Strong Electron-Phonon Coupling in the Excitonic Insulator TaNiSe.

An excitonic insulating (EI) state is a fantastic correlated electron phase in condensed matter physics, driven by screened electron-hole interaction. TaNiSe is an excitonic insulator with a critical temperature () of 328 K. In the current study, temperature-dependent Raman spectroscopy is used to investigate the phonon vibrations in TaNiSe. The following observations were made: (1) an abnormal blue shift around 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 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 TaSe octahedra are "frozen" and the NiSe tetrahedra are greatly distorted below . Thus, it seems that the distortion of NiSe tetrahedra plays an important role in the strong electron-phonon coupling (EPC) in TaNiSe, while the strong EPC, coupled with electron-hole interaction, opens the energy gap to form the EI state in TaNiSe.