Two-dimensional optical thermal ratchets based on Fibonacci spirals

Ray

2015

Phys. Rev. B

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.

Section: Ratchet Effects For Colloidal Particlesmentioning

confidence: 96%

Reversible ratchet effects for vortices in conformal pinning arrays

Ray

2015

Phys. Rev. B

“…In many of the experimentally studied systems where ratchet effects occur, such as vortices in type-II superconductors 10,12,14,15,19 or colloids 20,21 , the motion of the particles is effectively overdamped. Recently a new particlelike excitation called skyrmions was discovered in chiral magnets [22][23][24] .…”

Section: Introductionmentioning

confidence: 99%

Reversible vector ratchets for skyrmion systems

2017

Phys. Rev. B

We show that ac driven skyrmions interacting with an asymmetric substrate provide a realization of a new class of ratchet system which we call a vector ratchet that arises due to the effect of the Magnus term on the skyrmion dynamics. In a vector ratchet, the dc motion induced by the ac drive can be described as a vector that can be rotated clockwise or counterclockwise relative to the substrate asymmetry direction. Up to a full 360• rotation is possible for varied ac amplitudes or skyrmion densities. In contrast to overdamped systems, in which ratchet motion is always parallel to the substrate asymmetry direction, vector ratchets allow the ratchet motion to be in any direction relative to the substrate asymmetry. It is also possible to obtain a reversal in the direction of rotation of the vector ratchet, permitting the creation of a reversible vector ratchet. We examine vector ratchets for ac drives applied parallel or perpendicular to the substrate asymmetry direction, and show that reverse ratchet motion can be produced by collective effects. No reversals occur for an isolated skyrmion on an asymmetric substrate. Since a vector ratchet can produce motion in any direction, it could represent a new method for controlling skyrmion motion for spintronic applications.

“…Ratchet effects can also be realized using other forms of external driving such as by flashing the substrate on and off to create what is called a flashing ratchet [1][2][3] . Ratchets have been studied and experimentally realized for a variety of systems including colloidal particles on asymmetric substrates 4,5 , vortices in type-II superconductors interacting with nanostructured pinning sites [6][7][8] , and granular media on vibrated asymmetric substrates 9 . It is also possible to realize ratchet effects on symmetric substrates provided that the external driving has some form of asymmetry [10][11][12][13][14][15] .…”

Section: Introductionmentioning

confidence: 99%

Active matter ratchets with an external drift

2013

Phys. Rev. E

When active matter particles such as swimming bacteria are placed in an asymmetric array of funnels, it has been shown that a ratchet effect can occur even in the absence of an external drive. Here we examine active ratchets for two dimensional arrays of funnels or L-shapes where there is also an externally applied dc drive or drift. We show that for certain conditions, the ratchet effect can be strongly enhanced, and that it is possible to have conditions under which run-and-tumble particles with one run length move in the opposite direction from particles with a different run length. For the arrays of L-shapes, we find that the application of a drift force can enhance a transverse rectification in the direction perpendicular to the drift. When particle-particle steric interactions are included, we find that the ratchet effects can be either enhanced or suppressed depending on barrier geometry, particle run length, and particle density.