U-jrelationship in type-II superconductors

The Anderson-Kim model has been modified in terms of the U-j relationship for high-${\mathit{T}}_{\mathit{c}}$ superconductors. We have obtained U-j curves by an approach based on the temperature dependence of ${\mathit{j}}_{\mathit{c}}$. Our results are consistent with the physical picture of flux creep. Our data is obtained from magnetic-relaxation experiments on ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$ single crystals with the direction of the field parallel to the c axis. The decay of magnetization was studied for both increasing and decreasing fields, used to obtain the current density.