Archimedean Screw in Driven Chiral Magnets

TL;DR

This paper theoretically demonstrates that oscillating perpendicular magnetic fields induce a net screw-like rotation in chiral magnetic helices, enabling controlled spin and charge transport with potential for novel dynamic phases.

Contribution

It introduces the concept of an Archimedean screw mechanism in driven chiral magnets and analyzes its effects, including resonance enhancement and instability leading to a time quasicrystal phase.

Findings

Net screw-like rotation induced by oscillating magnetic fields.

Rotation velocity proportional to the square of field amplitude.

Helix becomes unstable forming a space-time oscillating phase.

Abstract

In chiral magnets a magnetic helix forms where the magnetization winds around a propagation vector $\mathbf{q}$. We show theoretically that a magnetic field $\mathbf{B}_{\perp}(t) \perp \mathbf{q}$, which is spatially homogeneous but oscillating in time, induces a net rotation of the texture around $\mathbf{q}$. This rotation is reminiscent of the motion of an Archimedean screw and is equivalent to a translation with velocity $v_{\text{screw}}$ parallel to $\mathbf{q}$. Due to the coupling to a Goldstone mode, this non-linear effect arises for arbitrarily weak $\mathbf{B}_{\perp}(t) $ with $v_{\text{screw}} \propto |\mathbf{B}_{\perp}|^2$ as long as pinning by disorder is absent. The effect is resonantly enhanced when internal modes of the helix are excited and the sign of $v_{\text{screw}}$ can be controlled either by changing the frequency or the polarization of $\mathbf{B}_{\perp}(t)$. The Archimedean screw can be used to transport spin and charge and thus the screwing motion is predicted to induce a voltage parallel to $\mathbf{q}$. Using a combination of numerics and Floquet spin wave theory, we show that the helix becomes unstable upon increasing $\mathbf{B}_{\perp}$ forming a `time quasicrystal' which oscillates in space and time for moderately strong drive.