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

We have measured the rf impedance of 10-nm thick superconducting granular NbN meander lines between 5 /spl mu/m and 20 /spl mu/m wide. The complex reflection coefficient was obtained directly using a vector network analyzer. The impedance (at 10 MHz for a 5 /spl mu/m wide line) is dominated by the large kinetic Inductance of the film, with a sheet inductance of order 1 nH, giving a total inductance of 400 nH for a 5 /spl mu/m/spl times/2 mm line. The kinetic inductance diverges as T/spl rarr/T/sub c/=6.5 K, as expected from Ginzburg-Landau theory (GL). Of particular interest is a substantial decrease of measured inductance with increasing dc current, contrary to the dependence expected from GL. This anomalous current dependence is not well understood, but 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.