FLUX DIFFUSION IN HIGH-TC SUPERCONDUCTORS

In type-II superconductors in the flux flow (J⊥≫Jc), flux creep (Jc⊥≈Jc), and thermally activated flux flow (TAFF) (J⊥≪Jc) regimes the induction B(r, t), averaged over several penetration depths λ, in general follows from a nonlinear equation of motion into which enter the nonlinear resistivities ρ⊥(B, J⊥, T) caused by flux motion and ρ{norm of matrix}(B, J{norm of matrix}, T) caused by other dissipative processes. J⊥ and J{norm of matrix} are the current densities perpendicular and parallel to B, B=|B|, and T is the temperature. For flux flow and TAFF in isotropic superconductors with weak relative spatial variation of B, this equation reduces to the diffusion equation {Mathematical expression} plus a correction term which vanishes when J{norm of matrix}=0 (this meansBׇ×B=0) or when ρ⊥ - ρ{norm of matrix} = 0 (isotropic normal conductor). When this diffusion equation holds the material anisotropy may be accounted for by a tensorial ρ⊥. The response of a superconductor to an applied current or to a change of the applied magnetic field is considered for various geometries. Such perturbations affect only a surface layer of thickness λ where a shielding current flows which pulls at the flux lines; the resulting deformation of the vortex lattice diffuses into the interior until a new equilibrium or a new stationary state is reached. The a.c. response, in particular the frequency with maximum damping, depends thus on the geometry and size of the superconductor. © 1990 Springer-Verlag.