A multimodal approach to uncovering magnetic symmetries: the case of EuIn₂As₂

Understanding and manipulating emergent phases, which are themes at the forefront of modern quantum materials research, rely upon correctly identifying underlying symmetries. This general principle has been particularly prominent in materials with coupled electronic and magnetic degrees of freedom, in which magnetic symmetries can cause drastic changes to electronic states, including protecting exotic topological phases. EuIn₂As₂ is a prominent example of the latter: it has been identified as a prime candidate to host the elusive axion state [1,2]. However, despite intense experimental efforts, no direct evidence for topology in EuIn₂As₂ is available, motivating a reexamination of standard assumptions.

I will show how combining scattering data with bespoke spatially-resolved symmetry-sensitive optical experiments and a group theory analysis led us to uniquely identify the two magnetic phases in EuIn₂As₂ [3]. While our data contradict previous proposals for magnetic structures, we demonstrate that all experimental results on EuIn₂As₂ can be reconciled into a unique picture. In addition to revealing the causes and consequences of exotic magnetism in EuIn₂As₂, I will highlight the importance of combining the information obtained through complementary experimental probes, with special emphasis on probes of time-reversal symmetry breaking. I will argue that such a multimodal approach is an invaluable asset in the quest for determining symmetries of complex ordered phases in a broad class of materials.

[1] Xu Y., “Higher-Order Topology of the Axion Insulator EuIn₂As₂,” Phys. Rev. Lett., 122 256402 (2019)

[2] Riberolles S. X. M. et al.: “Magnetic crystalline-symmetry-protected axion electrodynamics and field-tunable unpinned Dirac cones in EuIn₂As₂,” Nat Commun, 12 999 (2021)

[3] Donoway E, (…), Sunko V.: “Symmetry-breaking pathway towards the unpinned broken helix” Phys. Rev. X 2024, 14 (3), 031013.