Archimedean screw in driven chiral magnets

In chiral magnets a magnetic helix forms where the magnetization winds around a propagation vector q. We show theoretically that a magnetic field B-perpendicular to(t) perpendicular to q, which is spatially homogeneous but oscillating in time, induces a net rotation of the texture around q. This rotation is reminiscent of the motion of an Archimedean screw and is equivalent to a translation with velocity nu(screw) parallel to q. Due to the coupling to a Goldstone mode, this non-linear effect arises for arbitrarily weak B-perpendicular to(t) with nu(screw) proportional to vertical bar B-perpendicular to vertical bar(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 nu(screw) can be controlled either by changing the frequency or the polarization of B-perpendicular to(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-perpendicular to, forming a 'time quasicrystal' which oscillates in space and time for moderately strong drive.

Automated summary

In chiral magnets, a spatially homogeneous but oscillating magnetic field perpendicular to the propagation vector induces a net rotation of the texture around the vector, reminiscent of the motion of an Archimedean screw. This effect, proportional to the square of the field strength, can transport spin and charge, and can be resonantly enhanced by exciting internal modes of the helix. The helix can become unstable under stronger fields, forming a 'time quasicrystal' that oscillates in space and time.