Subharmonic gap structures and Josephson effect in MgB2/Nb micro-constrictions

TL;DR

This study investigates the superconducting properties of MgB2/Nb micro-constrictions, revealing subharmonic gap structures, Josephson effects, and band coupling through detailed conductance and current-voltage measurements across a wide temperature range.

Contribution

It provides the first detailed experimental evidence of Josephson effects and subharmonic gap structures in MgB2/Nb micro-constrictions, confirming band coupling and characterizing the superconducting gaps.

Findings

Observation of Josephson current below Nb critical temperature.

Identification of the 3D gap of MgB2 as 2.5 meV.

Temperature dependence of critical current follows Ambegaokar-Baratoff law.

Abstract

Superconducting micro-constrictions between Nb tips and high quality MgB$_{2}$ 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 T$_{C}^{Nb}$, the conductance of the MgB$_2$/normal-metal constrictions behaves as predicted by the BTK model for low resistance contacts while high resistance junctions show quasiparticle tunneling characteristics. Consistently, from the whole set of data we infer the value $\Delta_{\pi} = 2.5 \pm 0.2$ meV for the three-dimensional gap of MgB$_2$. Below T$_{C}^{Nb}$, low resistance contacts show Josephson current and subharmonic gap structures (SGS), due to multiple Andreev reflections. Simultaneous observations of both features, unambiguously indicate coupling of the 3D band of MgB$_2$ 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 $I_CR_N=(2.1 \pm 0.1)$ meV at low temperatures.