Flux-flow noise in type-II superconductors

Measurements of flux flow in type-II superconductors by a direct flux-detection method are described. A superconducting-quantum-interference-device magnetometer is employed to monitor the fluctuating component of flux changes in the central part of In0.9{Pb}0.1$ alloy samples as a function of the transport current. Experimental results show that the behavior of flux flow in the high-current region is significantly different from that near the onset. For high-current values, the noise-power spectral density can be fitted quite well by a Lorentzian form, and the rolloff frequency varies linearly with the current as well as the length, but not the width, of the detection loop. A simple model is proposed to account for these observations. The number of moving bundles is estimated from the data, and its low value suggests that only a small fraction of flux lines form sizable bundles that can be detected in this experiment. In the near-onset region, the average ac output of the detector shows several distinct peaks as the transport current is increased. Power spectra exhibit the form of 1/f noise at the peak positions, whereas deviations from 1/f noise are found at positions away from the peaks. The 1/f noise as well as deviations from 1/f can be explained by assuming a velocity spread of the moving bundles.