Temperature evolution of subharmonic gap structures in MgB2/Nb point-contacts

We have performed point-contact spectroscopy experiments on superconducting micro-constrictions between Nb tips and high quality MgB2 pellets. We measured the temperature evolution (between 4.2 K and 300 K) of the current-voltage (I-V) and of the dynamical conductance (dI/dV-V) characteristics. Above the Nb critical temperature TCNb, the conductance of the constrictions behaves as predicted by the BTK model for S/N contacts being Nb in its normal state below TCNb, the contacts show Josephson current and subharmonic gap structures, due to multiple Andreev reflections. These observations clearly indicate the coupling of the MgB2 3D π-band with the Nb superconducting order parameter. We found Δπ = 2.4 ± 0.2 meV for the three-dimensional gap of MgB2