DIFFUSIVE FLUCTUATIONS, LONG-TIME AND SHORT-TIME CROSS-CORRELATIONS IN THE MOTION OF VORTICE-PANCAKES IN DIFFERENT LAYERS OF YBCO PBCO SUPERLATTICES/

We have used magnetic flux noise measurements to probe the spatial num ber-density fluctuations of magnetic vortices in two YBCO/PBCO superla ttices having different coupling between the YBCO layers. The fluctuat ions are shown to arise from diffusive motion of the vortices. Measure ments at a fixed frequency show a maximum in the temperature-dependenc e of the fluctuation in a fixed subvolume of the sample. On the low-te mperature side of the maximum, the noise spectrum of fluctuations show s a pronounced f(-1.5) dependence (which is a universal rule for diffu sion noise), while on the high-temperature side it tends towards a whi te spectrum. At the highest temperatures we observe an excess noise wh ich we attribute to vortex annihilation-creation over an energy barrie r at the sample edges. We have improved the model of Rogers et al. [14 ] as applied to single BSCCO layers, in orders to investigate the long -time and short-time correlations between the motion of vortice pancak es in different layers in the superlattices. The new method turns out to be a powerful tool to obtain information about long-time and short- time cross-correlations between the diffusive motion of pancakes in di fferent layers, as a result of coupling. Preliminary measurements in m agnetic field indicate a strongly space limited diffusion above a crit ical temperature. Copyright (C) 1996 Elsevier Science Ltd.