Photo-induced phase transitions have been intensively studied owing to the ability to control a material of interest in a flexible manner, which can induce exotic phases unable to be attained at equilibrium     . The key mechanisms are still under debate, and how the couplings between the electron, lattice, and spin degrees of freedom are evolving during photo-induced phase transitions has currently been a central issue . However, the measurements for each degree in the ultrafast timescale require distinctively different and state-of-the-art techniques, such as terahertz spectroscopy for low frequency conductivity  , time- and angle-resolved photoemission spectroscopy for electronic band structures  , and time-resolved X-ray measurements for crystal structures   , which prevents directly revealing the connections between each degree in the nonequilibrium state. Here, we develop a new analysis method, frequency-domain angle-resolved photoemission spectroscopy, to gain precise insight into electron-phonon couplings in photo-induced insulator-to-metal transitions. Regarding the electron-phonon coupling in the photo-induced insulator-to-metal transition for Ta2NiSe5, we find that multiple coherent phonons are generated as a result of displacive excitations, and they show band-selective coupling to the electrons. The lattice modulation corresponding to the specific phonon mode, where Ta lattice is sheared along the a-axis, is the most relevant for the formation of excitonic insulator phase and is the most effective to modulate the valence band top in the photo-induced semimetallic phase. Furthermore, we developed a new theoretical method to analyse the mode and momentum selective electron-phonon couplings based on the density functional theory. Our novel analysis method can pave the way for quantum engineering utilizing correlations between electrons and phonons.
|Publication status||Published - 2020 Feb 23|
ASJC Scopus subject areas