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 TCNb, 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 Δπ=2.5±0.2 meV for the three-dimensional (3D) gap of MgB2. Below TCNb, 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 ICRN=(2.1±0.1) meV at low temperatures.
Subharmonic gap structures and Josephson effect in MgB2 /Nb microconstrictions
GIUBILEO, Filippo;BOBBA, Fabrizio;PIANO, SAMANTA;SCARFATO, Alessandro;CUCOLO, Anna Maria
2005
Abstract
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 TCNb, 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 Δπ=2.5±0.2 meV for the three-dimensional (3D) gap of MgB2. Below TCNb, 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 ICRN=(2.1±0.1) meV at low temperatures.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
