Local magnetic relaxation in high-temperature superconductors.

Abstract

A novel Hall probe array technique is used to measure the spatial distribution and time dependence of the magnetic induction in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ crystals. Analysis of the data based on the flux diffusion equation allows a direct, model-independent determination of the local activation energy $U$ and the logarithmic time scale ${t}_{0}$ for flux creep. The results indicate that the spatial variations of $U$ are small ( $\ifmmode\pm\else\textpm\fi{}\mathrm{kT}$) and that $U$ increases logarithmically with time. The time ${t}_{0}$ is inversely proportional to the field and it exhibits a nonmonotonic temperature dependence. These results confirm theoretical predictions based on the logarithmic solution of the flux diffusion equation.