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

摘要

MgB2 superconducting wires with critical temperature Tc approaching 40 K wereused for preparation of MgB2/Ag and MgB2/In junctions. The differentialconductance vs. voltage characteristics of N-S junctions exhibit clearcontribution of Andreev reflection. Using modified BTK theory for s-wavesuperconductors two order parameters 4 meV and 2.6 meV have been determinedfrom temperature dependencies. Surprisingly, larger order parameter vanishes atlower temperature ~20 K than smaller one with Tc 38 K. Both the magnitudes ofthe order parameters and their critical temperatures are in good agreement withtheoretical calculations of electron-phonon coupling in MgB$_2$ carried out byLiu et al.