[论文解读] Symmetry breaking and ascending in the magnetic kagome metal FeGe

Shangfei Wu, Mason Klemm+13 more|arXiv (Cornell University)|Sep 25, 2023

Topological Materials and Phenomena被引用 6

一句话总结

本研究报道了磁性凯库梅金属FeGe中罕见的对称性上升转变，即随着温度降低，晶格对称性反而提高，这是由于晶格、电荷和自旋自由度之间的竞争所致。通过中子拉莫尔衍射和拉曼光谱，作者识别出在T_N ≈ 400 K时出现单斜畸变，随后在T_CDW ≈ 110 K以下形成对称性增加的CDW相，该相由弱结构不稳定性与磁序和电荷序共存所驱动。

ABSTRACT

Spontaneous symmetry breaking-the phenomenon where an infinitesimal perturbation can cause the system to break the underlying symmetry-is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. In the paramagnetic state at 460K, we confirm that the crystal structure is indeed hexagonal kagome lattice. On cooling to TN, the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions on the order of 10^(-4) and the associated splitting of the double degenerate phonon mode of the pristine kagome lattice. Upon further cooling to TCDW, the kagome lattice shows a small negative thermal expansion, and the crystal structure becomes more symmetric gradually upon further cooling. Increasing the crystalline symmetry upon cooling is unusual, it originates from an extremely weak structural instability that coexists and competes with the CDW and magnetic orders. These observations are against the expectations for a simple model with a single order parameter, hence can only be explained by a Landau free energy expansion that takes into account multiple lattice, charge, and spin degrees of freedom. Thus, the determination of the crystalline lattice symmetry as well as the unusual spin-lattice coupling is a first step towards understanding the rich electronic and magnetic properties of the system and sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant.

研究动机与目标

• 研究磁性凯库梅金属FeGe中晶格、电荷和自旋自由度之间的相互作用。
• 确定在冷却过程中晶格对称性是否增加，这与传统的对称性破缺预期相反。
• 阐明FeGe中异常结构转变的起源，特别是CDW转变以下对称性增强的机制。
• 确立自旋-晶格耦合及竞争序参数在稳定非传统相变中的作用。
• 通过多技术实验探针，建立对凯库梅晶格体系中交织电子序的微观理解。

提出的方法

• 采用中子拉莫尔衍射测量磁性和结构转变，对晶格畸变具有高灵敏度。
• 在RR和RL几何构型下进行拉曼光谱测量，以探测声子模式及其随温度的演化。
• 应用耦合双洛伦兹声子模型结合格林函数形式，描述模式耦合与谱重转移。
• 采用非谐声子衰变模型拟合声子频率和线宽的温度依赖性，引入玻色-爱因斯坦统计。
• 进行朗道自由能分析，以考虑多个竞争序参数（磁序、CDW、晶格）的影响。
• 通过中子衍射测量晶格畸变，在单斜相中检测到约10⁻⁴量级的面内变化。

实验结果

研究问题

• RQ1 在关联电子体系中，晶格对称性是否可能在冷却过程中增加，与标准的对称性破缺预期相反？
• RQ2 FeGe中观测到的晶格畸变及对称性演化在相变过程中的起源是什么？
• RQ3 自旋-晶格耦合与竞争序参数如何影响FeGe的结构相图？
• RQ4 声子模式在多大程度上反映了凯库梅晶格中电荷密度波与磁序的相互作用？
• RQ5 多序参量朗道理论能否解释FeGe中晶格对称性非单调演化的现象？

主要发现

• FeGe的晶体结构在T_N ≈ 400 K时由六方相转变为单斜相，面内晶格畸变为~10⁻⁴，表明对称性破缺。
• 进一步冷却至T_CDW ≈ 110 K以下时，晶格对称性提高，表现出负热膨胀和对称性上升行为。
• 拉曼光谱显示在T_N处双简并的E₂g声子模式发生分裂，证实了单斜畸变。
• 在90 K的CDW相中，A_g声子模式在100、127和203 cm⁻¹处的积分强度出现峰值，于90 K达到最大值后在T_canting ≈ 60 K时消失。
• 耦合双洛伦兹模型成功描述了RL几何中约162 cm⁻¹处的肩峰，表明存在模式耦合与能级排斥。
• 非谐声子衰变模型能够拟合声子频率和线宽的温度依赖性，其耦合机制通过三声子过程与声学模式关联。