In superconducting films under an applied dc current, we analyze experimentally and theoretically the influence of engineered pinning on the vortex velocity at which the flux-flow dissipation undergoes an abrupt transition from low to high resistance. We argue, based on a nonuniform distribution of vortex velocity in the sample, that in strongly disordered systems the mean critical vortex velocity for flux-flow instability (i) has a nonmonotonic dependence on magnetic field and (ii) decreases as the pinning strength is increased. These findings challenge the generally accepted microscopic model of Larkin and Ovchinnikov (1979 J. Low. Temp. Phys. 34 409) and all subsequent refinements of this model which ignore the presence of pinning centers. 8

We study the instability of the superconducting state in a mesoscopic geometry for the low pinning material\ud Mo3Ge characterized by a large Ginzburg-Landau parameter. We observe that in the current-driven switching to\ud the normal state from a nonlinear region of the Abrikosov flux flow, the mean critical vortex velocity reaches a\ud limitingmaximum velocity as a function of the appliedmagnetic field.Based on time-dependent Ginzburg-Landau\ud simulations, we argue that the observed behavior is due to the high-velocity vortex dynamics confined on a\ud mesoscopic scale. We build up a general phase diagram which includes all possible dynamic configurations of\ud the Abrikosov lattice in a mesoscopic superconducto

We study the quasiparticle energy relaxation processes in superconducting Nb films of different thicknesses corresponding to different electron mean free paths in a state far from equilibrium, that is the highly dissipative flux-flow state driven up to the instability point. From the measured currentvoltage curves we derive the vortex critical velocity v * for several temperatures. From the v * (T ) values, the quasiparticle energy relaxation time τǫ is evaluated within the Larkin-Ovchinnikov model and numerical calculations of the quasiparticle energy relaxation rates are carried out to support the experimental findings. Besides the expected constant behavior of τǫ(T ) for the dirty samples, we observe a strong temperature dependence of the quasiparticle energy relaxation time in the clean samples. This feature is associated with the increasing contribution from the electron-phonon scattering process.

We measure current-voltage characteristics as function of magnetic field and temperature in Nb strips of different thickness and width. The instability voltage of the flux flow state related to the vortex critical velocity v * is studied and compared with the Larkin-Ovchinnikov theory. Beside the usual power-law dependence v * ≈ B −1/2 , in the low field range a new cross-over field, Bcr1, is observed below which v * decreases by further lowering the external magnetic field B. We ascribe this unexpected cross-over to vortex channeling due to a fan-like penetration of the applied magnetic field as confirmed by magneto-optic imaging. The observation of Bcr1 becomes a direct evidence of a general feature in type-II superconducting films at low fields, that is a channel-like vortex motion induced by the inhomogeneous magnetic state caused by the relatively strong pinning.

We study the flux flow state in superconducting materials characterized by rather strong intrinsic pinning, such as Nb, NbN, and nanostructured Al thin films, in which we drag the superconducting dissipative state into the normal state by current biasing. We modify the vortex pinning strength either by ion irradiation, by tuning the measuring temperature or by including artificial pinning centers. We measure critical flux flow voltages for all materials and the same effect is observed: switching to low flux flow dissipations at low fields for an intermediate pinning regime. This mechanism offers a way to additionally promote the stability of the superconducting state. VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4718309