NanLin Wang’s group in ICQM at Peking University and their collaborators report the discovery of photoinduced multistage phase of Ta2NiSe5 in Nature Communications

Ultrashort laser pulse is not only a powerful tool to excite and probe nonequilibrium electronic processes in transient states, but also emerges as a useful method to induce phase transitions that may or may not be thermally accessible. The latter has drawn increasing attention because it enables ultrafast optical manipulation and control over material properties. The ultrafast switching from an insulating to a metallic state is particularly attractive, for it has high potential for device applications.

Recently, NanLin Wang’s group in ICQM at Peking University and their collaborators reports the discovery of photoinduced multistage phase of Ta₂NiSe₅ utilizing transport measurement under ultrashort photoexcitation, and they revealed its structural change by combing coherent phonon spectroscopy and TEM experiment. This work was published in Nature Communications [Nature Communications 12, 2050 (2021) ]. Prof. Nanlin Wang in ICQM ang Associate research fellow Dong Wu in Songshan Lake Materials Laboratory are the corresponding authors. Qiaomei Liu, a Ph D student in ICQM, Prof. Dong Wu in Songshan Lake Materials Laboratory and Prof. ZiAn Li in Institute of Physics are the leading authors of this paper.

They have found that, 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 reveals an in-chain Ta atom displacement in the photoinduced new structure phase.

Their measurements provide useful information about exciton formation in Ta₂NiSe₅ system. The phase of Ta₂NiSe₅ has been widely considered as a prototype excitonic insulator. The binding energy of exciton appears to be very sensitive to the lattice distortion. Their measurement reveals the presence of a nonthermal stable phase with subtle difference in structure relative to the pristine Ta₂NiSe₅. Therefore, the subtle structural change could induce a large change in the exciton binding energy.

This work was supported by the National Natural Science Foundation of China, the National Key Research and Development Projects.

Fig. 1 (a) The layered crystal structure of Ta₂NiSe₅. (b) The resistance of the pristine (blue) and PI-LR state (green) after a writing excitation ~3.5 mJ/cm². Inset is the schematic of the sample and experimental configuration. (c) Shot-to-shot resistivity change under different pulse fluence.

Fig. 2 Pump–probe response spectra. (a) Transient photoinduced reflectivity at various states. (b)The corresponding FFTs from transient reflectivity spectra. (c)The temperature evolution of the coherent phonon spectra of pristine state (upper panel) and the PI-LR state (lower panel).

Fig. 3 TEM structural characterization of both pristine and laser-treated Ta₂NiSe₅. (a) Perspective view of Ta₂NiSe₅ atomic model. (b) Projection view along [110] zone-axis. (c) [110]-oriented electron diffraction pattern and morphology for pristine Ta₂NiSe₅, (d) Atomically-resolved high-angle annual dark-field image taken along [110] zone-axis corresponding to (c). (e) Enlarged part outline in (d). Descriptions for (f, g, h) are the same as (c–e) but for hidden state Ta₂NiSe₅.