Strong Electron-Phonon Coupling in the Excitonic Insulator Ta2NiSe5.

An excitonic insulating (EI) state is a fantastic correlated electron phase in condensed matter physics, driven by screened electron-hole interaction. Ta2NiSe5 is an excitonic insulator with a critical temperature (TC) of 328 K. In the current study, temperature-dependent Raman spectroscopy is used to investigate the phonon vibrations in Ta2NiSe5. The following observations were made: (1) an abnormal blue shift around TC is observed, which originates from the monoclinic to orthorhombic structural phase transition; (2) the splitting of a mode and two new Raman modes at 147 and 235 cm-1 have been observed with the formation of an EI state. With the help of first-principles calculations and temperature-dependent X-ray diffraction (XRD) experiments, it is found that the TaSe6 octahedra are "frozen" and the NiSe4 tetrahedra are greatly distorted below TC. Thus, it seems that the distortion of NiSe4 tetrahedra plays an important role in the strong electron-phonon coupling (EPC) in Ta2NiSe5, while the strong EPC, coupled with electron-hole interaction, opens the energy gap to form the EI state in Ta2NiSe5.