Archimedean Screw in Driven Chiral Magnets

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.

12 pages, 9 figures. Supplementary: 7 pages, 1 figure. V2: added references, added videos showing dynamics of driven system. V3: corrected mistake in Sec. VI: transport, and eq. F7 in App.F. In the limit $v_F\tau \ll q^{-1}$ current grows as ${\tau }^2$, rather than being independent of $\tau$ as previously claimed. Changes in v4: minor revisions following referee recommendations; typos corrected