Rearrangement of the vortex lattice due to instabilities of vortex flow

With increasing applied current, we show that the moving vortex lattice changes its structure from a triangular one to a set of parallel vortex rows in a pinning free superconductor. This effect originates from the change of the shape of the vortex core due to nonequilibrium effects (similar to the mechanism of vortex motion instability in the Larkin-Ovchinnikov theory). The moving vortex creates a deficit of quasiparticles in front of its motion and an excess of quasiparticles behind the core of the moving vortex. This results in the appearance of a wake (region with suppressed order parameter) behind the vortex, which attracts other vortices resulting in an effective direction-dependent interaction between vortices. When the vortex velocity v reaches the critical value v(c), quasiphase slip lines (lines with fast vortex motion) appear, which may coexist with slowly moving vortices between such lines. Our results are found within the framework of the time-dependent Ginzburg-Landau equations and are strictly valid when the coherence length xi(T) is larger or comparable with the decay length L-in of the nonequilibrium quasiparticle distribution function. We qualitatively explain experiments on the instability of vortex flow at low magnetic fields when the distance between vortices a > L-in >xi(T). We speculate that a similar instability of the vortex lattice should exist for v>v(c) even when a < L-in.