Photoinduced multistage phase transitions in Ta2NiSe5

Utrafast control of material physical properties represents a rapid developing field and has become one of the most attractive frontiers in condensed matter physics and materials science. Yet, accessing to the long-lived photoinduced electronic states is still in its early stage, especially with respect to the transition from an insulator to a steady metal phase. Here, by combing transport measurement with ultrashort photoexcitation and coherent phonon spectroscopy, we report on photoinduced multistage phase transitions in Ta2NiSe5. Upon excitation by weak pulse intensity, the system is triggered to a short-lived state accompanied by a structural change. Further increasing the excitation intensity beyond a threshold, a photoinduced steady new state is achieved where the resistivity drops by more than four orders at temperature 50 K. This new state is thermally stable up to at least 350 K and exhibits the lattice structure different from any of the thermally accessible equilibrium states. Transmission electron microscopy revealed a photoinduced structure transition showing in-chain Ta atoms displacement. We also found that nano-sheet samples with the thickness less than the optical penetration depth are required for attaining a complete transition with dramatic resistivity change, indicating an intrinsic cooperative multiple phase control of this electron-hole coupled system in an ultrafast fashion.