Archimedean screw in driven chiral magnets

• Published in: SciPost physics Vol. 11; no. 1; p. 009
• Main Authors: del Ser, Nina, Heinen, Lukas, Rosch, Achim
• Format: Journal Article
• Language: English
• Published: SciPost 01.07.2021
• ISSN: 2542-4653; 2542-4653
• DOI: 10.21468/SciPostPhys.11.1.009

In chiral magnets a magnetic helix forms where the magnetization winds around a propagation vector {q} q . We show theoretically that a magnetic field B_\bot(t) \bot q B ⊥ ( t ) ⊥ q , which is spatially homogeneous but oscillating in time, induces a net rotation of the texture around {q} q . This rotation is reminiscent of the motion of an Archimedean screw and is equivalent to a translation with velocity v_{\text{screw}} v screw parallel to q. Due to the coupling to a Goldstone mode, this non-linear effect arises for arbitrarily weak B_\bot(t) B ⊥ ( t ) with v_{\text{screw}} \propto |{ B_\perp}|^2 v screw ∝ | B ⊥ | 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}} v screw can be controlled either by changing the frequency or the polarization of B_\bot(t) B ⊥ ( 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 q. Using a combination of numerics and Floquet spin wave theory, we show that the helix becomes unstable upon increasing B_\bot B ⊥ , forming a `time quasicrystal’ which oscillates in space and time for moderately strong drive.