Low-frequency noise in the normal state of thin-film high-temperature superconductors

Abstract

The excess low-frequency (1/f) noise of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin-film microbridges has been studied in the normal state. The normalized noise-power spectral density, ${\mathit{S}}_{\mathit{V}}$/${\mathit{V}}^{2}$, shows quantitatively different behavior depending on the film-substrate lattice mismatch and the oxygen content. The noise behavior is largely attributable to the hopping of basal-plane oxygen vacancies subject to strain-dependent activation barriers. Both electromigration biases and oxygen plasma annealing can be effective techniques for lowering these noise levels, but in different ways that point to the origin of the noise. The low-temperature, \ensuremath{\sim}100 K, noise behavior, indicates that even films with high critical current densities have a substantial density of oxygen-defect states with very low activation energies for hopping.