Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

Observation of ordered vortices with Andreev bound states in Ba0.6K0.4Fe2As2

Nature Physics volume 7, pages 325–331 (2011)Cite this article

Abstract

For a type-II superconductor in an applied magnetic field greater than the lower critical field Hc1, the magnetic flux will penetrate into the superconductor and form quantized vortices with Andreev bound states in the vortex cores. The characteristics of the bound states are related to the pairing symmetry and band structure of the superconductor. Recently, a new family of high temperature (high-Tc) superconductors, the iron pnictides, has been discovered. Surprisingly, in electron-doped Ba(Fe1−xCox)2As2, no bound state was found in the vortices. Here, we use a low-temperature scanning tunnelling microscope to study the electronic structure of hole-doped Ba0.6K0.4Fe2As2. Two superconducting gaps (with gap values 2Δ/kBTc≈2.2 and 5.1, where kB is Boltzmann’s constant) were observed in the superconducting state. By applying magnetic fields, we observed ordered vortices with Andreev bound states in the vortex cores. The bound states and their spatial evolution can be qualitatively explained by our numerical calculations for multiband s-wave superconductivity.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Figure 1: Topographic STM image of the Ba0.6K0.4Fe2As2 single crystal cleaved in situ.
Figure 2: Spatially resolved spectra in zero magnetic field at 2 K.
Figure 3: Comparison between the bright and dark areas.
Figure 4: Fitting experimental data to a two-gap model with a larger superconducting gap ΔL(θ) and a smaller one ΔS(θ).
Figure 5: Vortex lattice observed in Ba0.6K0.4Fe2As2 at 2 K.
Figure 6: Vortex core states of Ba0.6K0.4Fe2As2, all spectra were taken at 2 K.

Similar content being viewed by others

References

  1. Kamihara, Y., Watanabe, T., Hirano, M. & Hosono, H. Iron-based layered superconductor La[O1−xFx]FeAs (x=0.05–0.12) with Tc=26 K. J. Am. Chem. Soc. 130, 3296–3297 (2008).

    Article  Google Scholar 

  2. Mazin, I. I., Singh, D. J., Johannes, M. D. & Du, M. H. Unconventional sign-reversing superconductivity in LaFeAsO1−xFx . Phys. Rev. Lett. 101, 057003 (2008).

    Article  ADS  Google Scholar 

  3. Caroli, C., de Gennes, P. G. & Matricon, J. Bound fermion states on a vortex line in a type II superconductor. Phys. Lett. 9, 307–309 (1964).

    Article  ADS  Google Scholar 

  4. Hess, H. F., Robinson, R. B., Dynes, R. C., Valles, J. M. Jr & Waszczak, J. V. Scanning-tunnelling-microscope observation of the Abrikosov flux lattice and the density of states near and inside a fluxoid. Phys. Rev. Lett. 62, 214–216 (1989).

    Article  ADS  Google Scholar 

  5. Shore, J. D., Huang, M., Dorsey, A. T. & Sethna, J. P. Density of states in a vortex core and the zero-bias tunneling peak. Phys. Rev. Lett. 62, 3089–3092 (1989).

    Article  ADS  Google Scholar 

  6. Hess, H. F., Robinson, R. B. & Waszczak, J. V. Vortex-core structure observed with a scanning tunneling microscope. Phys. Rev. Lett. 64, 2711–2714 (1990).

    Article  ADS  Google Scholar 

  7. Gygi, F. & Schlüter, M. Electronic tunneling into an isolated vortex in a clean type-II superconductor. Phys. Rev. B 41, 822–825 (1990).

    Article  ADS  Google Scholar 

  8. Hu, X., Ting, C. S. & Zhu, J. X. Vortex core states in a minimal two-band model for iron-based superconductors. Phys. Rev. B 80, 014523 (2009).

    Article  ADS  Google Scholar 

  9. Fischer, Ø., Kugler, M., Maggio-Aprile, I., Berthod, C. & Renner, C. Scanning tunneling spectroscopy of high-temperature superconductors. Rev. Mod. Phys. 79, 353–419 (2007).

    Article  ADS  Google Scholar 

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

    Article  ADS  Google Scholar 

  11. Eskildsen, M. R. et al. Vortices in superconducting Ba(Fe0.93Co0.07)2As2 studied via small-angle neutron scattering and bitter decoration. Phys. Rev. B 79, 100501(R) (2009).

    Article  ADS  Google Scholar 

  12. Luan, L. et al. Local measurement of penetration depth in the pnictide superconductor Ba(Fe0.95Co0.05)2As2 . Phys. Rev. B 81, 100501(R) (2010).

    Article  ADS  Google Scholar 

  13. Hanaguri, T., Niitaka, S., Kuroki, K. & Takagi, K. Unconventional s-wave superconductivity in Fe(Se, Te). Science 328, 474–476 (2010).

    Article  ADS  Google Scholar 

  14. Wray, L. et al. Momentum dependence of superconducting gap, strong-coupling dispersion kink, and tightly bound Cooper pairs in the high-Tc(Sr,Ba)1−x(K,Na)xFe2As2 superconductors. Phys. Rev. B 78, 184508 (2008).

    Article  ADS  Google Scholar 

  15. Nascimento, V. B. et al. Surface geometric and electronic structure of BaFe2As2(001). Phys. Rev. Lett. 103, 076104 (2009).

    Article  ADS  Google Scholar 

  16. Niestemski, F. C. et al. Unveiling the atomic and electronic structure at the surface of the parent pnictide SrFe2As2. Preprint at http://arxiv.org/abs/0906.2761 (2009).

  17. Massee, F. 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).

    Article  ADS  Google Scholar 

  18. Chuang, T-M. et al. Nematic electronic structure in the ‘parent’ state of the iron-based superconductor Ca(Fe1−xCox)2As2 . Science 327, 181–184 (2010).

    Article  ADS  Google Scholar 

  19. Ding, H. et al. Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2 . Europhys. Lett. 83, 47001 (2008).

    Article  ADS  Google Scholar 

  20. Ren, C. et al. Evidence for two energy gaps in superconducting Ba0.6K0.4Fe2As2 single crystals and the breakdown of the Uemura plot. Phys. Rev. Lett. 101, 257006 (2008).

    Article  ADS  Google Scholar 

  21. Evtushinsky, D. V. et al. Momentum dependence of the superconducting gap in Ba1−xKxFe2As2 . Phys. Rev. B 79, 054517 (2009).

    Article  ADS  Google Scholar 

  22. Zhang, Y. et al. Out-of-plane momentum and symmetry-dependent energy gap of the pnictide Ba0.6K0.4Fe2As2 superconductor revealed by angle-resolved photoemission spectroscopy. Phys. Rev. Lett. 105, 117003 (2010).

    Article  ADS  Google Scholar 

  23. Dynes, R. C., Garno, J. P., Hertel, G. B. & Orlando, T. P. Tunneling study of superconductivity near the metal–insulator transition. Phys. Rev. Lett. 53, 2437–2440 (1984).

    Article  ADS  Google Scholar 

  24. Alldredge, J. W. et al. Evolution of the electronic excitation spectrum with strongly diminishing hole density in superconducting Bi2Sr2CaCu2O8+δ . Nature Phys. 4, 319–326 (2008).

    Article  Google Scholar 

  25. Chubukov, A. V. & Eremin, I. Angle-resolved specific heat in iron-based superconductors: The case for a nodeless extended s-wave gap. Phys. Rev. B 82, 060504(R) (2010).

    Article  ADS  Google Scholar 

  26. Yuan, H. Q. et al. Nearly isotropic superconductivity in (Ba,K)Fe2As2 . Nature 457, 565–568 (2009).

    Article  ADS  Google Scholar 

  27. Maggio-Aprile, I., Renner, C., Erb, A., Walker, E. & Fischer, Ø. et al. Direct vortex lattice imaging and tunneling spectroscopy of flux lines on YBa2Cu3O7−δ . Phys. Rev. Lett. 75, 2754–2757 (1995).

    Article  ADS  Google Scholar 

  28. Pan, S. H. et al. STM studies of the electronic structure of vortex cores in Bi2Sr2CaCu2O8+δ . Phys. Rev. Lett. 85, 1536–1539 (2000).

    Article  ADS  Google Scholar 

  29. Hoogenboom, B. W., Renner, C., Revaz, B., Maggio-Aprile, I. & Fischer, Ø. Low-energy structures in vortex core tunneling spectra in Bi2Sr2CaCu2O8+δ . Physica C 332, 440–444 (2000).

    Article  ADS  Google Scholar 

  30. Shibata, K., Maki, M., Nishizaki, T. & Kobayashi, N. Scanning tunneling spectroscopy studies on vortices in YBa2Cu3Oy single crystals. Physica C 392–396, 323–327 (2003).

    Article  ADS  Google Scholar 

  31. Levy, G., Kugler, M., Manuel, A. A., Fischer, Ø. & Li, M. Fourfold structure of vortex-core states in Bi2Sr2CaCu2O8+δ . Phys. Rev. Lett. 95, 257005 (2005).

    Article  ADS  Google Scholar 

  32. Hayashi, N., Isoshima, T., Ichioka, M. & Machida, K. Low-lying quasiparticle excitations around a vortex in quantum limit. Phys. Rev. Lett. 80, 2921–2924 (1998).

    Article  ADS  Google Scholar 

  33. Nishimori, H. et al. First observation of the fourfold-symmetric and quantum regime vortex core in YNi2B2C by scanning tunneling microscopy and spectroscopy. J. Phys. Soc. Jpn 73, 3247–3250 (2004).

    Article  ADS  Google Scholar 

  34. Gao, Y., Huang, H-X., Chen, C., Ting, C. S. & Su, W-P. Vortex states in hole-doped iron-pnictide superconductors. Preprint at http://arxiv.org/abs/1008.3885 (2010).

Download references

Acknowledgements

We thank D. H. Lee, K. Kuroki, T. Xiang and T. Hanaguri for comments and suggestions. This work was supported by the Natural Science Foundation of China (No. 10774170, No. 10974086, No. 10734120), the Ministry of Science and Technology of China (973 Projects No. 2011CBA00100, No. 2011CB922101, No. 2010CB923002), and the Chinese Academy of Sciences.

Author information

Authors and Affiliations

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

    Lei Shan, Yong-Lei Wang, Bing Shen, Bin Zeng, Yan Huang, Huan Yang, Cong Ren & Hai-Hu Wen

  2. Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204-5002, USA

    Ang Li & Shuheng H. Pan

  3. National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China

    Da Wang, Qiang-Hua Wang & Hai-Hu Wen

Authors
  1. Lei Shan
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Yong-Lei Wang
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Bing Shen
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Bin Zeng
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. Yan Huang
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. Ang Li
    View author publications

    You can also search for this author in PubMed Google Scholar

  7. Da Wang
    View author publications

    You can also search for this author in PubMed Google Scholar

  8. Huan Yang
    View author publications

    You can also search for this author in PubMed Google Scholar

  9. Cong Ren
    View author publications

    You can also search for this author in PubMed Google Scholar

  10. Qiang-Hua Wang
    View author publications

    You can also search for this author in PubMed Google Scholar

  11. Shuheng H. Pan
    View author publications

    You can also search for this author in PubMed Google Scholar

  12. Hai-Hu Wen
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

The low-temperature STM was constructed by L.S., Y-L.W. and Y.H. with the help of S.H.P., A.L. and H-H.W., B.S. and B.Z. prepared the samples and measured the sample quality. L.S. designed and performed STM experiments and analysed data. Y-L.W. performed STM experiments and analysed data. D.W. and Q-H.W. contributed to numerical calculations. H-H.W. coordinated the whole work, contributed to sample preparation, STM experiments and data analysis. The paper was written jointly by L.S., H-H.W., Q-H.W. and S.H.P. All authors have discussed the results and the interpretation.

Corresponding authors

Correspondence to Lei Shan or Hai-Hu Wen.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Information (PDF 479 kb)

Rights and permissions

Reprints and permissions

About this article

Cite this article

Shan, L., Wang, YL., Shen, B. et al. Observation of ordered vortices with Andreev bound states in Ba0.6K0.4Fe2As2. Nature Phys 7, 325–331 (2011). https://doi.org/10.1038/nphys1908

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nphys1908

This article is cited by

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing