Reversible Ratchet Effects for Vortices in Conformal Pinning Arrays

TL;DR

This study demonstrates that conformal pinning arrays in superconductors significantly enhance vortex ratchet effects due to their gradient structure, outperforming square or random arrays, and also applies to colloidal particles, showing broad applicability.

Contribution

The paper introduces the use of conformal arrays for vortex pinning, revealing their superior ratchet effect and multiple reversals, a novel approach compared to traditional uniform or random arrays.

Findings

Conformal arrays produce the strongest ratchet effects among tested structures.

Enhanced ratchet effects are due to the gradient in pinning density allowing vortex funneling.

Multiple ratchet reversals occur as vortex density and ac amplitude vary.

Abstract

A conformal transformation of a uniform triangular pinning array produces a structure called a conformal crystal which preserves the six-fold ordering of the original lattice but contains a gradient in the pinning density. Here we use numerical simulations to show that vortices in type-II superconductors driven with an ac drive over gradient pinning arrays produce the most pronounced ratchet effect over a wide range of parameters for a conformal array, while square gradient or random gradient arrays with equivalent pinning densities give reduced ratchet effects. In the conformal array, the larger spacing of the pinning sites in the direction transverse to the ac drive permits easy funneling of interstitial vortices for one driving direction, producing the enhanced ratchet effect. In the square array, the transverse spacing between pinning sites is uniform, giving no asymmetry in the funneling of the vortices as the driving direction switches, while in the random array, there are numerous easy-flow channels present for either direction of drive. We find multiple ratchet reversals in the conformal arrays as a function of vortex density and ac amplitude, and correlate the features with a reversal in the vortex ordering, which is greater for motion in the ratchet direction. The enhanced conformal pinning ratchet effect can also be realized for colloidal particles moving over a conformal array, indicating the general usefulness of conformal structures for controlling the motion of particles.