Rectification and flux reversals for vortices interacting with triangular traps

Abstract

We simulate vortices in superconductors interacting with two-dimensional arrays of triangular traps. We find that, upon application of an ac drive, a net dc flow can occur which shows current reversals with increasing ac drive amplitude for certain vortex densities, in agreement with recent experiments and theoretical predictions. We identify the vortex dynamics responsible for the different rectification regimes. We also predict the occurrence of a novel transverse rectification effect in which a dc flow appears that is transverse to the direction of the applied ac drive. PACS numbers: 74.25.QtVortices interacting with periodic pinning structures have been attracting considerable attention since they are an ideal system in which to study the static and dynamical behaviors of collectively interacting particles coupled to periodic substrates. In these systems, a variety of commensurability effects and novel vortex crystals can occur as a function of the ratio of the number of vortices to the number of pinning sites [1][2][3]. The vortex crystals are composed of one or more vortices trapped at each pin with any remaining vortices located in the interstitial regions between the pinning sites. A variety of dynamical flow phases occur when a drive is applied, including the flow of interstitial vortices between the pinning sites [4,5] as well as channeling along rows of pinning [4]. Much of the physics of vortices interacting with periodic pinning is also observed for repulsively interacting colloids in periodic optical trap arrays [6][7][8]. In addition to the basic science issues, vortices interacting with periodic pinning arrays are relevant to possible technological applications of superconductors, such as critical current enhancement or controlled motion of flux for new types of devices.Several methods have been proposed for using periodic pinning or controlled disorder in superconductors to create vortex ratchets, rectifiers, and logic devices [9][10][11][12][13]. As in general ratchet systems, which can be deterministic or stochastic in nature [14], a vortex ratchet transforms an ac input into a dc response. Vortex ratchets can be created via asymmetric periodic pinning lines [9,10], asymmetric channels, or the asymmetry introduced by multiple ac drives in a system with a symmetric substrate [11,12]. Ratchets constructed of periodic twodimensional (2D) arrays of asymmetric pinning sites have also been proposed [13]. Recently, both positive and negative vortex rectification have been experimentally realized in periodic arrays of triangular pinning sites [15]. In Ref.[15], the triangles are arranged in a square lattice with the tips of the triangles oriented in the +y direction. When an ac force is applied in the y direction at low matching fields, rectification of the vortex motion in the +y direction occurs over a range of ac amplitudes, with a peak rectification at a particular amplitude. For higher matching fields, there is a −y rectification at lower ac drives, followed by a +y rectification at higher ac dri...

Vortex ratchet reversal: Role of interstitial vortices

“…[5][6][7] Vortex-ratchet reversal is due to collective effects such as deformations of the vortex lattice, the appearance of interstitial vortices in an effective pinning potential created by the pinned vortices, or the creation of interstitial or vacancy sites in ordered commensurate vortex configurations. [8][9][10][11] Some of the first results predicting ratchet reversals in interacting particle systems were proposed by Derényi and Vicsek. 8 One simple system which exhibits time irreversibility, and can be produced and studied in a systematic way, consists of an array of asymmetric magnetic pinning sites in proximity to a superconducting film.…”

Artificial Brownian motors: Controlling transport on the nanoscale

“…When designing and operating a device capable of transverse rectification, experimenters can vary the orientation of the pinning lattice axes, the symmetry axes of the individual traps ͑if any͒, and the direction of the injected current ͑i.e., of the Lorentz force͒. Many have explored by numerical simulation the geometries best suited for the experimental implementation of this concept ͑Zhu et al, 2001;Reichhardt and Reichhardt, 2005͒. The most recent realizations of transverse fluxon rectifiers fall into two main categories.…”

Transverse rectification in superconducting thin films with arrays of asymmetric defects

“…The vortex lattice dynamics simulations quoted in Ref. 10 could be a hint to figure out the transverse effect behavior. According to these works, taking into account that the asymmetric potentials are attractive potentials, the vortex trajectory is deflected for the triangles always in the same way, for instance, downwards.…”

“…8,9 The transverse ratchet effect for vortices moving in arrays of triangular pinning sites was specifically studied by Reichhardt-Olson and Reichhardt. 10 The transverse effect could be used for fabricating devices whose aim should be particle separation ͑DNA sorting, ion channeling, and so on͒. In this letter, we will deal with transverse and ratchet rectifier devices based on Nb films grown on top of an array of Cu nanotriangles.…”