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

Abstract

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.