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VOLUME 75 (2002) | ISSUE 5 | PAGE 283
Superconducting energy gap distribution in c-axis oriented MgB2 thin film from point contact study
PACS: 73.40.Jn, 74.76.Db, 74.80.Fp
Abstract
We have analyzed about hundred voltage dependent differential resistance dV/dI(V) curves of metallic point contacts between c-axis oriented MgB2 thin film and Ag, which exhibit clear Andreev reflection features connected with the superconducting gap. About one half of the curves show the presence of a second larger gap. The histogram of the double gap distribution reveals distinct maxima at 2.4 and 7 meV, while curves with a single gap features result in more broad maximum at 3.5 meV. The double gap distribution is in qualitative agreement with the distribution of gap values over the Fermi surface calculated by H. J. Choi et al. (cond-mat/0111183). The data unequivocally show the presence of two gaps \Delta _{S}=2.45\pm 0.15 meV and \Delta _{L}=7.0\pm 0.45 meV in MgB2 with gap ratio \Delta _{L}/\Delta _{S}=2.85\pm 0.15. Our observations prove further a widely discussed multi-gap scenario for MgB2, when two distinct gaps are seen in the clean limit, while a single averaged gap is present in the dirty one.