I. Zeljkovic Colloquium: Imaging symmetry breaking at the atomic scale in kagome superconductors

Kagome metals have emerged as a vibrant playground in materials physics, where geometric frustration, electronic correlations, and band topology intertwine to produce a variety of exotic properties. The kagome superconductors AV₃Sb₅ (A = K, Rb, Cs) and, more recently, CsCr₃Sb₅ have set off an avalanche of theoretical and experimental efforts to understand unusual electronic phenomena, including various density waves, unconventional superconductivity, and time-reversal symmetry breaking. In this talk, I will first provide a brief introduction to the rapidly developing field of kagome metals. I will then present our experiments on AV₃Sb₅ and CsCr₃Sb₅, where we uncovered a cascade of symmetry-broken electronic states. We observe clear signatures of spontaneous rotational symmetry breaking in the electronic structure, revealing the tendency of this family of materials toward electronic unidirectionality. Spatial mapping further uncovers unique features associated with a new form of density wave that acts as a precursor to superconductivity in CsCr₃Sb₅. Our experiments reveal a rich landscape of coexisting electronic states in kagome superconductors and highlight the crucial role of nanoscale imaging in advancing their understanding. Ilija Zeljkovic obtained his Ph.D. in Physics from Harvard University in 2013, where he built a low-temperature STM and used it to study the interplay of structural, chemical, and electronic properties of high-Tc superconductors. He is currently a Professor of Physics at Boston College. His research focuses on molecular beam epitaxy synthesis and nanoscale characterization of quantum materials using advanced scanning tunneling microscopy and spectroscopy techniques. The quantum materials of interest include unconventional superconductors, correlated electron systems, kagome metals, topological materials, and magnets. Zeljkovic is the recipient of the DARPA Young Faculty Award (2017), NSF CAREER Award (2017), ARO Young Investigator Award (2017), DOE Early Career Award (2019), Marko Jaric Award (2023), and is a Moore Experimental Physics Investigator (2024).