Spin excitation spectra in helimagnetic states: proper-screw, cycloid, vortex crystal, and hedgehog lattice

Abstract

We investigate the spin excitation spectra in chiral and polar magnets by the linear spin-wave theory for an effective spin model with symmetric and antisymmetric long-range interactions. In one dimension, we obtain the analytic form of the dynamical spin structure factor for proper-screw and cycloidal helical spin states with uniform twists, which shows a gapless mode with strong intensity at the helical wave number. When introducing spin anisotropy in the symmetric interactions, we numerically show that the stable spin spirals become elliptically anisotropic with nonuniform twists and the spin excitation is gapped. In higher dimensions, we find that similar anisotropy stabilizes multiple-$Q$ spin states, such as vortex crystals and hedgehog lattices. We show that the anisotropy in these states manifests itself in the dynamical spin structure factor: a strong intensity in the transverse components to the wave number appears only when the helical wave vector and the corresponding easy axis are perpendicular to each other. Our findings could be useful not only to identify the spin structure but also to deduce the stabilization mechanism by inelastic neutron scattering measurements.

23 pages, 21 figures

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• Stability of Skyrmion Crystal Phase in Centrosymmetric Distorted Triangular-Lattice Antiferromagnets
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• Hidden Topological Transitions in Emergent Magnetic Monopole Lattices
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• Hybrid skyrmion and anti-skyrmion phases in polar $C_{4v}$ systems
• Theory of collective excitations in the quadruple-$Q$ magnetic hedgehog lattices
• Current-induced motion of nanoscale magnetic torons over the wide range of the Hall angle
• Field-direction-dependent skyrmion crystals in noncentrosymmetric cubic magnets: A comparison between point groups $(O,T)$ and $T_{\mathrm{d}}$
• Keldysh pseudo-fermion functional renormalization group for quantum magnetism
• Dynamical and Static Structure Factors in Hedgehog-Antihedgehog Order in Icosahedral 1/1 Approximant Crystal
• Soliton penetration from edges in a monoaxial chiral magnet
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• Symmetry Rules on Multipole Interactions under Crystallographic Point Groups and Application to Multiple-$Q$ Multipole States