Letter | Published:

Observation of a phononic quadrupole topological insulator

Nature volume 555, pages 342345 (15 March 2018) | Download Citation

Abstract

The modern theory of charge polarization in solids1,2 is based on a generalization of Berry’s phase3. The possibility of the quantization of this phase4,5 arising from parallel transport in momentum space is essential to our understanding of systems with topological band structures6,7,8,9,10. Although based on the concept of charge polarization, this same theory can also be used to characterize the Bloch bands of neutral bosonic systems such as photonic11 or phononic crystals12,13. The theory of this quantized polarization has recently been extended from the dipole moment to higher multipole moments14. In particular, a two-dimensional quantized quadrupole insulator is predicted to have gapped yet topological one-dimensional edge modes, which stabilize zero-dimensional in-gap corner states14. However, such a state of matter has not previously been observed experimentally. Here we report measurements of a phononic quadrupole topological insulator. We experimentally characterize the bulk, edge and corner physics of a mechanical metamaterial (a material with tailored mechanical properties) and find the predicted gapped edge and in-gap corner states. We corroborate our findings by comparing the mechanical properties of a topologically non-trivial system to samples in other phases that are predicted by the quadrupole theory. These topological corner states are an important stepping stone to the experimental realization of topologically protected wave guides12,15 in higher dimensions, and thereby open up a new path for the design of metamaterials16,17.

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Acknowledgements

We acknowledge financial support from the Swiss National Science Foundation and the NCCR QSIT. T.L. acknowledges support from a Marie Curie fellowship and O.R.B. from the ETH postdoctoral fellowship FEL-26 15-2.

Author information

Author notes

    • Marc Serra-Garcia
    •  & Valerio Peri

    These authors contributed equally to this work.

Affiliations

  1. Institute for Theoretical Physics, ETH Zurich, 8093 Zürich, Switzerland.

    • Marc Serra-Garcia
    • , Valerio Peri
    • , Roman Süsstrunk
    • , Osama R. Bilal
    •  & Sebastian D. Huber
  2. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, USA.

    • Osama R. Bilal
  3. Advanced NEMS Group, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland.

    • Tom Larsen
    •  & Luis Guillermo Villanueva

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Contributions

S.D.H. conceived the research. M.S.-G., V.P. and O.R.B. designed the samples. M.S.-G., V.P., S.D.H. and R.S. conducted the experiments. L.G.V. and T.L. fabricated the samples.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Sebastian D. Huber.

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https://doi.org/10.1038/nature25156

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