Josephson ac and Step Structure in the Supercurrent Tunneling Characteristic

Superconductive tunneling junctions exhibiting the Josephson effect will carry appreciable currents (50 mA) with no voltage drop in the absence of any magnetic field. In applied fields of a few gauss the current-voltage characteristic becomes steplike, with appreciable currents now being carried in constant-voltage steps at regularly spaced voltages. It is shown that these steps are associated with resonant electro-magnetic modes of the junction, which acts as an open-ended cavity. The lowest frequency mode in junctions made so far occurs at 5 kMc/sec, and is set by the junction dimensions. The highest mode occurs near the half-energy gap of the superconductor used for the junctions---for tin, about 250 kMc/sec.