Detecting electron-phonon couplings during photoinduced phase transition

Takeshi Suzuki, Yasushi Shinohara, Yangfan Lu, Mari Watanabe, Jiadi Xu, Kenichi L. Ishikawa, Hide Takagi, Minoru Nohara, Naoyuki Katayama, Hiroshi Sawa, Masami Fujisawa, Teruto Kanai, Jiro Itatani, Takashi Mizokawa, Shik Shin, Kozo Okazaki

Research output: Contribution to journalArticlepeer-review

Abstract

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 [1] [2] [3] [4] [5]. 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 [6]. 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 [3] [7], time- and angle-resolved photoemission spectroscopy for electronic band structures [8] [9], and time-resolved X-ray measurements for crystal structures [10] [11] [12], 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.

Original languageEnglish
JournalUnknown Journal
Publication statusPublished - 2020 Feb 23

ASJC Scopus subject areas

  • General

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