Large non-linear kinetic inductance in superconductor/normal metal bilayer films

We study the kinetic inductance in thin superconducting/normal metal (S/N) bilayer films where the penetration depth is greater than the S thickness. Using the two-coil mutual inductance method, we show that it is possible to estimate the dependence of kinetic inductance on sheet current density. It is found that, as expected from Ginzburg-Landau theory, the dependence is very weak in bare superconducting films. However in S/N bilayers of NbN/Al and Nb/Al the kinetic inductance can be smoothly varied over at least a 2:1 range by application of a dc transport current. It is proposed that this novel non-linearity could be exploited in some microwave applications.