Observation of flat bands in breathing kagome semiconductors*

Quantum materials with kagome lattice – comprised of corner-sharing triangles forming a hexagon in the crystal structure - have been studied as the potential playgrounds for exploring the interplay among parameters such as geometry, topology, electronic correlations, magnetic, and charge density orders. Niobium halides, Nb₃X₈ (X = Cl, Br, I), which are predicted to be two-dimensional magnets, have recently received attention due to their breathing kagome geometry. In this talk, I will discuss the electronic structure of Nb₃X₈ system revealed by angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. ARPES results depict the presence of multiple flat and weakly dispersing bands. These bands are well reproduced by the theoretical calculations, which show that they have Nb d character indicating their origin from the Nb atoms forming the breathing kagome plane. These van der Waals materials can be easily thinned down via mechanical exfoliation to the ultrathin limit and such ultrathin samples are stable as shown from the time-dependent Raman spectroscopy measurements at room temperature. These results demonstrate that Nb₃X₈ system is an excellent material platform for studying breathing kagome induced flat band physics and its connection with magnetism.