Vortex dynamics in an annular Josephson ratchet ladder

摘要

We present numerically the motion of vortices placed in an annular Josephson ladder which has a periodic ratchet potential along the annular direction. The ratchet characteristics are provided by assigning both alternate critical currents and alternate plaquette areas. The vortices are subject to an external current applied uniformly from each superconducting grain in the inner ring to each grain in the outer ring. The current-voltage (I-V) curves show asymmetric features because of the spatially broken symmetry of the potential. When an alternating current is added to the external current, Shapiro steps appear in the I-V curves, showing asymmetric values of the step widths and on-set currents. For a certain range of the alternating currents, vortices rotate to the easy direction, even at zero driving current, that corresponds to the direction away from the steep slope and toward the gentle slope of the ratchet potential.