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, was 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