No 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 ∆S = 2.45 ± 0.15 meV and ∆L = 7.0 ± 0.45 meV in MgB2 with gap ratio ∆L/∆S = 2.85 ± 0.15. Our observations prove further a widely discussed multigap scenario for MgB2, when two distinct gaps are seen in the clean limit, where a single averaged gap is present in the dirty one.

The superconducting order parameter and electron-phonon interaction (EPI) in a MgB2 c-axis oriented thin film are investigated by point contacts (PCs). Two superconducting gaps ∆ Small ≈ 2.6 meV and ∆Large ≈ 7.4 meV have been established, which fairly well correspond to the theoretical gap ratio 1:3. The reconstructed PC EPI function along c-axis has above 30 meV similarities with the phonon DOS, while a pronounced maximum around 20 meV is resolved pointing out that the low frequency phonon mode can be of importance for pairing. We have estimated the Fermi velocity in MgB2 about 5×10 7 cm/s.

The possibility to reconstruct the electron-phonon interaction (EPI) function was demonstrated for S − c − N and S − c − S point contacts using the superconducting inelastic contribution to the excess current caused by Andreev reflection processes. Superconductors with both weak (Sn, Al) and strong (Pb, In) EPI were considered. It was shown that in the latter case it is necessary to account for the elastic component of current which is related to the frequency dependence of the superconducting energy gap arising due to electron-phonon renormalization of the energy spectrum of the superconductor.