Anomalous microwave response of high-temperature superconducting thin-film microstrip resonator in weak dc magnetic fields

摘要

We have studied an anomalous microwave (mw) response of superconducting YBa_2Cu_3O_(7 - #delta#) (YBCO) microstrip resonators in the presence of a weak dc magnetic field, H_(dc). The surface resistance (R_s) and reactance (X_s) show a correlated non-monotonic behavior as a function of H_(dc). R_s and X_s were found to initially decrease with elevated H_(dc) and then increase after H_(dc) reaches a crossover field, H_c, which is independent of the amplitude and frequency of the input mw signal within the measurements. The frequency dependence of R_s is almost linear at fixed H_(dc) with different magnitudes (< H_c, = H_c and > H_c). The impedance plane analysis demonstrates that r_H, which is defined as the ratio of the change in R_s(H_(dc)) and that in X_s(H(dc)), is about 0.6 at H_(dc) < H_c and 0.1 at H_(dc) < H_c. The H_(dc) dependence of the surface impedance is qualitatively independent of the orientation of H_(dc).