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Register a new userArchimedean Screw in Driven Chiral Magnets
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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.
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