[论文解读] Negative flatband magnetism in a spin-orbit coupled kagome magnet

Jia‐Xin Yin, Songtian S. Zhang+18 more|arXiv (Cornell University)|Jan 15, 2019

Topological Materials and Phenomena被引用 6

一句话总结

本研究利用扫描隧道显微镜揭示了凯莫夫磁体Co3Sn2S2中负平坦能带磁性的存在，表明自旋-轨道耦合诱导了与外加磁场方向相反的Zeeman位移。费米能级处的平坦能带表现出异常轨道磁化，这与其非平庸的贝里曲率相关联，确立了Co3Sn2S2作为自旋-轨道耦合平坦能带物理研究平台的地位。

ABSTRACT

It has long been speculated that electronic flat band systems can be a fertile ground for hosting novel emergent phenomena including unconventional magnetism and superconductivity. Although flat bands are known to exist in a few systems such as heavy fermion materials and twisted bilayer graphene, their microscopic roles and underlying mechanisms in generating emergent behavior remain elusive. Here we use scanning tunneling microscopy to elucidate the atomically resolved electronic states and their magnetic response in the kagome magnet Co3Sn2S2. We observe a pronounced peak at the Fermi level, which is identified to arise from the kinetically frustrated kagome flat band. Increasing magnetic field up to +-8T, this state exhibits an anomalous magnetization-polarized Zeeman shift, dominated by an orbital moment in opposite to the field direction. Such negative magnetism can be understood as spin-orbit coupling induced quantum phase effects tied to non-trivial flat band systems. We image the flat band peak, resolve the associated negative magnetism, and provide its connection to the Berry curvature field, showing that Co3Sn2S2 is a rare example of kagome magnet where the low energy physics can be dominated by the spin-orbit coupled flat band. Our methodology of probing band-resolved ordering phenomena such as spin-orbit magnetism can also be applied in future experiments to elucidate other exotic phenomena including flat band superconductivity and anomalous quantum transport.

研究动机与目标

• 研究平坦能带在凯莫夫晶格材料中主导非传统磁性的角色。
• 阐明自旋-轨道耦合平坦能带系统中异常磁响应的微观起源。
• 建立平坦能带电子结构与涌现量子现象（如负磁性）之间的直接联系。
• 证明Co3Sn2S2是少数低能物理由自旋-轨道耦合平坦能带主导的罕见体系。
• 开发用于探测奇异量子相（如平坦能带超导性和异常输运）的能带分辨探测方法。

提出的方法

• 采用高分辨率扫描隧道显微镜（STM）成像Co3Sn2S2中的原子尺度电子态。
• 测量局域态密度（LDOS）以识别费米能级处的平坦能带特征。
• 施加高达±8 T的磁场，探测平坦能带态的Zeeman响应。
• 分析平坦能带峰的磁化极化位移，以区分自旋与轨道贡献。
• 通过理论建模将观测到的负磁性与贝里曲率场相关联。
• 利用空间分辨光谱映射与平坦能带相关的非平庸量子相效应。

实验结果

研究问题

• RQ1 在磁场作用下，Co3Sn2S2平坦能带中异常磁化响应的起源是什么？
• RQ2 自旋-轨道耦合如何影响凯莫夫晶格平坦能带的磁行为？
• RQ3 观测到的负磁性是否可归因于轨道磁矩效应而非自旋极化？
• RQ4 该体系中平坦能带的电子结构与贝里曲率场之间存在何种关联？
• RQ5 平坦能带在多大程度上主导了Co3Sn2S2的低能物理？

主要发现

• 在局域态密度中费米能级处的显著峰被确认源于凯莫夫平坦能带。
• 在±8 T的磁场下，平坦能带峰表现出与磁场方向相反的Zeeman位移，表明存在负磁性。
• 异常磁化主要由与外加磁场方向相反的轨道磁矩主导。
• 负磁性归因于非平庸平坦能带中自旋-轨道耦合诱导的量子相效应。
• 平坦能带峰被直接成像，其磁响应与贝里曲率场直接关联。
• 确立了Co3Sn2S2是少数低能物理由自旋-轨道耦合平坦能带主导的罕见体系。