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

We have studied an anomalous microwave (mw) response of superconducting YBa ₂Cu ₃O _{7- δ} (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.