•Roland Hott, Alexander Zaitsev und Rudolf Schneider
Forschungszentrum Karlsruhe, Institut für Festkörperphysik, P.O.B. 3640, D-76021 Karlsruhe, Germany

We present a general scheme how to extract the response function Z(H) = å_m Z_m H^m for a nonlinear reaction of a quantity E(H) = Z(H) H on an input H from two-tone intermodulation distortion (IMD) experiments using a time-dependent excitation H(t) = h cos(w₀ t)cos(d₀ t). We applied this method with success to high-temperature superconductor (HTS) rf resonators to identify here the mechanism of nonlinear signal distortions. Comparison of the experimental 3rd-order IMD I₃(p_circ) measured as function of rf power p_circ with the theoretical I₃(h) given in terms of the excitation amplitude h gives direct access to h(p_circ). We derived very similar characteristic parameters from HTS resonators with extremely different geometries. They suggest a divergence of Z(H) which can be identified with the critical magnetic field H_c2 of the superconductor. From comparison of the theoretical function Z(p_circ) with the experimental surface impedance Z_S(p_circ) = R_S(p_circ)+iX_S(p_circ) of a HTS disk resonator we deduce Z(p_circ) µ R_S(p_circ) identifying the rf power induced dissipation increase as dominant IMD source.