Magnetic relaxation, current-voltage characteristics, and possible dissipation mechanisms for high-Tcsuperconducting thin films of Y-Ba-Cu-O

We propose a mechanism that may account for the temperature-insensitive relaxation of the magnetic-shielding current in epitaxial thin films of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. We show that such relaxation is related to the shape of the current-voltage (J-E) characteristic of the superconductor in its critical state. The weak temperature dependence of the relaxation implies a temperature-insensitive J-E characteristic that resembles that of conventional type-II superconductors when a spatial variation of critical current density (${\mathit{J}}_{\mathit{c}}$) is present. We suggest such a distribution of ${\mathit{J}}_{\mathit{c}}$ as an explanation for the apparently large and temperature-insensitive relaxation observed in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$.