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.