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.
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Nagamatsu, J., Nakagawa, N., Muranaka, T., Zenitani, Y. & Akimitsu, J. Superconductivity at 39 K in magnesium diboride. Nature 410, 63–64 (2001)
Bardeen, J., Cooper, L. N. & Schrieffer, J. R. Theory of superconductivity. Phys. Rev. 108, 1175–1204 (1957)
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)
Bouquet, F. et al. Specific heat of single crystal MgB2: a two-band superconductor with two different anisotropies. Phys. Rev. Lett. 89, 257001 (2002)
Tsuda, S. et al. Evidence for a multiple superconducting gap in MgB2 from high-resolution photoemission spectroscopy. Phys. Rev. Lett. 87, 177006 (2001)
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)
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).
Szabó, P. et al. Evidence for two superconducting energy gaps in MgB2 by point-contact spectroscopy. Phys. Rev. Lett. 87, 137005 (2001)
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)
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)
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)
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)
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)
Takahashi, T., Sato, T., Souma, S., Muranaka, T. & Akimitsu, J. High-resolution photoemission study of MgB2 . Phys. Rev. Lett. 86, 4915–4917 (2001)
Bud'ko, S. L. et al. Boron isotope effect in superconducting MgB2 . Phys. Rev. Lett. 86, 1877–1880 (2001)
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)
Buzea, C. & Yamashita, T. Review of the superconducting properties of MgB2 . Supercond. Sci. Technol. 14, R115–R146 (2001)
Joas, C., Eremin, I., Manske, D. & Bennemann, K. H. Theory for phonon-induced superconductivity in MgB2 . Phys. Rev. B 65, 132518 (2002)
McMillan, W. L. Tunneling model of the superconducting proximity effect. Phys. Rev. 175, 537–542 (1968)
Seneor, P. et al. Spectroscopic evidence for anisotropic s-wave pairing symmetry in MgB2 . Phys. Rev. B 65, 012505 (2001)
Uchiyama, H. et al. Electronic structure of MgB2 from angle-resolved photoemission spectroscopy. Phys Rev. Lett. 88, 157002 (2002)
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)
Fedorov, A. V. et al. Temperature dependent photoemission studies of optimally doped Bi2Sr2CaCu2O8 . Phys. Rev. Lett. 82, 2179–2182 (1999)
Ding, H. et al. Momentum dependence of the superconducting gap in Bi2Sr2CaCu2O8 . Phys. Rev. Lett. 74, 2784–2787 (1995)
Reinert, F. et al. Observation of a BCS spectral function in a conventional superconductor by photoelectron spectroscopy. Phys. Rev. Lett. 87, 047001 (2001)
Machida, Y. et al. Ambient-pressure synthesis of single-crystal MgB2 and their superconducting anisotropy. Phys. Rev. B 67, 094507 (2003)
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.
The authors declare that they have no competing financial interests.
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
Nature Communications (2021)
npj Computational Materials (2019)
Free surfaces recast superconductivity in few-monolayer MgB2: Combined first-principles and ARPES demonstration
Scientific Reports (2017)
Scientific Reports (2017)
Size and symmetry of the superconducting gap in the f.c.c. Cs3C60 polymorph close to the metal-Mott insulator boundary
Scientific Reports (2015)