Superconducting gap parameters of MgB2 obtained on MgB2/Ag and MgB2/In junctions

Abstract

MgB2 superconducting wires with critical temperature Tc approaching 40 K were used for preparation of MgB2/Ag and MgB2/In junctions. The differential conductance vs. voltage characteristics of N-S junctions exhibit clear contribution of Andreev reflection. Using modified BTK theory for s-wave superconductors two order parameters 4 meV and 2.6 meV have been determined from temperature dependencies. Surprisingly, larger order parameter vanishes at lower temperature ~20 K than smaller one with Tc 38 K. Both the magnitudes of the order parameters and their critical temperatures are in good agreement with theoretical calculations of electron-phonon coupling in MgB$_2$ carried out by Liu et al.