Flux-flow noise in aPb0.8In0.2superconducting alloy

Abstract

An investigation of flux-noise in ${\mathrm{Pb}}_{0.8}$${\mathrm{In}}_{0.2}$ samples is presented. Good fits are obtained for experimental noise spectra using our recent model of fluxoid motion interrupted by pinning centers. A distribution of subpulse times is introduced in accordance with the model, and the data require that longer subpulse times be included when the samples undergo annealing which increases grain size. Since at fixed field the relative distribution of subpulse times does not shift with Lorentz force or fluxoid velocity, we conclude that pinning centers which are effective at interrupting fluxoid motion at one Lorentz force and fluxoid velocity appear to be effective at all forces and velocities in the linear flux-flow region. In our model, the noise at zero frequency, $W(0)$, is no longer simply proportional to the average intrinsic flux bundle size, but includes averages over distances between pinning centers. We refer to the appropriate new quantity as the generalized flux bundle size and we find that this quantity is affected by the sample metallurgy, increasing in proportion to grain size. The generalized flux bundle size also begins to decrease rapidly in the field vicinity of the volume pinning force maximum, and becomes small at ${H}_{c2}$. The data give an indication that there is a shift to shorter subpulse times in the region of the pinning force maximum. The concept of a "flux bundle" is discussed in light of the present results.