磁性 Kagome 金属 FeGe 的对称性破缺与提升,Physical Review X

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磁性 Kagome 金属 FeGe 的对称性破缺与提升
Physical Review X ( IF 11.6 ) Pub Date : 2024-03-08 , DOI: 10.1103/physrevx.14.011043
Shangfei Wu , Mason L. Klemm , Jay Shah , Ethan T. Ritz , Chunruo Duan , Xiaokun Teng , Bin Gao , Feng Ye , Masaaki Matsuda , Fankang Li , Xianghan Xu , Ming Yi , Turan Birol , Pengcheng Dai , Girsh Blumberg

自发对称破缺——无穷小的扰动可能导致系统破坏潜在对称性的现象——是理解相互作用的固态系统的基石概念。在一系列典型的温度驱动相变中，由于随着温度升高，熵的稳定效应变得占主导地位，因此较高温度的相更加对称。然而，当系统存在多个自由度时，相反的情况很少见，但也是可能的。在这里，我们利用中子拉莫尔衍射和拉曼光谱展示了磁性 kagome 金属 FeGe 冷却时对称性上升现象的例子。 FeGe 具有 Kagome 晶格结构，在尼尔温度以下具有简单的 A 型反铁磁序

{时间}_{氮} \approx 400 K

和电荷密度波 (CDW) 跃迁

{时间}_{CDW} \approx 110 K

，随后在 60 K 左右发生自旋倾斜跃迁。在 460 K 的顺磁态下，我们确认晶体结构确实是六方 kagome 晶格。冷却至约

{时间}_{氮}

，晶体结构从六方晶系转变为单斜晶系，面内晶格畸变约为

10^{- 4}

以及原始戈薇晶格的双简并声子模式的相关分裂。进一步冷却至

{时间}_{CDW}

，戈薇晶格表现出较小的负热膨胀，并且晶体结构在进一步冷却时逐渐变得更加对称。冷却时晶体对称性增加的趋势是不寻常的。它源于极弱的结构不稳定性，与碰撞波和磁序共存并竞争。这些观察结果与具有单阶参数的简单模型的预期相反，因此只能通过考虑多个晶格、电荷和自旋自由度的朗道自由能展开来解释。因此，晶格对称性以及不寻常的自旋晶格耦合的确定是理解系统丰富的电子和磁性特性的第一步，并且它为晶格自由度不存在的交织有序提供了新的线索。统治时间更长。

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Symmetry Breaking and Ascending in the Magnetic Kagome Metal FeGe

Spontaneous symmetry breaking—the phenomenon in which 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 upon cooling in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. FeGe has a kagome lattice structure with simple A-type antiferromagnetic order below Néel temperature

T_{N} \approx 400 K

and a charge density wave (CDW) transition at

T_{CDW} \approx 110 K

, followed by a spin-canting transition at around 60 K. In the paramagnetic state at 460 K, we confirm that the crystal structure is indeed a hexagonal kagome lattice. On cooling to around

T_{N}

, 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

T_{CDW}

, the kagome lattice shows a small negative thermal expansion, and the crystal structure gradually becomes more symmetric upon further cooling. A tendency of 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 and 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 it sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant.

更新日期：2024-03-08

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