Nonlinear microwave impedance of short and long Josephson junctions

The nonlinear dependence on applied ac field (b(omega)) or current (i(omega )) of the microwave (ac) impedance R-omega +iX(omega) of both short and lon g Josephson junctions is calculated under a variety of excitation condition s. The dependence on the junction width is studied, for both field symmetri c (current antisymmetric) and field antisymmetric (current symmetric) excit ation configurations. The resistance shows steplike features every time a f luxon (soliton) enters the junction, with a corresponding phase slip seen i n the reactance. For finite widths the interference of fluxons leads to som e interesting effects which are described. Many of these calculated results are observed in microwave impedance measurements on intrinsic and fabricat ed Josephson junctions in the high temperature superconductors. When a de f ield (b(dc)) or current (i(dc)) is applied, interesting phase locking effec ts are observed in the ac impedance Z(omega). In particular an almost perio dic dependence on the de bias is seen similar to that observed in microwave experiments at very low de field bias. These results are generic to all sy stems with a cos(phi) potential in the overdamped limit and subjected to an ac drive. [S0163-1829(99)00610-4].