LASSP Special Seminar- Veronika Sunko

A Multimodal Approach to Uncovering Magnetic Symmetries: The Case of Euln2As2

Understanding and manipulating emergent phases, which are themes at the forefront of modern quantum materials research, rely upon correctly identifying underlying symmetried. This general principle has been particularly prominent in materials with coupled electronic and magnetic degrees of freedom, in which symmetries can cause drastic changes to electronic states, including protecting exotic topological phases. Euln2As2 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 Euln2As2 is available, motivating a reexamination of standard assumptions.

I will show how combining scattering data with bespoke spatially-resolved symmetry-sensitive optical experiements and a group theory analysis led us to uniquely identify the two magnetic phases in Euln2As2 [3]. While our data contradic previous proposals for magnetic structures, we demonstrate that all experimental results on Euln2As2 can be reconciled into a unique picture. In addition to revealing the causes and consequences of exotic magnetism in Euln2As2, 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 braod class of materials.

References:

[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 fieldtunable
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.

Bio:

Veronika Sunko is a postdoc at UC Berkeley. She got her master degree from University of Zagreb in Croatia, and did her PhD in collaboration between the University of St Andrews and the Max Planck Institute for Chemical Physics of Solids in Dresden. For her thesis work she studied the electronic properties of delafossite oxides using angle resolved photoemission and uncovered new electronic features in this class of materials. She won several prizes for her thesis work, including the Springer Thesis prize and the Richard L. Greene Dissertation Award. During her postdoctoral work as a Miller Fellow she has been exploring new methods to probe symmetries of quantum materials using light, and utilizing them to reveal magnetic phases in several Eu-based compounds. In September 2025 she will start a new group at the Institute of Science and Technology Austria (ISTA), which will combine complementary experimental probes of symmetry to uncover new states in quantum materials.