GHN colloquium with Dr. Veronika Sunko (UC Berkeley)

Summary

The GHN Colloquium talk series features the female scientists of the Grete Hermann Network (GHN) - a newly founded international network of female researchers in condensed matter physics and neighboring research areas. Distinguished female researchers are invited to give a lecture on their research and current projects, as well as about their career paths, to inspire young female scientists in particular, and to exchange ideas. After the official talk there will be an internal GHN networking event.

On January 30, 2024, we are happy to welcome Dr. Veronika Sunko from the UC Berkeley who will give an online scientific talk on A multimodal approach to uncovering magnetic symmetries: the case of an axion insulator candidate EuIn₂As₂.

Talk abstract

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, exhibiting a quantized magneto-electric effect. 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. While our data contradict previous proposals for magnetic structures, we demonstrate that all experimental results on EuIn₂As₂ can be reconciled into a unique coherent picture. We show that a higher temperature phase, previously thought to correspond to a spin-helix is in fact characterized by a ‘nodal amplitude modulated’ structure, unprecedented in materials dominated by local-moment magnetism. We further show that the ground state is similar to the previously proposed ‘broken helix,’ with one key difference: the relative orientation of the magnetic moments and the lattice can be freely tuned, resulting in an ‘unpinned broken helix’. Because of this freedom, the symmetries protecting the axion phase are not generically present, but we show how they can be retrieved by application of uniaxial stress. We complement this symmetry-based approach with an effective spin-Hamiltonian describing the ground state, which identifies the key role of itinerant electrons in stabilizing the exotic magnetism, and motivates investigation of electron density as a tuning knob for magnetism in EuIn₂As₂.
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 believe that such multimodal approach will prove invaluable to determine symmetries of complex ordered phases in a broad class of materials.

About Veronika Sunko

Veronika Sunko is a Miller Fellow 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 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.