Multimodal Approach Reveals the Symmetry-Breaking Pathway to the Broken Helix inEuIn2⁢As2,Physical Review X

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Multimodal Approach Reveals the Symmetry-Breaking Pathway to the Broken Helix inEuIn2⁢As2
Physical Review X ( IF 11.6 ) Pub Date : 2024-07-22 , DOI: 10.1103/physrevx.14.031013
E. Donoway _{1,

2} , T. V. Trevisan _{1,

2} , A. Liebman-Peláez _{1,

2} , R. P. Day _{1,

2} , K. Yamakawa _{1,

2} , Y. Sun _{1,

2} , J. R. Soh ₃ , D. Prabhakaran ₄ , A. T. Boothroyd ₄ , R. M. Fernandes ₅ , J. G. Analytis _{1,

2,

6,

7} , J. E. Moore _{1,

2} , J. Orenstein _{1,

2} , V. Sunko _{1,

2}

Affiliation

Understanding and manipulating emergent phases, which are themes at the forefront of quantum-materials research, rely on identifying their underlying symmetries. This general principle has been particularly prominent in materials with coupled electronic and magnetic degrees of freedom, in which magnetic order influences the electronic band structure and can lead to exotic topological effects. However, identifying symmetry of a magnetically ordered phase can pose a challenge, particularly in the presence of small domains. Here we introduce a multimodal approach for determining magnetic structures, which combines symmetry-sensitive optical probes, scattering, and group-theoretical analysis. We apply it to

E u I n 2 A s 2

, a material that has received attention as a candidate axion insulator. While first-principles calculations predict this state on the assumption of a simple collinear antiferromagnetic structure, subsequent neutron-scattering measurements reveal a much more intricate magnetic ground state characterized by two coexisting magnetic wave vectors reached by successive thermal phase transitions. The proposed high- and low-temperature phases are a spin helix and a state with interpenetrating helical and Néel antiferromagnetic order termed a “broken helix,” respectively. Employing a multimodal approach, we identify the magnetic structure associated with these two phases of

E u I n 2 A s 2

. We find that the higher-temperature phase is characterized by a variation of the magnetic moment amplitude from layer to layer, with the moment vanishing entirely in every third Eu layer. The lower-temperature structure is similar to the broken helix, with one important difference: Because of local strain, the relative orientation of the magnetic structure and the lattice is not fixed. Consequently, the symmetry required to protect the axion phase is not generically protected in

E u I n 2 A s 2

, but we show that it can be restored if the magnetic structure is tuned with uniaxial strain. Finally, we present a spin Hamiltonian that identifies the spin interactions that account for the complex magnetic order in

E u I n 2 A s 2

. Our work highlights the importance of a multimodal approach in determining the symmetry of complex order parameters.

中文翻译：

多模态方法揭示了 EuIn2As2 中断裂螺旋的对称性破坏途径

理解和操纵紧急阶段是量子材料研究的最前沿主题，依赖于确定其潜在的对称性。这一一般原理在电子和磁自由度耦合的材料中尤为突出，其中磁序会影响电子能带结构，并可能导致奇特的拓扑效应。然而，识别磁序相的对称性可能是一项挑战，尤其是在存在小畴的情况下。在这里，我们介绍了一种确定磁性结构的多模态方法，该方法结合了对称敏感的光学探针、散射和群论分析。我们将其应用于

EuIn2As2

，这是一种作为候选轴子绝缘体而受到关注的材料。虽然第一性原理计算在假设简单的共线反铁磁结构上预测了这种状态，但随后的中子散射测量揭示了一个更复杂的磁基态，其特征是通过连续的热相变达到两个共存的磁波矢量。提出的高温和低温相是自旋螺旋和具有相互穿脱的螺旋和 Néel 反铁磁序的状态，分别称为“断螺旋”。采用多模态方法，我们确定了与