Phase-sensitive measurements of nonlinearity in high-temperature superconductor thin films

We present new frequency-dependent measurements of both the magnitude and phase of the nonlinear response in superconducting thin films at microwave frequencies obtained using a large-signal network analyzer. Our measurements show that the nonlinear inductance dominates the nonlinear response in thin YBCO films at 76 K, and our analysis yields two current-density scales corresponding to the real and imaginary components of the nonlinear response. The current-density scale associated with the dominant inductive response likely results from intrinsic pair-breaking, while the current-density scale associated with the nonlinear resistive term is smaller than the expected value due to pair-breaking, and could originate from vortex motion or other extrinsic effects.