Flux-flow voltage noise and normal-state resistance fluctuations in epitaxial (Dy, Y)Ba 2Cu 3O 7 thin films

Abstract

We have studied low-frequency noise in the normal and mixed states of high- T c thin films. In the normal state, resistance fluctuations with a 1/ f power spectrum are observed from 100 K up to 300 K. The normal state noise is largely attributable to hopping or reordering of oxygen vacancies which in turn lead to fluctuations in local carrier density in the copper oxygen planes. Analysis shows a substantial density of oxygen defect states with activation energies below 0.5 eV, followed by a rapid increase at higher energies. In the mixed state, we have measured the current-induced, vortex-motion voltage noise. When the current–voltage characteristics are nonlinear there exist two distinct regimes. The first is a low magnetic field regime where measurements of the variations of voltage noise with magnetic field reveal reproducible, microstructure-dependent noise magneto-fingerprints. Rapid variations of the noise as a function of magnetic field and ac current drive experiments indicated that the noise is associated with metastability of pinned vortex configurations. Above magnetic fields which roughly match vortex separation with the grain size in the films, the magnitude and fine structure of the noise magneto-fingerprints are suppressed, suggesting a crossover to a more weakly pinned vortex state. The second regime is a low bias voltage regime where the observed noise scales linearly with bias voltage and has a 1/ f-like spectrum. The linear voltage scaling is reminiscent of vortex shot noise although no characteristic transit time could be determined. At higher voltage biases, the noise power drops off rapidly. We attribute the sharp drop off to a loss of pinning at higher temperatures and fields which results in the absence of metastability in the vortex pinned states. The temperature and magnetic field dependence of the noise are discussed in the context of various models including vortex shot noise, channel, critical current fluctuation, and vortex-glass models.