Anomalous current-dependence of kinetic inductance in ultrathin NbN meander lines

We have measured the rf impedance of BO-nan thick superconducting granular NbN meander lines between 5 mu m and 20 mu m wide. The complex reflection coefficient was obtained directly using a vector network analyzer. The impedance (at 10 MHz for a 5 mu m wide line) is dominated by the large kinetic inductance of the film, with a sheet Inductance of order a HIM, giving a total inductance of 400 nH for a 5 mu m x 2 mm line, The kinetic inductance diverges as T-->T-c=6.5 K, as expected ham Ginzburg-Landau theory (GL), Of particular interest is a substantial decrease of measured inductance with increasing de current, contrary to the dependence expected from GL. This anomalous current dependence is not well understood, hut may be related to vortex dynamics and Josephson effects in these granular films, as supported by preliminary Josephson array simulations. These results have important implications for potential devices such as tunable rf phase shifters and kinetic inductance photodetectors.