Anisotropy of flux dynamics for YBa2Cu3O7

ac susceptibility measurements were made to study the effects of an ac field on dynamics of textured YBa2Cu3O7. The effective activation barrier U, time scale t(0), and velocity scale v(0), of flux diffusion are obtained based on the experiment and nonlinear flux diffusion theory. The result that U(H(ac)parallel to ab) proportional to H-ac(-0.26) and U(H(ac)parallel to C) proportional to - In H-ac shows that the barriers are anisotropic. The time and velocity scale manifest their anisotropy in such a way that [t(0)(or v(0)(-1))(H(ac)parallel to C)]/[t(0)(or v(0)(-1))(H(ac)parallel to ab)] approximate to 10(3), t(0) proportional to (H(ac)parallel to ab)(1.27), and t(0) proportional to(H(ac)parallel to C)(-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.