Correlation between Types of Junction and Submillimeter-Wave Responses in Point-Contact Josephson Junctions

It has been found that point-contact Josephson junctions can be clearly classified into bridge-type and tunnel-type by introducing new parameters characterizing their I-V curves. Nearly-ideal tunnel-type junctions with Nb–SuOx–Sn (Film) point-contacts were obtained. Tunnel-type junctions showed larger gap energies than bridge-type junctions in Nb–SnOx–Sn and Nb–NbOx–NbN point-contacts. The submillimeter-wave responses were measured in Nb–Nb point-contacts, and it was found that the contacts with the best performance were bridge-type junctions with a sharp gap structure; tunnel-type junctions showed rather bad performance. On irradiation with laser light, the center of the Shapiro step moved to the lower-current side in bridge-type junctions but to the higher-current side in tunnel-type junctions. These differences can be explained by differences in the heating effects between bridge-type and tunnel-type junctions.