[Show abstract][Hide abstract] ABSTRACT: We study theoretically dynamical phases of vortices in superconducting films
with arrays of obstacles. By performing a series of molecular dynamics
simulations and analytical calculations, we demonstrate the existence of a
phase of soliton-like vortex-density waves appearing in a wide range of
parameters. These waves are formed by a self-assembled phase separation process
induced by strongly nonlinear density fluctuations of the moving vortex matter
above a certain critical driving current. At high vortex concentrations, the
waves move at an approximately current-independent speed resulting in a wide
plateau in the voltage-current characteristics. At stronger drives, the vortex
system enters into a fully jammed (zero-voltage) phase. By combining ac and dc
drives, the interplay between the vortex-density-wave and jammed phases leads
to the observation of negative absolute mobility of vortices, which induces the
superconducting film into a negative resistance state.
[Show abstract][Hide abstract] ABSTRACT: The transport of interacting Brownian particles in a periodic asymmetric (ratchet) substrate is studied numerically. In a zero-temperature regime, the system behaves as a reversible step motor, undergoing multiple sign reversals of the particle current as any of the following parameters are varied: the pinning potential parameters, the particle occupation number, and the excitation amplitude. The reversals are induced by successive changes in the symmetry of the effective ratchet potential produced by the substrate and the fraction of particles which are effectively pinned. At high temperatures and low frequencies, thermal noise assists delocalization of the pinned particles, rendering the system to recover net motion along the gentler direction of the substrate potential. The joint effect of high temperature and high frequency, on the other hand, induces an additional current inversion, this time favoring motion along the direction where the ratchet potential is steeper. The dependence of these properties on the ratchet parameters and particle density is analyzed in detail.