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 number-density fluctuations of magnetic vortices in two YBCO/PBCO superlattices having different coupling between the YBCO layers. The fluctuations are shown to arise from diffusive motion of the vortices. Measurements at a fixed frequency show a maximum in the temperature-dependence of the fluctuation in a fixed subvolume of the sample. On the low-temperature side of the maximum, the noise spectrum of fluctuations shows a pronounced f-1.5 dependence (which is a universal rule for diffusion noise), while on the high-temperature side it tends towards a white spectrum. At the highest temperatures we observe an excess noise which we attribute to vortex annihilation-creation over an energy barrier at the sample edges. We have improved the model of Rogers et al.  as applied to single BSCCO layers, in orders to investigate the long-time and short-time correlations between the motion of vortice pancakes 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 different layers, as a result of coupling. Preliminary measurements in magnetic field indicate a strongly space limited diffusion above a critical temperature.