Dynamic ordering and instability of the vortex lattice in Nb films exhibiting moderately strong pinning

We study the nonlinear effects in the current-voltage characteristics of Nb superconducting thin films, induced both by the current dependence of the pinning force and by the electric-field dependence of the flux-flow resistance of Abrikosov vortices driven by high electric currents. Despite of the quite strong pinning in our samples, by increasing the bias current, in a temperature dependent magnetic-field range, the moving vortex system undergoes a dynamical transition from a disordered to an ordered vortex lattice. Such transition leads to a quite sharp reduction of the dynamic pinning force corresponding to a peak in the current depen-dence of the differential flux-flow resistance. On the same samples, in a higher voltage-velocity regime, for a different temperature dependent magnetic-field range, an instability of the moving vortex lattice also occurs, with a sudden jump in theI-Vcurve from the non linear flux-flow branch to the linear normal resistive state. Within the Larkin-Ovchinnikov velocity dependence of the flux-flow resistance, this flux-flow instability has been studied as function of the magnetic field and temperature in order to get out its nonequilibrium electronic nature. Finally, we propose a dynamic phase diagram to describe the vortex lattice motion driven by high electric currents in the presence of such a disordered pinning landscape.