Density-gradient mechanism of vortex plastic creep

The electromagnetic response of type-II superconductors exposed to a magnetic field is governed by the dynamics of the vortex matter. We found that there exists a force of non-Peach-Kohler type which acts on edge dislocations in vortex lattices and depends on the gradients of vortex density. This force is absent in atomic solids where the density of the crystal lattice is constant even in the presence of dislocations. By mapping the classic theory of dislocations onto vortex lattices, we find the critical current j(c)(pl) and the activation energy U-pl associated with the dislocation-mediated plastic creep. The latter proves to decrease with magnetic field as B-3/4, in concordance with experimental data. Such behavior of U-pl is just opposite to the case of elastic (collective) creep, where U-el always grows with B. At high enough field U-pl becomes less than U-el, thus plastic creep overwhelms the elastic one and becomes the dominant mechanism of vortex mobility. This explains the appearance of the "fishtail" (double-peak) shape of magnetization curves in high-T-c superconductors.