Electron-Phonon Coupling Mode in Excitonic Insulator

Ta2NiSe5 is considered a promising excitonic insulator (EI) candidate with slight phonon contributions, since it exhibits a tiny orthorhombic-to-monoclinic structural distortion at 328 K without any superlattice structure. Our synchrotron inelastic x-ray scattering measurements reveal strong electron-optical-phonon coupling occurring at temperatures higher than the transition temperature. Density functional theoretical calculations indicate that two coupled optical modes arise due to the vibration of Ta and Se ions. Further, the two modes are frozen such that Ta and Se approach each other, forming atomic-displacement-type electric dipoles in the monoclinic phase. The characteristic of electronic toroidal moment formation by the antiferro-arrangements of electric dipoles is the universality of EI between Ta2NiSe5 and 1T-TiSe2.