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.

  • Letter
  • Published:

The origin of multiple superconducting gaps in MgB2

Abstract

Magnesium diboride, MgB2, has the highest transition temperature (Tc = 39 K) of the known metallic superconductors1. Whether the anomalously high Tc can be described within the conventional BCS (Bardeen–Cooper–Schrieffer) framework2 has been debated. The key to understanding superconductivity lies with the ‘superconducting energy gap’ associated with the formation of the superconducting pairs. Recently, the existence of two kinds of superconducting gaps in MgB2 has been suggested by several experiments3,4,5,6,7,8,9; this is in contrast to both conventional and high-Tc superconductors. A clear demonstration of two gaps has not yet been made because the previous experiments lacked the ability to resolve the momentum of the superconducting electrons. Here we report direct experimental evidence for the two-band superconductivity in MgB2, by separately observing the superconducting gaps of the σ and π bands (as well as a surface band). The gaps have distinctly different sizes, which unambiguously establishes MgB2 as a two-gap superconductor10,11.

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

Access options

Rent or buy this article

Prices vary by article type

from$1.95

to$39.95

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

Figure 1: Experimental band structure near EF of MgB2 obtained by ARPES.
Figure 2: Temperature dependence of ARPES spectra near EF of MgB2.

Similar content being viewed by others

References

  1. Nagamatsu, J., Nakagawa, N., Muranaka, T., Zenitani, Y. & Akimitsu, J. Superconductivity at 39 K in magnesium diboride. Nature 410, 63–64 (2001)

    Article  ADS  CAS  Google Scholar 

  2. Bardeen, J., Cooper, L. N. & Schrieffer, J. R. Theory of superconductivity. Phys. Rev. 108, 1175–1204 (1957)

    Article  ADS  MathSciNet  CAS  Google Scholar 

  3. Bouquet, F., Fisher, R. A., Phillips, N. E., Hinks, D. G. & Jorgensen, J. D. Specific heat of Mg11B2: evidence for a second energy gap. Phys. Rev. Lett. 87, 047001 (2001)

    Article  ADS  CAS  Google Scholar 

  4. Bouquet, F. et al. Specific heat of single crystal MgB2: a two-band superconductor with two different anisotropies. Phys. Rev. Lett. 89, 257001 (2002)

    Article  ADS  CAS  Google Scholar 

  5. Tsuda, S. et al. Evidence for a multiple superconducting gap in MgB2 from high-resolution photoemission spectroscopy. Phys. Rev. Lett. 87, 177006 (2001)

    Article  ADS  CAS  Google Scholar 

  6. Chen, X. K., Konstantinović, M. J., Irwin, J. C., Lawrie, D. D. & Franck, J. P. Evidence for two superconducting gaps in MgB2 . Phys. Rev. Lett. 87, 157002 (2001)

    Article  ADS  CAS  Google Scholar 

  7. Quilty, J. W., Lee., S., Tajima, S. & Yamanaka, A. c-axis Raman scattering in MgB2: observation of a dirty-limit gap in the π-bands. Preprint cond-mat/0206506 at 〈http://xxx.lanl.gov〉 (2002).

  8. Szabó, P. et al. Evidence for two superconducting energy gaps in MgB2 by point-contact spectroscopy. Phys. Rev. Lett. 87, 137005 (2001)

    Article  ADS  Google Scholar 

  9. Gonnelli, R. S. et al. Direct evidence for two-band superconductivity in MgB2 single crystals from directional point-contact spectroscopy in magnetic fields. Phys. Rev. Lett. 89, 247004 (2002)

    Article  ADS  CAS  Google Scholar 

  10. Liu, A. Y., Mazin, I. I. & Kortus, J. Beyond Eliashberg superconductivity in MgB2: anharmonicity, two-phonon scattering, and multiple gaps. Phys. Rev. Lett. 87, 087005 (2001)

    Article  ADS  CAS  Google Scholar 

  11. Choi, H. J., Roundy, D., Sun, H., Cohen, M. L. & Louie, S. G. The origin of the anomalous superconducting properties of MgB2 . Nature 418, 758–760 (2002)

    Article  ADS  CAS  Google Scholar 

  12. Karapetrov, G., Iavarone, M., Kwok, W. K., Crabtree, G. W. & Hinks, D. G. Scanning tunneling spectroscopy in MgB2 . Phys. Rev. Lett. 86, 4374–4377 (2001)

    Article  ADS  CAS  Google Scholar 

  13. Schmidt, H., Zasadzinski, J. F., Gray, K. E. & Hinks, D. G. Energy gap from tunneling and metallic contacts onto MgB2: possible evidence for a weakened surface layer. Phys. Rev. B 63, 220504 (2001)

    Article  ADS  Google Scholar 

  14. Takahashi, T., Sato, T., Souma, S., Muranaka, T. & Akimitsu, J. High-resolution photoemission study of MgB2 . Phys. Rev. Lett. 86, 4915–4917 (2001)

    Article  ADS  CAS  Google Scholar 

  15. Bud'ko, S. L. et al. Boron isotope effect in superconducting MgB2 . Phys. Rev. Lett. 86, 1877–1880 (2001)

    Article  ADS  CAS  Google Scholar 

  16. Kotegawa, H., Ishida, K., Kitaoka, Y., Muranaka, T. & Akimitsu, J. Evidence for strong-coupling s-wave superconductivity in MgB2: 11B NMR study. Phys. Rev. Lett. 87, 127001 (2001)

    Article  ADS  CAS  Google Scholar 

  17. Buzea, C. & Yamashita, T. Review of the superconducting properties of MgB2 . Supercond. Sci. Technol. 14, R115–R146 (2001)

    Article  ADS  CAS  Google Scholar 

  18. Joas, C., Eremin, I., Manske, D. & Bennemann, K. H. Theory for phonon-induced superconductivity in MgB2 . Phys. Rev. B 65, 132518 (2002)

    Article  ADS  Google Scholar 

  19. McMillan, W. L. Tunneling model of the superconducting proximity effect. Phys. Rev. 175, 537–542 (1968)

    Article  ADS  Google Scholar 

  20. Seneor, P. et al. Spectroscopic evidence for anisotropic s-wave pairing symmetry in MgB2 . Phys. Rev. B 65, 012505 (2001)

    Article  ADS  Google Scholar 

  21. Uchiyama, H. et al. Electronic structure of MgB2 from angle-resolved photoemission spectroscopy. Phys Rev. Lett. 88, 157002 (2002)

    Article  ADS  CAS  Google Scholar 

  22. Kortus, J., Mazin, I. I., Belashchenko, K. D., Antropov, V. P. & Boyer, L. L. Superconductivity of metallic boron in MgB2 . Phys. Rev. Lett. 86, 4656–4659 (2001)

    Article  ADS  CAS  Google Scholar 

  23. Fedorov, A. V. et al. Temperature dependent photoemission studies of optimally doped Bi2Sr2CaCu2O8 . Phys. Rev. Lett. 82, 2179–2182 (1999)

    Article  ADS  CAS  Google Scholar 

  24. Ding, H. et al. Momentum dependence of the superconducting gap in Bi2Sr2CaCu2O8 . Phys. Rev. Lett. 74, 2784–2787 (1995)

    Article  ADS  CAS  Google Scholar 

  25. Reinert, F. et al. Observation of a BCS spectral function in a conventional superconductor by photoelectron spectroscopy. Phys. Rev. Lett. 87, 047001 (2001)

    Article  MathSciNet  Google Scholar 

  26. Machida, Y. et al. Ambient-pressure synthesis of single-crystal MgB2 and their superconducting anisotropy. Phys. Rev. B 67, 094507 (2003)

    Article  ADS  Google Scholar 

Download references

Acknowledgements

We thank J. Akimitsu for discussions. We also thank H. Höchst for experimental assistance. This work was supported by grants from the MEXT of Japan, JSPS and US NSF. S.S. thanks JSPS for financial support. The Synchrotron Radiation Center is supported by US NSF.

Author information

Authors and Affiliations

Authors

Corresponding author

Correspondence to T. Takahashi.

Ethics declarations

Competing interests

The authors declare that they have no competing financial interests.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Souma, S., Machida, Y., Sato, T. et al. The origin of multiple superconducting gaps in MgB2. Nature 423, 65–67 (2003). https://doi.org/10.1038/nature01619

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nature01619

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

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