Point-contact spectroscopy of magnesium diboride with different counter-electrodes

We report on tunneling and Andreev-reflection conductance spectra of 39 K superconducting magnesium diboride, obtained with Pb and Au counter-electrodes. Two distinct steps at close to 2.7 and 7.1 meV appear in a low-resistance metallic-type Au-MgB2 junction characteristic, whereas a tunneling-like spectrum measured for the same junction, annealed by the application of dc current, exhibits only a rounded contribution of the larger gap. Junctions with a superconducting lead counter-electrode pressed into a bulk MgB2 sample reveal two conductance peaks that are interpreted as the result of the formation of highly-transmitting break junctions inside the magnesium diboride ceramic. Our results strongly support the two-band model with two different gap values on quasi-two-dimensional \sigma (7.1 meV) and three-dimensional \pi (2.7 meV) Fermi surface sheets of MgB2.