Superconducting energy gap distribution in c-axis oriented MgB2 thin film from point contact study

We have analyzed about a 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 single-gap features result in a 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 +/- 0.15 meV and Delta(L) = 7.0 +/- 0.45 meV in MgB2 with the gap ratio Delta(L)/Delta(S) = 2.85 +/- 0.15. Our observations further prove a widely discussed multigap scenario for MgB2, where two distinct gaps are seen in the clean limit, while a single averaged gap is present in the dirty one. (C) 2002 MAIK Nauka / Interperiodica.