a Study of the Nonresonant Microwave Absorption in Copper Oxide Superconductors.

Microwave absorption at ~9 GHz, 0.1-100 mW and 5 K by two species of copper oxide superconductors, YBaCuO and TlSrCaCuO, has been investigated by both a non-modulated direct absorption technique and the conventional field modulated first derivative technique. The resonance-like broad signal and the noise -like fluctuations were shown to follow the same dependence on the magnetic history of the sample. In particular, they broadened and shifted in field position in the direction of the field applied upon cooling. Furthermore, field cooled samples were found to exhibit a dependence on their orientation relative to the direction of the field applied upon cooling. When the sample was rotated after field cooling, both the position and width of the microwave absorption signal were affected. The position showed a cosine function dependence on the angular orientation of the sample. On the other hand, as previously reported in the literature, the derivative signal showed a strong dependence on the modulation amplitude and on the direction of sweep of the magnetic field. However, the modulation of the magnetic field was determined to have no effect on the direct absorption spectrum. Furthermore, for modulation amplitudes larger than ~8 Oe, some jumps appeared in the derivative spectrum. The results were explained in terms of the Ebner -Stroud model, where the broad signal is a superposition of a multitude of series of many equally spaced narrow signals. Each of the series is due to one of the Josephson junctions completing one of the supercurrent loops formed in the specimen and connecting the random granular network. The shift in the minimum of absorption and the broadening of the signal were explained in terms of the effect of the flux trapped in the intragranular regions on the intergranular regions.