Coherent motion of vortices driven by alternating currents in an annular Josephson ratchet ladder

K H Lee

doi:10.1063/1.5111172

Abstract

Vortex dynamics have been investigated numerically in a double-ring annular ladder composed of Josephson ratchet network along the circular direction. One repeat unit of the periodic ratchet potential consists of two adjacent plaquettes of different sizes, where the critical currents of their rung junctions are also assigned with different values. Each vortex generated between the two rings by a transverse magnetic field oscillates through several plaquettes or rotates along the ladder, depending on the magnitude of direct and alternating currents applied to the radial direction. Because of the broken spatial symmetry in the potential landscape, onsets of the vortex motion and widths of the Shapiro steps in the current-voltage characteristics show polarity-dependent behavior. The directional motion of vortices, already known in rectangular Josephson ratchet networks driven by alternating currents, has also been confirmed to appear as directional rotation in this annular ratchet ladder. A notable feature is that such dynamics can occur with a system’s commensurate condition that makes a Shapiro step at zero direct current, which leads to coherent rotational motion of vortices. Here, we report that for each magnetic flux applied to the system there is a finite range of alternating currents that allow vortices to move coherently.

Highlights

Topological defects such as dislocations in crystalline materials, vortices in superfluids and superconductors, and magnetic skyrmions in spintronics devices have been widely studied during the last decades in the field of solid state physics
Studies on vortex dynamics under various potential landscapes formed in two-dimensional superconductors or Josephson junction arrays have made great step forward in application technology related to these topological defects
The dynamics of vortices located between the two rings of an annular Josephson ratchet ladder have been investigated

Summary

INTRODUCTION

Topological defects such as dislocations in crystalline materials, vortices in superfluids and superconductors, and magnetic skyrmions in spintronics devices have been widely studied during the last decades in the field of solid state physics. Vortex configuration and their rearrangement caused by external factors could be confirmed by measuring local tunnel currents by using STM.. Vortex configuration and their rearrangement caused by external factors could be confirmed by measuring local tunnel currents by using STM.18,19 Such a direct imaging technique could provide a good way to understand the time development of Josephson vortices formed in the proximity area of two nearby superconducting islands.. One notable thing about the vortex motion in Josephson networks is the ratchet dynamics, such as motion rectification in microsized systems This refers to the phenomenon that Brownian particles under spatially asymmetric potentials exhibit directional motion by thermal random fluctuations or by alternating current bias.. One interesting feature here is that if the driving ac frequency is commensurate with the Josephson frequency of the ratchet system, vortices are engaged in coherent rotational motion, which results in a Shapiro step in the current-voltage curves

NUMERICAL METHOD BASED ON THE RCSJ MODEL

VORTEX CONFIGURATIONS IN AN ANNULAR JOSEPHSON RATCHET LADDER

CONCLUSION