Specific heat of Nb{sub 3}Sn: The case for a single gap

Journal Article · Wed Oct 01 04:00:00 EDT 2014 · APL Materials

DOI:https://doi.org/10.1063/1.4896935· OSTI ID:22499203

Zhou, Jian; Sung, Zu Hawn; Lee, Peter J.; Larbalestier, D. C. ^[1]

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1. National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310 (United States)

The important influence of multiple gaps in the superconductivity of MgB{sub 2} and Fe-based compounds, especially because of the possibility that manipulation of a second gap can significantly raise the upper critical field H{sub c2}, has refocused attention on Nb{sub 3}Sn because anomalies in both specific heat and point-contact tunneling studies have led to the proposal that Nb{sub 3}Sn 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 T{sub c} within very difficult-to-fully homogenize samples. A curiosity of the A15 Nb{sub 3}Sn 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.

• OSTI ID: 22499203
• Journal Information: APL Materials, Journal Name: APL Materials Journal Issue: 10 Vol. 2; ISSN AMPADS; ISSN 2166-532X
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ANNEALING
CHEMICAL COMPOSITION
CRITICAL FIELD
ELECTRIC CONTACTS
ELECTRONS
MAGNESIUM BORIDES
MIXTURES
PHONONS
PROPOSALS
SPECIFIC HEAT
SUPERCONDUCTIVITY
SUPERCONDUCTORS
TUNNEL EFFECT