Specific heat of Nb3Sn: The case for a second energy gap
Abstract
We present new specific heat data for Nb3Sn, a well-known technically applied superconductor with a critical temperature Tc ~ 18 K, in the temperature range from 1.2 to 200 K in zero magnetic field, and from 1.5 to 22 K in fields H <= 16 T. The particularly dense and homogeneous polycrystalline sample used for this study is characterized in detail. We determine the bulk upper critical field Hc2(T) from specific heat data, and the Sommerfeld constant Gamma from the entropy S(T). We investigate in detail a low-temperature anomaly already noticed in previous investigations in zero field, and find that this feature can be quantitatively ascribed to the presence of a second superconducting gap 2 DeltaS(0) ~ 0.8 kB Tc, in addition to the main one 2 DeltaL(0) ~ 4.9 kB Tc. The signature of this minor gap, which affects 7.5% of the electronic density-of-states, vanishes in fields above ~ 7 T.
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