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 T-C(Nb), the conductance of the constrictions behaves as predicted by the BTK model for S/N contacts being Nb in its normal state below T-C(Nb), the contacts show Josephson current and subharmonic gap structures, due to multiple Andreev reflections. These observations clearly indicate the coupling of the MgB2 3D pi-band with the Nb superconducting order parameter. We found Delta(pi) = 2.4 +/- 0.2 meV for the three-dimensional gap of MgB2. (C) 2007 Elsevier B.V. All rights reserved.