Nonlinear Flux Diffusion and ac Susceptibility in High-T_c Superconductors

It is shown that the flux motion in high-T_c superconductors can be well consistently analysed by a unified materials equation. The different models of U ( J ) so far suggested can be shown as specific forms or limits of this unified materials equation. The scaling law of flux pinning derived from this equation agrees with recent experimental observations on high-J_c NdBa₂Gu₃O_7-δ single crystal. The amplitude dependence of ac susceptibility derived from this equation is compared with several experimental results of high-T_c superconductors and a general power law is found.