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.

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

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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.

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Correspondence to T. Takahashi.

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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

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