Anisotropy of flux dynamics for YBa2Cu3O7

ac susceptibility measurements were made to study the effects of an ac field on dynamics of textured ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The effective activation barrier U, time scale ${\mathit{t}}_{0}$, and velocity scale ${\mathit{v}}_{0}$, of flux diffusion are obtained based on the experiment and nonlinear flux diffusion theory. The result that U(${\mathit{H}}_{\mathrm{ac}}$\ensuremath{\parallel}ab)\ensuremath{\propto}${\mathit{H}}_{\mathrm{ac}}^{\mathrm{\ensuremath{-}}0.26}$ and U(${\mathit{H}}_{\mathrm{ac}}$\ensuremath{\parallel}C)\ensuremath{\propto}-ln ${\mathit{H}}_{\mathrm{ac}}$ shows that the barriers are anisotropic. The time and velocity scale manifest their anisotropy in such a way that [${\mathit{t}}_{0}$ (or ${\mathit{v}}_{0}^{\mathrm{\ensuremath{-}}1}$)(${\mathit{H}}_{\mathrm{ac}}$\ensuremath{\parallel}C)]/[${\mathit{t}}_{0}$ (or ${\mathit{v}}_{0}^{\mathrm{\ensuremath{-}}1}$) (${\mathit{H}}_{\mathrm{ac}}$\ensuremath{\parallel}ab)]\ensuremath{\approxeq}${10}^{3}$, ${\mathit{t}}_{0}$\ensuremath{\propto}(${\mathit{H}}_{\mathrm{ac}}$\ensuremath{\parallel}ab${)}^{1.27}$, and ${\mathit{t}}_{0}$\ensuremath{\propto}(${\mathit{H}}_{\mathrm{ac}}$\ensuremath{\parallel}C${)}^{\mathrm{\ensuremath{-}}0.6}$. These results can be understood if flux lines are considered to be collective diffusion in the vortex glass phase near the viscous flux-flow state. \textcopyright{} 1996 The American Physical Society.