The voltage-noise measurements in epitaxial YBa2Cu30 x films have been extended to the case of a high magnetic field. In a magnetic field, two noise peaks, one from the vortex motion and the other from the resistance fluctuations, were observed. The location of the vortex-motion induced peak was found to be well above the vortex-glass transition. Our interpretation is that a noise peak manifests itself in a crossover from the pinned vortex-liquid to the unpinned vortex-liquid state.
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fluctuations [6], critical current fluctuations [7], granularity [8], and so on.
In our earlier work [9], the temperature dependence of the noise spectrum in the absence of a magnetic field showed a single peak at T t corresponding to the resistance fluctuation, while in a magnetic field an additional peak appeared at Tf near the zero-resistance temperature. It was shown that the field-induced noise is due to the vortex motion interacting with the pinning potential. An analysis based on the classical model suggested that the noise peak at Tf is caused by the two opposite temperature dependences of the flux-bundle size and the sample resistance. The upper limits of the flux-bundle size was also estimated.
In this work, the noise measurement has been extended to a higher magnetic field. The present data resembled the previous ones very closely, but no peaks due to the vortex motion were observed in magnetic fields higher than 1 T. In general the magnitude of noise is a measure of the randomness