Behavior of Thin-Film Superconducting Bridges in a Microwave Field

Quantitative measurements are presented on the behavior of thin-film superconducting bridges in the presence of an applied microwave field. Classical rectification is expected when an rf signal is applied to the sample because of its nonlinear $I-V$ characteristic. This is verified experimentally at 10 kc/sec. Deviation from classical rectification starts to occur at frequencies as low as 10 Mc/sec, and increases with increasing frequency up to ∼2 Gc/sec, where the critical current starts to increase with the applied microwave field. The increase of the critical current with the field is more pronounced at higher frequencies and at temperatures closer to the transition temperature. Dependence of this behavior on frequency, power, and temperature, as well as on the width of the bridge, was studied in detail. The possibility of "microwave-induced condensation" is briefly discussed. Additional measurements were carried out on the other effects of the microwave field, namely, the production of constant-voltage steps in the $I-V$ characteristic. It was found that the size of the steps increases smoothly over three decades of microwave power and then drops rapidly to zero. A brief comparison between bridge samples and Josephson tunnel junctions is included.