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

Abstract

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. The authors show that van der Waals Nb3Cl8 combines switchable electric polarization, unusual flat electronic bands and chiral lattice vibrations, offering a versatile platform for studying quantum materials and developing nanoscale devices.