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

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 ofmore » course contains off-stoichiometric A15, Nb, and other phases when the Nb:Sn ratio departs from the true 3:1 stoichiometry.« less
Authors:
 [1] ; ; ; ;  [2]
  1. Department of Physics, Kyungpook National University, Daegu 702-701 (Korea, Republic of)
  2. National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310 (United States)
Publication Date:
OSTI Identifier:
22499203
Resource Type:
Journal Article
Resource Relation:
Journal Name: APL Materials; Journal Volume: 2; Journal Issue: 10; Other Information: (c) 2014 Author(s); Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ANNEALING; CHEMICAL COMPOSITION; CRITICAL FIELD; ELECTRIC CONTACTS; ELECTRONS; MAGNESIUM BORIDES; MIXTURES; PHONONS; PROPOSALS; SPECIFIC HEAT; SUPERCONDUCTIVITY; SUPERCONDUCTORS; TUNNEL EFFECT