Abstract
Superconducting microconstrictions between Nb tips and high-quality 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 and . Above the Nb critical temperature , the conductance of the /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 for the three-dimensional (3D) gap of . Below , 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 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 at low temperatures.
- Received 4 August 2005
DOI:https://doi.org/10.1103/PhysRevB.72.174518
©2005 American Physical Society