Ta2NiSe5 is considered as an excitonic insulator candidate where Coulomb interaction between electrons and holes leads to an ostensible exciton condensation. However, phonons are considered to play an important role [1]. Using inelastic x-ray scattering, an anomalous shortening of the phonon lifetime near Brillouin zone center is observed, indicating the instability of the phonons. Combining the electronic density of state extracted from angle-resolved photoemission spectra with the imaginary part of phonon self energy, the electron-phonon coupling strength is estimated to fall in a strong coupling regime. We also report structural hardening with S-doping indicated in the low-energy phonon spectra. In this process, rapidly dwindling electron phonon coupling is seen to accompany the precipitous drop of the structural transition temperature.

[1] C. Chen et al. arXiv:2203.06817