Subharmonic gap structures and Josephson effect in MgB₂∕Nb microconstrictions

Superconducting microconstrictions between Nb tips and high-quality MgB₂ pellets have been realized by means of a point-contact inset, driven by a micrometric screw. Measurements of the current-voltage characteristics and of the dynamical conductance versus bias have been performed in the temperature range between 4.2 K and 500 K. Above the Nb critical temperature T_C^Nb, the conductance of the MgB₂/normal-metal constrictions behaves as predicted by the Blonder-Tinkham-Klapwijk model for low-resistance contacts while high-resistance junctions show quasiparticle tunneling characteristics. Consistently, from the whole set of data we infer the value Δ_π=2.5±0.2 meV for the three-dimensional (3D) gap of MgB₂. Below T_C^Nb, low-resistance contacts show Josephson current and subharmonic gap structures, due to multiple Andreev reflections. Simultaneous observations of both features unambiguously indicate a coupling of the 3D band of MgB₂ with the Nb superconducting order parameter. We found that the temperature dependence of the Josephson critical current follows the classical Ambegaokar-Baratoff behavior with a value I_CR_N=(2.1±0.1) meV at low temperatures.