[论文解读] Triple-Well Charge Density Wave Transition Driven by Cooperation between Peierls-like Effect and Antiferromagnetic Order in FeGe

Binhua Zhang, Junyi Ji+2 more|arXiv (Cornell University)|Jul 20, 2023

Topological Materials and PhenomenaPhysics and Astronomy被引用 3

一句话总结

本研究揭示，FeGe中的三阱电荷密度波（CDW）相变是由一种类似Peierls效应与反铁磁序之间的协同机制所驱动，其中Ge 1 p_z能带下移超过0.5 eV以及费米能级钉扎，稳定了CDW相；而磁序则削弱了Fe-Ge杂化并降低Ge能带能量，从而解决了长期以来在原始相中无声子不稳定性却仍能保持CDW稳定性的难题。

ABSTRACT

Kagome materials provide a promising platform to explore intriguing correlated phenomena including magnetism, charge density wave (CDW), and nontrivial band topology. Recently, a CDW order was observed in antiferromagnetic kagome metal FeGe, sparking enormous research interests in intertwining physics of CDW and magnetism. Two of the core questions are (i) what are the driving forces of the CDW transition in FeGe and (ii) whether magnetism play a critical role in the transition. Such questions are critical as conventional mechanisms of van Hove singularities and Fermi surface nesting fail to explain the stable pristine phase, as well as the role of magnetism. Here, supported by density functional theory and tight-binding models, we unravel the triple-well CDW energy landscape of FeGe, indicating that both the pristine and CDW phases are locally stable. We point out that an entire downward shift of Ge band, instead of the previously proposed Fe bands, competes with the lattice distortion energy, driving the triple-well CDW transition. It is indeed a cooperation between the Peierls-like effect and the Fermi energy pinning phenomenon, which is distinct from the conventional Peierls effect that drives a double-well transition. Moreover, we demonstrate that the antiferromagnetic order also plays a critical role in driving the CDW transition, through weakening the Fe-Ge hybridization by exchange splitting and lowering the position of Ge-bands with respect to the Fermi energy. Our work thus not only deepens the understanding of the CDW mechanism in FeGe, but also indicates an intertwined connection between the emergent magnetism and CDW in kagome materials.

研究动机与目标

• 解决FeGe中稳定且多重稳定的CDW相的形成机制，该相在原始相中缺乏声子不稳定性。
• 确定传统CDW机制（如费米面嵌套或van Hove奇点）是否能解释观测到的CDW相变。
• 研究反铁磁序在驱动或稳定此kagome金属中CDW相中的作用。
• 揭示CDW相变中三阱能谷结构的微观起源，该结构与传统的双阱Peierls相变有本质区别。

提出的方法

• 采用第一性原理密度泛函理论（DFT）计算，研究FeGe在原始相与CDW相中的电子结构及能量景观。
• 构建紧束缚（TB）模型以模拟电子能带结构，其中轨道能级与跃迁积分由DFT结果导出，从而分析晶格畸变下的能带分裂。
• 应用四态能量映射法，从DFT总能量中提取磁交换相互作用，使用√3 × √3 × 1超胞和4×4×10 k点网格。
• 采用30×30×1超胞和每格点100,000次蒙特卡罗（MC）步长的蒙特卡罗模拟，验证磁序及其对CDW相变的影响。
• 在单离子各向异性计算中引入自旋-轨道耦合，以确保磁性建模的准确性。
• 分析聚焦于Ge 1 p_z轨道能带，追踪其在成键过程中的能级偏移与占据变化，以识别类似Peierls效应与费米能级钉扎。

实验结果

研究问题

• RQ1 在原始相中无声子不稳定性的情况下，是什么驱动了FeGe中三阱CDW能谷结构？
• RQ2 该体系中的类似Peierls效应与传统Peierls相变有何不同？Ge 1 p_z能带在此过程中起什么作用？
• RQ3 反铁磁序通过电子结构调制，在CDW相变中起到了多大程度的贡献？
• RQ4 为何传统CDW机制（费米面嵌套与van Hove奇点）无法解释FeGe中的CDW现象？

主要发现

• FeGe中的CDW相变表现出三阱能谷结构，原始相与CDW相均为局域稳定态，解释了高温相中无声子不稳定性。
• 主要驱动力为一种类似Peierls效应，即在Ge 1成键过程中Ge 1 p_z能带下移超过0.5 eV，从而降低体系总能量。
• 与其他原子的杂化导致费米能级钉扎，使Ge 1 p_z能带完全占据，进一步稳定CDW相。
• 反铁磁序通过交换分裂降低Fe-Ge杂化并使Ge能带相对费米能级下移，从而在稳定CDW相中起关键作用。
• 类似Peierls效应与磁序的协同作用，形成了一种与传统双阱Peierls相变截然不同的独特三阱相变。
• 理论建模证实，CDW相的稳定并非源于Fe衍生的van Hove奇点或嵌套效应，而是Ge能带位移与磁序相互作用的结果。