[论文解读] Flat band-engineered spin-density wave and the emergent multi-$k$ magnetic state in the topological kagome metal Mn$_{3}$Sn

Xiao Wang, Fengfeng Zhu+17 more|arXiv (Cornell University)|Jun 7, 2023

Topological Materials and Phenomena被引用 6

一句话总结

本研究通过费米面嵌套效应揭示了在拓扑凯莫格金属Mn₃Sn中由平带工程诱导的自旋密度波（SDW）序。中子衍射与DFT计算共同证实，在280 K以下，系统发生向双k非共面非公度磁性态的相变，且通过互调谐波（2kₗ + kₜ）明确确立了多k磁性基态。

ABSTRACT

Magnetic kagome metals, in which topologically non-trivial band structures and electronic correlation are intertwined, have recently emerged as an exciting platform to explore exotic correlated topological phases, that are usually not found in weakly interacting materials described within the semi-classical picture of electrons. Here, via a comprehensive single-crystal neutron diffraction and first-principles density functional theory study of the archetypical topological kagome metal Mn$_3$Sn, which is also a magnetic Weyl fermion material and a promising chiral magnet for antiferromagnetic spintronics, we report the realisation of an emergent spin-density wave (SDW) order, a hallmark correlated many-body phenomenon, that is engineered by the Fermi surface nesting of topological flat bands. We further reveal that the phase transition, from the well-known high-temperature coplanar and non-collinear k = 0 inverse triangular antiferromagnetic order to a double-$k$ non-coplanar modulated incommensurate magnetic structure below $T_1$ = 280 K, is primarily driven by the SDW instability. The double-$k$ nature of this complex low-temperature magnetic order, which can be regarded as an intriguing superposition of a longitudinal SDW with a modulation wavevector k$_L$ and a transverse incommensurate helical magnetic order with a modulation wavevector k$_T$, is unambiguously confirmed by our observation of the inter-modulation high-order harmonics of the type of 2k$_L$+k$_T$. This discovery not only solves a long-standing puzzle concerning the nature of the phase transition at $T_1$, but also provides an extraordinary example on the intrinsic engineering of correlated many-body phenomena in topological matter. The identified multi-$k$ magnetic state can be further exploited for the engineering of the new modes of magnetization and chirality switching in antiferromagnetic spintronics.

研究动机与目标

• 解决Mn₃Sn在T₁ = 280 K处相变本质的长期谜题。
• 探究拓扑平带在凯莫格金属中驱动自旋密度波（SDW）不稳定性的作用。
• 表征Mn₃Sn中涌现的多k磁序并确定其波矢结构。
• 建立强关联拓扑材料中电子拓扑、费米面嵌套与关联磁序之间的联系。

提出的方法

• 采用多种散射几何（xsf、zsf、znsf）的单晶中子衍射，探测磁结构及波矢分量。
• 在D23束线进行高分辨率Q扫描，精确测定调制波矢并识别互调谐波。
• 采用包含自旋-轨道耦合及室温k = 0磁序的第一性原理密度泛函理论（DFT）计算，模拟电子能带结构与费米面。
• 分析不同通道的磁散射强度，提取磁矩分量并验证双k结构。
• 对高阶衍射峰进行指数标定，以区分单k与多k磁序，特别检验2kₗ + kₜ与3kₜ的差异。
• 对比有无磁序情况下的计算费米面与态密度（DOS），评估嵌套条件与平带效应。

实验结果

研究问题

• RQ1 Mn₃Sn在T₁ = 280 K处的相变由何驱动，是否与自旋密度波（SDW）不稳定性相关？
• RQ2 低温磁序的真实性质是什么——具体而言，是否为双k非共面非公度结构？
• RQ3 能否在实验中观测到互调高阶谐波（如2kₗ + kₜ），并用于确认多k磁性基态？
• RQ4 位于费米能级附近的拓扑平带如何影响Mn₃Sn中的磁不稳定性与SDW形成？
• RQ5 凯莫格晶格中平带的费米面嵌套在多大程度上促进了关联多体磁序的形成？

主要发现

• T₁ = 280 K处的相变主要由自旋密度波（SDW）不稳定性驱动，而非传统磁有序。
• 低温磁性态为双k非共面非公度结构，包含纵向SDW（kₗ）与横向螺旋序（kₜ）。
• 实验观测到形式为2kₗ + kₜ的互调高阶谐波，并被明确识别，证实了磁序的多k特性。
• 布里渊区K–M–K路径上的平带在费米能级附近（E_F + 81 meV）产生态密度的尖锐峰，促进强费米面嵌套。
• 在(h,0,l)、(h,k,0)或(h,h,l)平面中未发现费米面嵌套条件，但靠近E_F的平带通过其不同部分间的嵌套驱动了SDW不稳定性。
• 磁结构由拓扑平带与电子关联的协同作用稳定，展示了在拓扑物质中内在工程关联多体现象的实现。