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

We have studied an anomalous microwave (mw) response of superconducting YBa 2Cu3O7-delta (YBCO) microstrip resonators in the presence of a weak dc magn etic field, H-dc. The surface resistance (R-s) and reactance (X-s) show a c orrelated non-monotonic behavior as a function of H-dc. R-s and X-s were fo und to initially decrease with elevated H-dc and then increase after H-dc r eaches a crossover field, H-c, which is independent of the amplitude and fr equency of the input mw signal within the measurements. The frequency depen dence 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), w hich is defined as the ratio of the change in R-s(H-dc) and that in X-s(H-d c), is about 0.6 at H-dc < H-c and 0.1 at H-dc > H-c. The H-dc dependence o f the surface impedance is qualitatively independent of the orientation of H-dc. (C) 1999 Elsevier Science B.V. All rights reserved.