Magneto-optical probes shed light on the unconventional transport and magnetism of Eu – based semi-metals
Dr. Veronika Sunko
Department of Physics
UC Berkeley
Understanding the mechanisms and consequences of the coupling between disparate degrees of freedom is at the forefront of quantum materials research. A prominent example is coupling between magnetic and electronic degrees of freedom, offering the exciting opportunity to control the near Fermi level electronic structure and topology by manipulating magnetic order. However, the same coupling may promote a complex magnetic state, which is challenging to identify experimentally.
Using a series of symmetry-sensitive optical techniques we investigate two Eu-based semi-metals which recently attracted attention due to theoretical predictions of topologically non-trivial electronic structure, as well as experimental observations of striking transport and magnetic properties. Firstly, we study EuCd2P2 [1], a material which stands out due to the unusual temperature dependence of its resistivity: metallic behavior at high temperatures is followed by a 100-fold increase of resistivity with decreasing temperature [2]. Remarkably, resistivity peaks at 18K, well above the Neel temperature of 11K. Using a powerful combination of experimental probes, we reveal a far richer picture of EuCd2P2 magnetism than has been previously reported, helping to resolve this mystery. Furthermore, we investigate the magnetic structure and dynamics of EuIn2As2, a material considered to be an axion insulator candidate. Neutron scattering revealed an onset of a complex ‘broken helix’ magnetic state at 16K, preceded by a helical spin order stabilized in a narrow temperature window of 16K-17K [3]. We build upon this work to demonstrate subtle modifications of the two magnetic structures and show how those structures arise due to the frustration between long-range magnetic exchange promoted by the conduction electrons and the magneto-crystalline anisotropy. The same interactions account for unusual Goldstone-like magnon excitations of the broken helix, which we observe.
Taken together, our results emphasize the richness of the Eu-based low carrier density materials, and the importance of the interplay of the conduction electrons and the localized moments in determining their electronic and magnetic properties.
[1] Sunko, V., Sun, Y., et al., 2022, arXiv, 2208.05499
[2] Wang et al., 2021, Adv. Mater. 33(10), 2005755
[3] Riberolles, S. X. M., Nat Commun 2021, 12 (1), 999