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 (T_C) 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 T_C 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 T_C. 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.