Asymmetric weak-pinning superconducting channels: Vortex ratchets

Abstract

The controlled motion of objects through narrow channels is important in many fields. We have fabricated asymmetric weak-pinning channels in a superconducting thin-film strip for controlling the dynamics of vortices. The lack of pinning allows the vortices to move through the channels with the dominant interaction determined by the shape of the channel walls. We present measurements of vortex dynamics in the channels and compare these with similar measurements on a set of uniformwidth channels. While the uniform-width channels exhibit a symmetric response for both directions through the channel, the vortex motion through the asymmetric channels is quite different, with substantial asymmetries in both the static depinning and dynamic flux flow. This vortex ratchet effect has a rich dependence on magnetic field and driving force amplitude.PACS numbers: 74.25. Qt, 74.25.Sv, 74.25.Op Recently there has been much interest in developing artificial ratchets for generating directed motion using tailored asymmetries [1]. Such ratchets could be used as pathways for producing net transport of matter at the nanoscale. In addition, artificial ratchets can serve as model systems for understanding similar ratchet phenomena in biological systems while allowing for experimental control over many of the ratchet parameters [2]. A variety of ratchets have been considered, but one particular type that has been implemented in several different systems is the rocking ratchet, where a spatial asymmetry is engineered into the potential energy landscape governing particle motion and an external control variable can be adjusted to tilt this potential. The application of an oscillatory drive of the control variable with zero mean can result in the net motion of particles through the potential because of the different rates for overcoming the barriers in the two directions through the ratchet.Implementations of ratchets in solid-state devices include asymmetric structures of electrostatic gates above a two-dimensional electron gas [3], and arrays of Josephson junctions with asymmetric critical currents [4]. Structures have also been developed for producing a ratchet effect with vortices in superconducting thin films involving either asymmetric arrangements of pinning centers [5,6] or asymmetric magnetic pinning structures [7]. In this Communication, we describe a vortex ratchet using two-dimensional guides to generate asymmetric channels for vortex motion. In our structures, the potential asymmetries arise from differences in the interaction strength between vortices and the channel walls, resulting in a substantial ratchet effect for the motion of vortices through the channels. Our design is related to a previous vortex ratchet proposal [8], although our ratchet is in a somewhat different parameter regime.Nanoscale channels for guiding vortices through superconducting films with a minimal influence from pin- ning have been developed for studies of vortex matter in confined geometries, including experiments on melting [9], commensurabili...

Artificial Brownian motors: Controlling transport on the nanoscale

“…These conclusions have been corroborated experimentally by Togawa et al ͑2005͒ andYu et al ͑2007͒. The more recent work fabricated triangular channels from bilayer films of amorphous niobium germanium, an extremely weak-pinning superconductor, and niobium nitride ͑NbN͒, with relatively strong pinning.…”

Reversible ratchet effects for vortices in conformal pinning arrays

“…Ratchet effects on asymmetric substrates have been extensively studied in colloidal systems [3][4][5] , granular matter 6,7 , and polymers 8,9 . Ratchet effects also appear in ac-driven vortices in type-II superconductors in the presence of an asymmetric substrate [10][11][12][13][14][15] , such as a quasi-one-dimensional periodic array produced by asymmetrically modulating the sample thickness 10,[16][17][18][19] , etching funnel-shaped channels for vortex flow 11,[20][21][22][23][24] , introducing asymmetry to the sample edges 25 , or adding periodic pinning arrays in which the individual pinning sites have some form of intrinsic asymmetry 13,14,[26][27][28][29][30][31][32][33][34][35] . At lower vortex densities when collective interactions between vortices are weak, the ratchet effect produces a dc flow of vortices in the easy flow direction of the asymmetric substrate; however, when collective effects are present it is possible to have reversals of the ratchet effect where for one set of parameters the vortices move in the easy direction while for another set of parameters they move in the hard direction [13][14][15][16]…”

Rectification of vortex motion in a circular ratchet channel

“…This is based on a technique for using weak-pinning channels with tailored edges to produce asymmetric vortex confining potentials. Such an arrangement resulted in substantial asymmetric vortex response for linear channels on a strip geometry 6 . The Corbino sample consists of a Si substrate with a 200-nm thick film of weak-pinning a-NbGe and a 50-nm thick film of strong-pinning NbN on top.…”

“…In the present paper, we study the dynamics of vortices in a circular channel formed by asymmetric triangular (funnel) cells (TCs) (see Fig. 1) [note that earlier this approach to form asymmetric channels in experiment (i.e., using weak-pinning channels) was employed in a stripe geometry 6 ]. Due to the radially flowing current in a Corbino setup [33][34][35][36][37][38][39][40] , the driving force is not uniform inside the cell which is different from linear ratchet channels.…”