The flux flow properties of epitaxial niobium films with different pinning strengths are investigated by dc electrical resistance measurements and mapped to results derived within the framework of a theoretical model. Investigated are the cases of weak random pinning in as-grown films, strong random pinning in Ga ion-irradiated films, and strong periodic pinning induced by a nanogroove array milled by focused ion beam. The generic feature of the current-voltage curves of the films consists in instability jumps to the normal state at some instability current density j * as the vortex lattice reaches its critical velocity v * . While v * (B) monotonically decreases for as-grown films, the irradiated films exhibit a non-monotonic dependence v * (B) attaining a maximum in the low-field range. In the case of nanopatterned films, this broad maximum is accompanied by a much sharper maximum in both, v * (B) and j * (B), which we attribute to the commensurability effect when the spacing between the vortex rows coincides with the location of the grooves. We argue that the observed behavior of v * (B) can be explained by the pinning effect on the vortex flow instability and support our claims by fitting the experimental data to theoretical expressions derived within a model accounting for the field dependence of the depinning current density.

The interplay between vortex guiding and the Hall effect in superconducting Nb films with periodically arranged nanogrooves is studied via four-probe measurements in standard and Hall configurations and accompanying theoretical modeling. The nanogrooves are milled by focused ion beam and induce a symmetric pinning potential of the washboard type. The resistivity tensor of the films is determined in the limit of small current densities at temperatures close to the critical temperature for the fundamental matching configuration of the vortex lattice with respect to the pinning nanolandscape. The angle between the current direction with respect to the grooves is set at seven fixed values between 0 • and 90 • . A sign change is observed in the temperature dependence of the Hall resistivity ρ − ⊥ of as-grown films in a narrow temperature range near Tc. By contrast, for all nanopatterned films ρ − ⊥ is nonzero in a broader temperature range below Tc, allowing us to discriminate between two contributions in ρ − ⊥ , namely one contribution originating from the guided vortex motion and the other one caused by the Hall anomaly just as in as-grown Nb films. All four measured resistivity components are successfully fitted to analytical expressions derived within the framework of a stochastic model of competing isotropic and anisotropic pinning. This provides evidence of the model validity for the description of the resistive response of superconductor thin films with washboard pinning nanolandscapes. arXiv:1604.01161v1 [cond-mat.supr-con]