Impact of S substitution for Se on the photoinduced insulator-to-metal transition in an excitonic insulator Ta2NiSe5

A layered transition-metal chalcogenide Ta2NiSe5 is a candidate of excitonic insulator and exhibits a unique photoinduced insulator-to-metal transition. S substitution for Se is expected to drive the system towards BEC regime of the excitonic insulator phase diagram and to change the nature of the photoinduced phase. Here, we report a systematic time-resolved angle-resolved photoemission spectroscopy study on Ta2Ni(Se0.6S0.4)5. The gap collapse is delayed relative to the photoinduced conduction band population and spectral broadening as commonly observed in Ta2NiSe5 relatives. The delay time of the gap collapse in the S doped system is shorter than the pure or Co substituted systems indicating reduction of the electron-lattice coupling in the photoinduced states.