Letter | Published:

Observation of a robust zero-energy bound state in iron-based superconductor Fe(Te,Se)

Nature Physics volume 11, pages 543546 (2015) | Download Citation

Abstract

In superconductors, electrons are paired and condensed into the ground state. An impurity can break the electron pairs into quasiparticles with energy states inside the superconducting gap. The characteristics of such in-gap states reflect accordingly the properties of the superconducting ground state1. A zero-energy in-gap state is particularly noteworthy, because it can be the consequence of non-trivial pairing symmetry1 or topology2,3. Here we use scanning tunnelling microscopy/spectroscopy to demonstrate that an isotropic zero-energy bound state with a decay length of 10 Å emerges at each interstitial iron impurity in superconducting Fe(Te,Se). More noticeably, this zero-energy bound state is robust against a magnetic field up to 8 T, as well as perturbations by neighbouring impurities. Such a spectroscopic feature has no natural explanation in terms of impurity states in superconductors with s-wave symmetry, but bears all the characteristics of the Majorana bound state proposed for topological superconductors2,3, indicating that the superconducting state and the scattering mechanism of the interstitial iron impurities in Fe(Te,Se) are highly unconventional.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

References

  1. 1.

    , & Impurity-induced states in conventional and unconventional superconductors. Rev. Mod. Phys. 78, 373–433 (2006).

  2. 2.

    & Superconducting proximity effect and Majorana fermions at the surface of a topological insulator. Phys. Rev. Lett. 100, 096407 (2008).

  3. 3.

    , , , & Unconventional superconductivity on a topological insulator. Phys. Rev. Lett. 104, 067001 (2010).

  4. 4.

    et al. Imaging the effects of individual zinc impurity atoms on superconductivity in Bi2Sr2Ca(Cu, Zn)2O8+δ. Nature 403, 746–750 (2000).

  5. 5.

    et al. Interplay of magnetism and high-Tc superconductivity at individual Ni impurity atoms in Bi2Sr2Ca(Cu, Zn)2O8+δ. Nature 411, 920–924 (2001).

  6. 6.

    et al. Close relationship between superconductivity and the bosonic mode in Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As. Nature Phys. 9, 42–48 (2013).

  7. 7.

    et al. In-gap quasiparticle excitations induced by non-magnetic Cu impurities in Na(Fe0.96Co0.03Cu0.01)As revealed by scanning tunnelling spectroscopy. Nature Commun. 4, 2749 (2013).

  8. 8.

    et al. Bound states of defects in superconducting LiFeAs studied by scanning tunneling spectroscopy. Phys. Rev. B 86, 174503 (2012).

  9. 9.

    , , & Unconventional s-wave superconductivity in Fe(Se, Te). Science 328, 474–476 (2010).

  10. 10.

    et al. Anisotropic energy gaps of iron-based superconductivity from intraband quasiparticle interference in LiFeAs. Science 336, 563–567 (2012).

  11. 11.

    et al. Charge-carrier localization induced by excess Fe in the superconductor Fe1+yTe1−xSex. Phys. Rev. B 80, 174509 (2009).

  12. 12.

    et al. Disorder-driven electronic localization and phase separation in superconducting Fe1+yTe0.5Se0.5 single crystals. Phys. Rev. B 82, 144523 (2010).

  13. 13.

    , , & Superconductivity at Tc 14 K in single-crystalline FeTe0.61Se0.39. Phys. Rev. B 80, 092502 (2009).

  14. 14.

    et al. Isotropic superconducting gaps with enhanced pairing on electron Fermi surfaces in FeTe0.55Se0.45. Phys. Rev. B 85, 094506 (2012).

  15. 15.

    et al. Cleavage surfaces of the BaFe2−xCoxAs2 and FeySe1−xTex superconductors: A combined STM plus LEED study. Phys. Rev. B 80, 140507(R) (2009).

  16. 16.

    et al. First-order magnetic and structural phase transitions in Fe1+ySexTe1−x. Phys. Rev. B 79, 054503 (2009).

  17. 17.

    et al. Friedel-like oscillations from interstitial iron in superconducting Fe1+yTe0.62Se0.38. Phys. Rev. Lett. 108, 107002 (2012).

  18. 18.

    et al. Scanning tunneling spectroscopy and vortex imaging in the iron pnictide superconductor BaFe1.8Co0.2As2. Phys. Rev. Lett. 102, 097002 (2009).

  19. 19.

    et al. Observation of ordered vortices with Andreev bound states in Ba0.6K0.4Fe2As2. Nature Phys. 7, 325–331 (2011).

  20. 20.

    , & Gap symmetry and structure of Fe-based superconductors. Rep. Prog. Phys. 74, 124508 (2011).

  21. 21.

    , , , & Probing the local effects of magnetic impurities on superconductivity. Science 275, 1767–1770 (1997).

  22. 22.

    , , & Impurity-induced bound states in iron-based superconductors with s-wave coskx cosky pairing symmetry. Phys. Rev. B 80, 064513 (2009).

  23. 23.

    Nonmagnetic impurity resonances as a signature of sign-reversal pairing in FeAs-based superconductors. Phys. Rev. Lett. 103, 186402 (2009).

  24. 24.

    Iron-based superconductors as odd-parity superconductors. Phys. Rev. X 3, 031004 (2013).

  25. 25.

    et al. Direct observation of nodes and twofold symmetry in FeSe superconductor. Science 332, 1410–1413 (2011).

Download references

Acknowledgements

The authors thank Z. Fang, X. Dai, T. Xiang, D-H. Lee, T-K. Lee, G-M. Zhang and P. Coleman for stimulating discussions. This work is supported by the State of Texas through TcSUH, the Chinese Academy of Sciences, US Air Force Office of Scientific Research (FA9550-09-1-0656), Robert A. Welch Foundation (E-1146), US DOE (DE-SC0002554, DE-FG02-99ER45747), National Science Foundation of China (11322432, 11190020), and Ministry of Science and Technology of China (2012CB933000).

Author information

Author notes

    • J-X. Yin
    •  & Zheng Wu

    These authors contributed equally to this work.

Affiliations

  1. Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

    • J-X. Yin
    • , Z-Y. Ye
    • , Jing Gong
    • , X-Y. Hou
    • , Lei Shan
    • , X-J. Liang
    • , X-X. Wu
    • , J-P. Hu
    • , H. Ding
    •  & S. H. Pan
  2. Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204, USA

    • J-X. Yin
    • , Zheng Wu
    • , J-H. Wang
    • , Z-Y. Ye
    • , Ang Li
    • , Jian Li
    • , C-S. Ting
    • , P-H. Hor
    •  & S. H. Pan
  3. Collaborative Innovation Center of Quantum Matter, Beijing 100190, China

    • Lei Shan
    • , H. Ding
    •  & S. H. Pan
  4. Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA

    • Z-Q. Wang
  5. Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA

    • J-P. Hu

Authors

  1. Search for J-X. Yin in:

  2. Search for Zheng Wu in:

  3. Search for J-H. Wang in:

  4. Search for Z-Y. Ye in:

  5. Search for Jing Gong in:

  6. Search for X-Y. Hou in:

  7. Search for Lei Shan in:

  8. Search for Ang Li in:

  9. Search for X-J. Liang in:

  10. Search for X-X. Wu in:

  11. Search for Jian Li in:

  12. Search for C-S. Ting in:

  13. Search for Z-Q. Wang in:

  14. Search for J-P. Hu in:

  15. Search for P-H. Hor in:

  16. Search for H. Ding in:

  17. Search for S. H. Pan in:

Contributions

J-X.Y. carried out the STM/S experiments with contributions from Z.W., J-H.W., Z-Y.Y., J.G., X-Y.H., L.S., A.L. and X-J.L.; Z.W. synthesized and characterized the sequence of samples; J-X.Y., S.H.P. and H.D. performed the data analysis, figure development and wrote the paper with contributions from J-P.H., Z-Q.W., C-S.T., P-H.H., J.L. and X-X.W.; S.H.P. supervised the project. All authors discussed the results and the interpretation.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to S. H. Pan.

Supplementary information

PDF files

  1. 1.

    Supplementary Information

    Supplementary Information

About this article

Publication history

Received

Accepted

Published

DOI

https://doi.org/10.1038/nphys3371

Further reading