Experimental observation and simulation of unusual microwave response for the superconducting microstrip resonator at small dc magnetic field

An unusual microwave response of the surface impedance Z(s) of high-T-c thi n films at an applied small de magnetic field (B-dc) at 77 K, namely a decr ease of Z(s), is observed with the microstrip resonator technique. The reso nant frequency is 1.107 GHz. The direction of B-dc is parallel or perpendic ular to the a-b plane. B-dc ranges from 0 to 200 G. It is found that the su rface resistance (R-s) at B-dc parallel to the a-b plane first decreases wi th B-dc and then increases above a crossover field. The R-s behaviour for B -dc perpendicular to the a-b plane is the same but with a different crossov er field. The two behaviours can be collapsed to one curve by scaling the c rossover fields. The changes of surface reactance X-s correlated linearly w ith the changes of R-s in the ranges of B-dc. The ratios r(H) of changes of R-s and X-s (r(H) = Delta R-s/Delta X-s) are 0.5 at B-dc less than the cro ssover field and 0.1 at B-dc greater than the crossover field. The measurem ents also show that the crossover field is independent of rf input power. A phenomenological model is also proposed to explain this unusual behaviour. By adjusting fitting parameters the computed results agree with the experi mental results qualitatively.