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

A layered transition-metal chalcogenide Ta 2 NiSe 5 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 Ta 2 Ni(Se 0.6 S 0.4 ) 5 . The gap collapse is delayed relative to the photoinduced conduction band population and spectral broadening as commonly observed in Ta 2 NiSe 5 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.