反克尔米子晶体中的磁四极拓扑绝缘体
Magnonic Quadrupole Topological Insulator in Antiskyrmion Crystals
摘要：
当保护高阶拓扑相的晶体对称性在样品的边界处不被保留时，无间隙铰链模式或间隙内角态不能稳定。因此，需要仔细设计样品终止。类似地，磁性纹理，其量子波动决定了所支持的磁振子激发，倾向于松弛到新的配置，当引入边界时，也可能打破晶体对称性。在这里，我们发现，反kyrmion晶体提供了一个实验平台，实现磁振子拓扑四极绝缘体，其标志性签名是强大的磁振子角态。此外，我们表明，调整施加的磁场可以触发沿样品边缘携带分数拓扑电荷的反kyrmions的自组装。至关重要的是，这些分数反回旋子恢复了强制磁振子角态出现所需的对称性。使用嵌套的威尔逊圈，适应于磁振子系统支持的非共线磁纹理的机械，我们证明了体四极矩，边缘偶极矩，和角落的费用的量化。
英文摘要：
When the crystalline symmetries that protect a higher-order topological phase are not preserved at the boundaries of the sample, gapless hinge modes or in-gap corner states cannot be stabilized. Therefore, careful engineering of the sample termination is required. Similarly, magnetic textures, whose quantum fluctuations determine the supported magnonic excitations, tend to relax to new configurations that may also break crystalline symmetries when boundaries are introduced. Here we uncover that antiskyrmion crystals provide an experimentally accessible platform to realize a magnonic topological quadrupole insulator, whose hallmark signature are robust magnonic corner states. Furthermore, we show that tuning an applied magnetic field can trigger the self-assembly of antiskyrmions carrying a fractional topological charge along the sample edges. Crucially, these fractional antiskyrmions restore the symmetries needed to enforce the emergence of the magnonic corner states. Using the machinery of nested Wilson loops, adapted to magnonic systems supported by noncollinear magnetic textures, we demonstrate the quantization of the bulk quadrupole moment, edge dipole moments, and corner charges.
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