Anomalous RF induced steps in resonant Josephson tunnel junctions for voltage standard maintenance

The authors theoretically investigated the current steps in the I-V (current-voltage) characteristics of a long Josephson tunnel junction driven by microwaves at frequencies near its geometrical resonance. They used a numerical algorithm based on a lossy transmission line model of the Josephson tunnel junction to calculate the DC current density distribution. In addition to the fundamental microwave frequency, harmonic contents of the tunneling current are also considered. The calculations show a strong asymmetric behavior of the current steps with respect to the quasi-particle curve. It is suggested that the excitation of higher harmonic waves gives rise to these anomalous microwave-induced steps. Due to the nonuniform DC-tunneling-current density distribution, the amplitudes of these current steps are not usually larger than 10% of the critical current. Since the critical current in long junctions is of the order of milliamperes, these amplitudes are considered relatively large