Abstract
The important influence of multiple gaps in the superconductivity of MgB2 and Fe-based compounds, especially because of the possibility that manipulation of a second gap can significantly raise the upper critical field Hc2, has refocused attention on Nb3Sn because anomalies in both specific heat and point-contact tunneling studies have led to the proposal that Nb3Sn is also a two-gap superconductor. Here, we search for evidence of the second gap in a careful study of the influence of the homogenization temperature on the sample uniformity. We show that it is very difficult to fabricate samples that are both homogeneous and stoichiometric. We find so-called “second-gap” anomalies disappear only after high temperature and long-term annealing. Such a well-annealed sample shows only a strong, electron-phonon-coupled, single-gap behavior. In contrast, samples reacted and annealed at lower temperatures, as in the earlier two-gap studies, show small chemical composition variations of the A15 phase. We propose that the second gap sightings are actually due to variation of Tc within very difficult-to-fully homogenize samples. A curiosity of the A15 Nb3Sn phase is that almost any mixture of Nb and Sn tries to form a stoichiometric A15 composition, but the residue of course contains off-stoichiometric A15, Nb, and other phases when the Nb:Sn ratio departs from the true 3:1 stoichiometry.
Highlights
The important influence of multiple gaps in the superconductivity of MgB2 and Febased compounds, especially because of the possibility that manipulation of a second gap can significantly raise the upper critical field Hc2, has refocused attention on Nb3Sn because anomalies in both specific heat and point-contact tunneling studies have led to the proposal that Nb3Sn is a two-gap superconductor
We propose that the second gap sightings are due to variation of Tc within very difficult-to-fully homogenize samples
While the theory of multiple gap superconductivity had been developed previously, MgB2 was the first example in which multiple gap behavior was clearly observed and where it was essential to understand the superconducting properties, even as the otherwise conventional BCS electron-phonon mechanism was confirmed both theoretically and experimentally.[2]
Summary
Samples reacted and annealed at lower temperatures, as in the earlier two-gap studies, show small chemical composition variations of the A15 phase. Several crystals from the same batch were used for specific heat measurement in fields up to 18 T and de Haas-van Alphen oscillation studies of the Fermi surfaces.[16,22] Clearly, the nature of the superconducting gap should be sample-independent if samples of Nb3Sn are well defined.
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