Breathing ferroelectricity, flat-bands and multiple chiral phonons in van der Waals breathing kagome Nb₃Cl₈

Van der Waals layered Nb₃X₈ (X=Cl, Br, I) are predicted to integrate multiple intriguing physical properties, notably multiferroicity and flat-bands that are highly promising for novel-principle memories and superconductors. However, direct experimental validations such as the proposed breathing ferroelectricity are lacking. Even the electronic band structure concerning flat-bands still remains inconsistent. Here, we systematically explore the structure and electrical/optical properties through a multifaceted study of Nb₃Cl₈. We demonstrate that, with an intralayer breathing Kagome lattice and an alternating van der Waals gap between adjacent layers, Nb₃Cl₈ can exhibit breathing ferroelectricity originating from a breathing-in/out distortion of alternate triangles via a Kagome state. More strikingly, our optical characterizations disclose that Nb₃Cl₈ shows the exciton-like absorption peaks involving flat-bands in thin states but semiconductor-like absorption edge in thick states possibly due to both interlayer interaction and thickness-induced stronger photon absorption, whose energies match our theoretical calculations, and possesses strong optical activity and multiple chiral phonons. Temperature dependencies of phonon modes and flat-bands consistently reveal several temperature-induced electronic-phase or magnetic-related transitions. The coexistence of these interrelated properties in Nb₃Cl₈ not only offers a platform for exploring fundamental condensed matter physics but also holds great promise for developing multifunctional nanodevices.