Unusual power dependence of two-tone intermodulation in high-T-c superconducting microwave resonators

We study the power dependence of two-tone intermodulation arising in high-T -c superconducting microwave resonators as a function of each tone separate ly. In the regime where the intermodulation power varies as the square of t he input power for equally strong tones, we observe unexpected behavior of the power dependence on the individual tones. We show that this behavior ca n be understood in terms of a nonlinear inductance which varies linearly wi th the magnitude of the input current. These findings provide a consistent phenomenological picture of the unusual power dependence and should place c onstraints on a microscopic description of nonlinear behavior in high-T-c f ilms where the intermodulation products vary like the square of the input p ower. (C) 1999 American Institute of Physics. [S0003-6951(99)02205-6].