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 thin films at an applied small dc magnetic field ( B dc ) at 77 K, namely a decrease of Z s , is observed with the microstrip resonator technique. The resonant frequency is 1.107 GHz. The direction of B dc is parallel or perpendicular to the a - b plane. B dc ranges from 0 to 200 G. It is found that the surface resistance ( R s ) at B dc parallel to the a - b plane first decreases with 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 crossover field. The two behaviours can be collapsed to one curve by scaling the crossover fields. The changes of surface reactance X s correlated linearly with 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 = R s / X s ) are 0.5 at B dc less than the crossover field and 0.1 at B dc greater than the crossover field. The measurements 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 experimental results qualitatively.