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The complex nature of superconductivity in MgB2 as revealed by the reduced total isotope effect

Nature volume 411, pages 457–460 (2001)Cite this article

Abstract

Magnesium diboride, MgB2, was recently observed to become superconducting1 at 39 K, which is the highest known transition temperature for a non-copper-oxide bulk material. Isotope-effect measurements, in which atoms are substituted by isotopes of different mass to systematically change the phonon frequencies, are one of the fundamental tests of the nature of the superconducting mechanism in a material. In a conventional Bardeen–Cooper–Schrieffer (BCS) superconductor, where the mechanism is mediated by electron–phonon coupling, the total isotope-effect coefficient (in this case, the sum of both the Mg and B coefficients) should be about 0.5. The boron isotope effect was previously shown to be large2 and that was sufficient to establish that MgB2 is a conventional superconductor, but the Mg effect has not hitherto been measured. Here we report the determination of the Mg isotope effect, which is small but measurable. The total reduced isotope-effect coefficient is 0.32, which is much lower than the value expected for a typical BCS superconductor. The low value could be due to complex materials properties, and would seem to require both a large electron–phonon coupling constant and a value of μ* (the repulsive electron–electron interaction) larger than found for most simple metals.

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Figure 1: The superconducting transitions for the isotopically substituted MgB2 samples.
Figure 2: The superconducting transition temperatures for samples with different Mg isotopic masses for the 10B and 11B substituted samples.
Figure 3: The allowed values of μ* and Tc as a function of λ based on the McMillan equation if αt ranges from 0.30 to 0.34.

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Acknowledgements

This work was supported by the US Department of Energy, Office of Science.

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Authors and Affiliations

  1. Materials Science Division, Argonne National Laboratory, Argonne, 60438, Illinois, USA

    D. G. Hinks, H. Claus & J. D. Jorgensen

  2. Department of Physics, University of Illinois at Chicago, Chicago, 60607, Illinois, USA

    H. Claus

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Correspondence to D. G. Hinks.

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Hinks, D., Claus, H. & Jorgensen, J. The complex nature of superconductivity in MgB2 as revealed by the reduced total isotope effect. Nature 411, 457–460 (2001). https://doi.org/10.1038/35078037

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