Peculiarities of the vortex dynamics in a granular niobium superconducting bridge

The voltage-current ($V\text{--}I$) characteristics of a submicron-wide superconducting bridge made of granular Nb-film exhibit puzzling regions characterized by negative differential resistance (NDR), $\ensuremath{\partial}V/\ensuremath{\partial}I<0$. Our local magnetic force microscopy studies suggest that above the critical current, the voltage develops across the bridge owing to the quantum vortices moving along the weakest intergranular links, which form a nanoscale network. The simulations based on the time-dependent Ginzburg-Landau model show that in such a network, the interaction between vortices may slow down the global vortex dynamics, leading to a flattening and even NDR regions in $V\text{--}I$ characteristics.