Disentangling the electron-lattice dichotomy of the excitonic insulating phase in Ta2Ni(Se1−xSx)5 with sulfur substitution and potassium deposition

Ta₂NiSe₅ is a promising candidate for hosting an excitonic insulator (EI) phase, a novel electronic state driven by electron-hole Coulomb attraction. However, the role of electron-lattice coupling in the formation of the EI phase remains controversial. Here, we use angle-resolved photoemission spectroscopy (ARPES) to study the band structure evolution of Ta₂Ni(Se_1−xS_x)₅ with sulfur substitution and potassium deposition, which modulate the band gap and the carrier concentration, respectively. We find that the Ta 5d states originating from the bottom of the conduction band persist at the top of the valence band in the low-temperature monoclinic phase, indicating the importance of exciton condensation in opening the gap in the semi-metallic band structure. We also observe that the characteristic overlap between the conduction and valence bands can be restored in the monoclinic lattice by mild carrier injection, suggesting that the lattice distortion in the monoclinic phase is not the main factor for producing the insulating gap, but rather the exciton condensation in the electronic system is the dominant driving force. Our results shed light on the electron-lattice decoupling and the origin of the EI phase in Ta₂Ni(Se_1−xS_x)₅.