螺旋磁态的自旋激发谱：旋螺、旋轮线、涡旋晶体和刺猬晶格
Spin excitation spectra in helimagnetic states: proper-screw, cycloid, vortex crystal, and hedgehog lattice
摘要：
本文利用线性自旋波理论研究了具有对称和反对称长程相互作用的有效自旋模型中手征磁体和极性磁体的自旋激发谱。在一维情况下，我们得到了旋量均匀的旋量旋量态和旋量态的动力学自旋结构因子的解析形式，在旋量波数处，旋量旋量态呈现出强强度的无隙模.当在对称相互作用中引入自旋各向异性时，我们数值计算表明，稳定的自旋螺线变成椭圆各向异性的非均匀扭曲和自旋激发是带隙的。在更高的维度，我们发现，类似的各向异性稳定多个$Q$自旋状态，如涡旋晶体和刺猬晶格。我们发现，在这些状态中的各向异性体现在动力学自旋结构因子：只有当螺旋波矢量和相应的易轴相互垂直时，才出现波数的横向分量中的强强度。我们的研究结果不仅可以用来确定自旋结构，而且还可以通过非弹性中子散射测量来推断稳定化机制。
英文摘要：
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.
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