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Superconducting micro-constrictions between Nb tips and high qualityMgB$_{2}$ pellets have been realized by means of a point-contact inset, drivenby a micrometric screw. Measurements of the current-voltage characteristics andof the dynamical conductance versus bias have been performed in the temperaturerange between 4.2 K and 500 K. Above the Nb critical temperature T$_{C}^{Nb}$,the conductance of the MgB$_2$/normal-metal constrictions behaves as predictedby the BTK model for low resistance contacts while high resistance junctionsshow quasiparticle tunneling characteristics. Consistently, from the whole setof data we infer the value $\Delta_{\pi} = 2.5 \pm 0.2$ meV for thethree-dimensional gap of MgB$_2$. Below T$_{C}^{Nb}$, low resistance contactsshow Josephson current and subharmonic gap structures (SGS), due to multipleAndreev reflections. Simultaneous observations of both features, unambiguouslyindicate coupling of the 3D band of MgB$_2$ with the Nb superconducting orderparameter. We found that the temperature dependence of the Josephson criticalcurrent follows the classical Ambegaokar-Baratoff behavior with a value$I_CR_N=(2.1 \pm 0.1)$ meV at low temperatures.Comment: 8 pages, 5 figures. Replaced with published versio

physical review. b, condensed matter and materials physics

Subharmonic gap structures and Josephson effect in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">Mg</mml:mi><mml:msub><mml:mi mathvariant="normal">B</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mo>∕</mml:mo><mml:mi mathvariant="normal">Nb</mml:mi></mml:mrow></mml:math>microconstrictions

Subharmonic gap structures and Josephson effect inMgB2∕Nbmicroconstrictions