14, pages 930–935 (2018)Cite this article

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Abstract

In monolayer graphene, the two inequivalent sublattices of carbon atoms combine with the electron spin to give electrons a nearly fourfold degenerate internal isospin. At high magnetic fields, the isospin degeneracy increases the already large intrinsic degeneracy of the two-dimensional Landau levels, making low-disorder graphene systems a versatile platform for studying multicomponent quantum magnetism. Here, we describe magnetocapacitance experiments of ultraclean monolayer graphene devices in which a hexagonal boron nitride substrate breaks the symmetry between carbon sublattices. We observe a phase transition in the isospin system, which is marked by unusual transitions in odd-denominator fractional quantum Hall states for filling factors ν near charge neutrality and by the unexpected appearance of incompressible even-denominator fractional quantum Hall states at ν = ±1/2 and ν = ±1/4. We propose a scenario in which the observed states are multicomponent fractional quantum Hall states incorporating correlations between electrons on different carbon sublattices, associated with a quantum Hall analogue of the Néel-to-valence bond solid transition that occurs at charge neutrality.

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Fig. 1: Incompressible FQH states at ν = ±1/2.
Fig. 2: Incompressible FQH states at ν = ±1/4.
Fig. 3: Odd-denominator fractional quantum Hall phase transitions associated with the ν = −1/2 state.
Fig. 4: Low-B data and mean field phase diagram of the ν = 0 state.
Fig. 5: Sublattice level crossing and multicomponent ν = ±1/2 states within a simplified model.

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  • 29 October 2018

    In the version of this Article originally published, the sketch of ‘PSP’ in Fig. 1a was incorrect; it has now been replaced. Please see the correction note to compare the original and corrected figure.

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Acknowledgements

We acknowledge discussions with D. Abanin, G. Murthy, Z. Papic, I. Sodemann and M. Zaletel and the experimental assistance of S. Hannahs. Magnetocapacitance measurements were funded by the National Science Foundation under DMR-1654186. A portion of the nanofabrication and transport measurements were funded by Army Research Office under proposal 69188PHH. A.F.Y. acknowledges the support of the David and Lucile Packard Foundation. E.M.S. acknowledges the support of the Elings Prize Fellowship in Science of the California Nanosystems Institute at the University of California, Santa Barbara (UCSB). The research reported here made use of shared facilities of the UCSB Materials Research Science and Engineering Center (NSF DMR 1720256), a member of the Materials Research Facilities Network (www.mrfn.org). Measurements above 14 T were performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative agreement no. DMR-1157490 and the State of Florida. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the Ministry of Education, Culture, Sports, Science and Technology, Japan, and the Japan Society for the Promotion of Science KAKENHI grant no. JP15K21722.

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  1. These authors contributed equally: A. A. Zibrov, E. M. Spanton.

Authors and Affiliations

  1. Department of Physics, University of California, Santa Barbara, CA, USA

    A. A. Zibrov, H. Zhou, C. Kometter & A. F. Young

  2. California Nanosystems Institute, University of California at Santa Barbara, Santa Barbara, CA, USA

    E. M. Spanton

  3. Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba, Japan

    T. Taniguchi & K. Watanabe

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Contributions

E.M.S. fabricated devices A, B and C, and H.Z. fabricated device D.T.T. and K.W. synthesized the hexagonal boron nitride crystals. A.F.Y. and C.K. built the measurement electronics. A.A.Z., E.M.S. and A.F.Y. acquired and analysed the experimental data. A.A.Z., E.M.S. and A.F.Y. wrote the paper.

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Correspondence to A. F. Young.

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Zibrov, A.A., Spanton, E.M., Zhou, H. et al. Even-denominator fractional quantum Hall states at an isospin transition in monolayer graphene. Nature Phys 14, 930–935 (2018). https://doi.org/10.1038/s41567-018-0190-0

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