LATTICE STRUCTURE, FLUX DYNAMICS, AND LOW-FREQUENCY NOISE IN HIGH-TEMPERATURE SUPERCONDUCTORS

Noise sources associated with the dynamics of motion of the flux bundles and its lattice interactions in YBaCuO high-Tc superconductors (HTS) have been examined. A model is proposed where, in the layered structure of the material, the flux bundles or fluxoids exhibit a continuous crossover from a motion over activation barriers to diffusive transfer between pinning sites. The results of the model were compared to various experimental measurements on thin films YBaCuO with distinct low frequency behavior. In one of the experiments the low frequency 1/f noise spectral power could, in general, be reproduced. In another experiment, agreement between measurements and theoretical predictions was achieved when a defect density functional D(n) (where n is the density of defects in a single plane) was derived. In the flux flow regime, D(n) has the simple form D(n)=(delta/xi)(1/n1/2 xi)delta-1 where xi is the superconducting coherence length in the Cu-O planes and delta-(U(o)/KT) is the pinning energy of a flux line normalized to its thermal energy. These results are clear indications that the noise frequency dependence is strongly related to the dynamics of motion of free vortices and the number fluctuation of bound vortices.