SciPost Physics | 2021
Archimedean screw in driven chiral magnets
Abstract
<jats:p>In chiral magnets a magnetic helix forms where the\nmagnetization winds around a propagation vector\n<jats:inline-formula><jats:alternatives><jats:tex-math>{q}</jats:tex-math><mml:math xmlns:mml= http://www.w3.org/1998/Math/MathML display= inline ><mml:mi>q</mml:mi></mml:math></jats:alternatives></jats:inline-formula>.\nWe show theoretically that a magnetic field <jats:inline-formula><jats:alternatives><jats:tex-math>B_\\bot(t) \\bot q</jats:tex-math><mml:math xmlns:mml= http://www.w3.org/1998/Math/MathML display= inline ><mml:mrow><mml:msub><mml:mi>B</mml:mi><mml:mi>⊥</mml:mi></mml:msub><mml:mo stretchy= false form= prefix >(</mml:mo><mml:mi>t</mml:mi><mml:mo stretchy= false form= postfix >)</mml:mo><mml:mi>⊥</mml:mi><mml:mi>q</mml:mi></mml:mrow></mml:math></jats:alternatives></jats:inline-formula>, which is\nspatially homogeneous but oscillating in time, induces a net rotation of\nthe texture around <jats:inline-formula><jats:alternatives><jats:tex-math>{q}</jats:tex-math><mml:math xmlns:mml= http://www.w3.org/1998/Math/MathML display= inline ><mml:mi>q</mml:mi></mml:math></jats:alternatives></jats:inline-formula>.\nThis rotation is reminiscent of the motion of an Archimedean screw and\nis equivalent to a translation with velocity\n<jats:inline-formula><jats:alternatives><jats:tex-math>v_{\\text{screw}}</jats:tex-math><mml:math xmlns:mml= http://www.w3.org/1998/Math/MathML display= inline ><mml:msub><mml:mi>v</mml:mi><mml:mtext mathvariant= normal >screw</mml:mtext></mml:msub></mml:math></jats:alternatives></jats:inline-formula>\nparallel to q. Due to the coupling to a Goldstone mode, this\nnon-linear effect arises for arbitrarily weak <jats:inline-formula><jats:alternatives><jats:tex-math>B_\\bot(t)</jats:tex-math><mml:math xmlns:mml= http://www.w3.org/1998/Math/MathML display= inline ><mml:mrow><mml:msub><mml:mi>B</mml:mi><mml:mi>⊥</mml:mi></mml:msub><mml:mo stretchy= false form= prefix >(</mml:mo><mml:mi>t</mml:mi><mml:mo stretchy= false form= postfix >)</mml:mo></mml:mrow></mml:math></jats:alternatives></jats:inline-formula> with\n<jats:inline-formula><jats:alternatives><jats:tex-math>v_{\\text{screw}} \\propto |{ B_\\perp}|^2</jats:tex-math><mml:math xmlns:mml= http://www.w3.org/1998/Math/MathML display= inline ><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mtext mathvariant= normal >screw</mml:mtext></mml:msub><mml:mo>∝</mml:mo><mml:mo stretchy= false form= prefix >|</mml:mo><mml:msub><mml:mi>B</mml:mi><mml:mo>⊥</mml:mo></mml:msub><mml:msup><mml:mo stretchy= false form= prefix >|</mml:mo><mml:mn>2</mml:mn></mml:msup></mml:mrow></mml:math></jats:alternatives></jats:inline-formula>\nas long as pinning by disorder is absent. The effect is resonantly\nenhanced when internal modes of the helix are excited and the sign of\n<jats:inline-formula><jats:alternatives><jats:tex-math>v_{\\text{screw}}</jats:tex-math><mml:math xmlns:mml= http://www.w3.org/1998/Math/MathML display= inline ><mml:msub><mml:mi>v</mml:mi><mml:mtext mathvariant= normal >screw</mml:mtext></mml:msub></mml:math></jats:alternatives></jats:inline-formula>\ncan be controlled either by changing the frequency or the polarization\nof <jats:inline-formula><jats:alternatives><jats:tex-math>B_\\bot(t)</jats:tex-math><mml:math xmlns:mml= http://www.w3.org/1998/Math/MathML display= inline ><mml:mrow><mml:msub><mml:mi>B</mml:mi><mml:mi>⊥</mml:mi></mml:msub><mml:mo stretchy= false form= prefix >(</mml:mo><mml:mi>t</mml:mi><mml:mo stretchy= false form= postfix >)</mml:mo></mml:mrow></mml:math></jats:alternatives></jats:inline-formula>. The Archimedean screw can be used to transport spin and\ncharge and thus the screwing motion is predicted to induce a voltage\nparallel to q. Using a combination of numerics and Floquet spin wave\ntheory, we show that the helix becomes unstable upon increasing <jats:inline-formula><jats:alternatives><jats:tex-math>B_\\bot</jats:tex-math><mml:math xmlns:mml= http://www.w3.org/1998/Math/MathML display= inline ><mml:msub><mml:mi>B</mml:mi><mml:mi>⊥</mml:mi></mml:msub></mml:math></jats:alternatives></jats:inline-formula>,\nforming a `time quasicrystal’ which oscillates in space and time for\nmoderately strong drive.</jats:p>