Superconducting Bridges Made of Films Superimposed with Ferromagnetic Metals

Superconducting bridges were constructed using tin films superimposed with ferromagnetic nickel. The critical currents were strongly depressed in the gapless state compared with the “mean-field” critical currents given by Ginzburg-Landau equations. In the presence of an applied microwave field, (1) quasi-periodic power dependence of induced current steps was observed, (2) no subharmonic step appeared, (3) the step structure resembled to that of point contacts. These observations in our superconducting bridges are different from those in Anderson-Dayem bridges reported earlier. Our bridges in the gapless state are concluded to be very weak links and have a sinusoidal current-phase relationship.