High-frequency impedance of superconducting tunnel junctions

Comparison is made between the high-frequency impedance of a Josephson tunnel junction predicted by the RSJ model and that predicted by the Werthamer theory. Results obtained analytically, numerically, and by electronic simulation illustrate several fundamental differences between the two models. In particular, the small-signal rf resistance of a tunnel junction described by the Werthamer theory exhibits strong variations with temperature and frequency. In addition, the Werthamer theory predicts that a tunnel junction will have a plasma-like resonance in the absence of any electrode capacitance, and that the junction can have a nonzero reactance when current-biased outside of a constant-voltage step.