Point-contact spectroscopy of magnesium diboride with different counter-electrodes

TL;DR

This study investigates the superconducting gaps of magnesium diboride using tunneling and Andreev-reflection spectroscopy with different counter-electrodes, providing evidence for the two-band model with distinct gap values.

Contribution

It presents experimental tunneling spectra with various electrodes, supporting the two-band superconductivity model in MgB2 with detailed gap measurements.

Findings

Identification of two distinct superconducting gaps at 2.7 and 7.1 meV.

Observation of different conductance features depending on junction type.

Evidence supporting the two-band superconductivity in MgB2.

Abstract

We report on tunneling and Andreev-reflection conductance spectra of 39 K superconducting magnesium diboride, obtained with Pb and Au counter-electrodes. Two distinct steps at close to 2.7 and 7.1 meV appear in a low-resistance metallic-type Au-MgB2 junction characteristic, whereas a tunneling-like spectrum measured for the same junction, annealed by the application of dc current, exhibits only a rounded contribution of the larger gap. Junctions with a superconducting lead counter-electrode pressed into a bulk MgB2 sample reveal two conductance peaks that are interpreted as the result of the formation of highly-transmitting break junctions inside the magnesium diboride ceramic. Our results strongly support the two-band model with two different gap values on quasi-two-dimensional \sigma (7.1 meV) and three-dimensional \pi (2.7 meV) Fermi surface sheets of MgB2.