Characterization of Josephson and quasi-particle currents in MgB ₂/MgB₂ and Pb/Pb contact junctions

The electrical properties of Josephson junctions formed by pressing two oxidized MgB₂ thin films together were measured. This fabrication method allowed us to characterize the properties of MgB₂ junctions with native or thermal oxide barriers without exposing the barrier and bottom electrode to the high temperature deposition process required to synthesize the top MgB₂ electrode in the conventional trilayer junctions. These junctions, with electrode T_cs of ∼ 32K, have critical currents up to ∼ 25K, broadened energy gaps at over 30K, subharmonic gap structures at moderate voltages (∼0.6-2.0mV) and Fiske modes at low voltage (<0.6mV). We show that this method can be used to quantify the barrier properties and the extent of suppression of the superconductor order parameter at the surface. Our results suggest that the near interface superconducting order parameter is reduced at both native and thermal oxide surfaces, with a greater reduction at the thermal oxide surface. For comparison, the same experimental equipment and protocol were used to investigate Pb contact junctions. The junctions containing a thermal oxide barrier were found to have electrical properties similar to those of multiply connected junctions modulated by their self-field, as initially observed by Clarke and Fulton in a SLUG (superconducting low inductance undulatory galvanometer) (Clarke and Fulton 1969 J. Appl. Phys. 40 4470).