Letter | Published:

Tunnelling spectroscopy of Andreev states in graphene

Nature Physics volume 13, pages 756760 (2017) | Download Citation

Abstract

A normal conductor placed in good contact with a superconductor can inherit its remarkable electronic properties1,2. This proximity effect microscopically originates from the formation in the conductor of entangled electron–hole states, called Andreev states3,4,5,6,7,8. Spectroscopic studies of Andreev states have been performed in just a handful of systems9,10,11,12,13. The unique geometry, electronic structure and high mobility of graphene14,15 make it a novel platform for studying Andreev physics in two dimensions. Here we use a full van der Waals heterostructure to perform tunnelling spectroscopy measurements of the proximity effect in superconductor–graphene–superconductor junctions. The measured energy spectra, which depend on the phase difference between the superconductors, reveal the presence of a continuum of Andreev bound states. Moreover, our device heterostructure geometry and materials enable us to measure the Andreev spectrum as a function of the graphene Fermi energy, showing a transition between different mesoscopic regimes. Furthermore, by experimentally introducing a novel concept, the supercurrent spectral density, we determine the supercurrent–phase relation in a tunnelling experiment, thus establishing the connection between Andreev physics at finite energy and the Josephson effect. This work opens up new avenues for probing exotic topological phases of matter in hybrid superconducting Dirac materials16,17,18.

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Acknowledgements

We acknowledge helpful discussions with W. Belzig, J. C. Cuevas, V. Fatemi, Ç. Girit, P. Joyez, A. L. Yeyati, J.-D. Pillet, H. Pothier, V. Shumeiko and C. Urbina. This work has been primarily supported by the US DOE, BES Office, Division of Materials Sciences and Engineering under Award DE-SC0001819 and by the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4541 to P.J.-H. J.I.-J.W. was partially supported by a Taiwan Merit Scholarship TMS-094-1-A-001. This work made use of the MRSEC Shared Experimental Facilities supported by NSF under award No. DMR-0819762 and of Harvard’s CNS, supported by NSF under Grant ECS-0335765.

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Author notes

    • Landry Bretheau
    •  & Joel I-Jan Wang

    These authors contributed equally to this work.

Affiliations

  1. Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • Landry Bretheau
    • , Joel I-Jan Wang
    • , Riccardo Pisoni
    •  & Pablo Jarillo-Herrero
  2. National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan

    • Kenji Watanabe
    •  & Takashi Taniguchi

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Contributions

J.I.-J.W., L.B. and P.J.-H. designed the experiment. J.I.-J.W. and R.P. fabricated the devices. L.B. and J.I.-J.W. carried out the measurements. L.B. analysed and interpreted the data. K.W. and T.T. supplied hBN crystals. L.B. and J.I.-J.W. wrote the manuscript with input from all the authors.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Landry Bretheau or Joel I-Jan Wang or Pablo Jarillo-Herrero.

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https://doi.org/10.1038/nphys4110

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