Letter

Experimental realization of the topological Haldane model with ultracold fermions

  • Nature volume 515, pages 237240 (13 November 2014)
  • doi:10.1038/nature13915
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Abstract

The Haldane model on a honeycomb lattice is a paradigmatic example of a Hamiltonian featuring topologically distinct phases of matter1. It describes a mechanism through which a quantum Hall effect can appear as an intrinsic property of a band structure, rather than being caused by an external magnetic field2. Although physical implementation has been considered unlikely, the Haldane model has provided the conceptual basis for theoretical and experimental research exploring topological insulators and superconductors2,3,4,5,6. Here we report the experimental realization of the Haldane model and the characterization of its topological band structure, using ultracold fermionic atoms in a periodically modulated optical honeycomb lattice. The Haldane model is based on breaking both time-reversal symmetry and inversion symmetry. To break time-reversal symmetry, we introduce complex next-nearest-neighbour tunnelling terms, which we induce through circular modulation of the lattice position7. To break inversion symmetry, we create an energy offset between neighbouring sites8. Breaking either of these symmetries opens a gap in the band structure, which we probe using momentum-resolved interband transitions. We explore the resulting Berry curvatures, which characterize the topology of the lowest band, by applying a constant force to the atoms and find orthogonal drifts analogous to a Hall current. The competition between the two broken symmetries gives rise to a transition between topologically distinct regimes. By identifying the vanishing gap at a single Dirac point, we map out this transition line experimentally and quantitatively compare it to calculations using Floquet theory without free parameters. We verify that our approach, which allows us to tune the topological properties dynamically, is suitable even for interacting fermionic systems. Furthermore, we propose a direct extension to realize spin-dependent topological Hamiltonians.

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Acknowledgements

We thank H. Aoki for drawing our attention to the relevance of their proposal for optical lattices and N. Cooper, S. Huber, L. Tarruell, L. Wang and A. Zenesini for discussions. We acknowledge the SNF, the NCCR-QSIT and the SQMS (ERC advanced grant) for funding.

Author information

Affiliations

  1. Institute for Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland

    • Gregor Jotzu
    • , Michael Messer
    • , Rémi Desbuquois
    • , Martin Lebrat
    • , Thomas Uehlinger
    • , Daniel Greif
    •  & Tilman Esslinger

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Contributions

The data were measured by G.J., M.M., R.D. and D.G. and analysed by G.J., M.M., R.D., T.U. and D.G. The theoretical framework was developed by G.J. and M.L. All work was supervised by T.E. All authors contributed to planning the experiment, discussions and the preparation of the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Tilman Esslinger.

Extended data

Supplementary information

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  1. 1.

    Supplementary Information

    This file contains Supplementary Text and Data, Supplementary Figures 1-13, Supplementary Table 1 and additional references.

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