Electron-Phonon Coupling Mode in Excitonic Insulator

TL;DR

This study investigates the electron-phonon interactions in Ta2NiSe5, revealing strong coupling and coupled optical modes that contribute to its excitonic insulator properties, with implications for understanding electronic toroidal moments.

Contribution

It provides experimental and theoretical evidence of coupled optical phonon modes and their role in the excitonic insulator state in Ta2NiSe5, highlighting atomic-displacement electric dipoles.

Findings

Strong electron-optical-phonon coupling above transition temperature

Identification of two coupled optical modes involving Ta and Se ions

Formation of atomic-displacement electric dipoles in the monoclinic phase

Abstract

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.