Flux motion in very anisotropic high-temperature superconductors

The vortex lattice in extremely anisotropic (high-temperature) superconductors breaks up into two-dimensional (2D) vortices located in the CuO₂ layers. In terms of such an anomalous vortex structure recent experiments questioning the mechanism for the mixed-state resistance resistance, can be explained. Under certain field conditions the layers of 2D vortices become decoupled and may be considered as independent thin films. Assuming that the pinning is caused by oxygen vacancies, one can apply the concepts of collective pinning. It is argued that the thermally assisted flux-flow is caused by hopping of vortex dislocation pairs. An expression for the activation barrier is derived and compared with the results of ac-susceptibility measurements on Bi : 2 2 1 2 single crystals.