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Schlagwörter:
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Zusammenfassung:
The characteristics of Fe-based superconductors are manifested in their
electronic, magnetic properties, and pairing symmetry of the Cooper
pair, but the latter remain to be explored. Usually in these materials,
superconductivity coexists and competes with magnetic order, giving
unconventional pairing mechanisms. We report on the results of the bulk
magnetization measurements in the superconducting state and the
low-temperature specific heat down to 0.4 K for BaFe2-xNixAs2 single
crystals. The electronic specific heat displays a pronounced anomaly at
the superconducting transition temperature and a small residual part at
low temperatures in the superconducting state. The normal-state
Sommerfeld coefficient increases with Ni doping for x = 0.092, 0.096,
and 0.10, which illustrates the competition between magnetism and
superconductivity. Our analysis of the temperature dependence of the
superconducting-state specific heat and the London penetration depth
provides strong evidence for a two-band s-wave order parameter. Further,
the data of the London penetration depth calculated from the lower
critical field follow an exponential temperature dependence,
characteristic of a fully gapped superconductor. These observations
clearly show that the superconducting gap in the nearly optimally doped
compounds is nodeless.
