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Archimedean screw in driven chiral magnets

by Nina del Ser, Lukas Heinen and Achim Rosch

This Submission thread is now published as

Submission summary

Authors (as registered SciPost users): Lukas Heinen · Achim Rosch · Nina del Ser
Submission information
Preprint Link: scipost_202102_00005v2  (pdf)
Date accepted: 2021-07-01
Date submitted: 2021-05-31 10:20
Submitted by: del Ser, Nina
Submitted to: SciPost Physics
Ontological classification
Academic field: Physics
  • Condensed Matter Physics - Theory
Approaches: Theoretical, Computational


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

Author comments upon resubmission

We thanks the referees for their comments and attach our resubmission to SciPost Physics.

List of changes

We have brought a number of changes to the paper, the most notable ones being the following:
1) Corrected equation numbering (previously Eq.(1)-the free energy of the system- was missing a number).
2) Fig.1 panels exchanged to better reflect the order in which we refer to them in the text.
3) Reference [14] “Seki et al.” on page 3, bottom paragraph corrected to “Onose et al.”
4) Page 4, paragraph 2, last sentence: removed “requires some damping for its stability” as damping is already a given in experimental systems.
5) Page 5, under Eq(3): clarified that z is the direction assumed by vector q as the direction of spontaneous symmetry breaking in the absence of an external magnetic field B0.
6) Page 5, under Eq.(4): gave a definition for gyromagnetic ratio gamma and explained our choice of sign convention.
7) Page 6, first new paragraph under Eq.(8): exchange the word order to make it clear that Nx, Ny are the demagnetization factors.
8) Page 10, Fig. 4: updated the caption to emphasize that for driving frequency ω=2, the first Floquet zone lies between -1<Re[ ω]<1.
9) Page 11, under Eq.(17): added the sentence “The restriction of the Floquet space is formally justified because we investigate the system in the limit of small B_⊥ and our results for eigenenergies and decay rates are formally exact to quadratic order in B_⊥." to justify why the restriction of Floquet copies to m=-1,0,1 is valid for our case. See also reply to referee 2 for further details.
10)Page 11&12, Eq.(19),(21) and text between Eq(21)-(22) added missing “0” superscript to unperturbed energies ϵ_(i,k)^0, and missing minus signs for negative momentum.
11) Pages 15,16 and 17: changed all spin-orbit coupling constants λ→λ_SO to avoid confusion with Floquet eigenenergies from the previous section.
12)Page 17, Figure (9) and caption as well as Eq.(34) and the accompanying text all corrected for a mistake in the calculation of the current in the dirty limit. In this limit, current actually depends quadratically on τ, instead of being independent as previously claimed. Eq. (68) in App. F from which these results are derived was also corrected.
13)Page 18, paragraph 3: motivated by referee comments, we calculated ω_screw and v_screw for MnSi (micromagnetic parameter, as well as driving field and damping stated with references provided in the text), obtaining 10 MHz and 200 mm/s respectively, a factor 10 larger than for CSO.
14)Page 18, paragraph 4: again, motivated by referee comments, using experimental order of magnitude estimates for the spin-orbit coupling, the mean free path of electrons and electron density in MnSI and arrived at a current density order-of-magnitude estimate of 10^4-10^7 A/m^2. References where these experimental values can be found were also provided in the text.

Published as SciPost Phys. 11, 009 (2021)

Reports on this Submission

Anonymous Report 1 on 2021-6-14 (Invited Report)

  • Cite as: Anonymous, Report on arXiv:scipost_202102_00005v2, delivered 2021-06-14, doi: 10.21468/SciPost.Report.3059


I have checked all the changes which the authors made and found that all these minor revisions never affect the conclusion and discussion as well as the validity and importance of this work, while they improved the quality of presentations and arguments. I will not change my decision of strong recommendation for publishing this manuscript. I appreciate that the authors seriously considered and addressed the comments in my previous report.

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Anonymous on 2021-06-23  [id 1515]

I have checked all the authors' replies and revised manuscript. The authors had revised the manuscript accordingly to all of the comments, which had improved the clarity and the strength of the arguments. I state that now the manuscript is worth to be published in this Journal.