A ferroelectric material often exhibits a soft transvers optical (TO) phonon mode which governs it phase transition. Charge coupling to this ferroelectric soft mode may further mediate emergent physical properties, including superconductivity and defect tolerance. However, direct experimental evidence for such coupling is scarce. Here we show that a photo-launched coherent phonon couples strongly to electronic transitions across the bandgap in the van der Waals (vdW) two-dimensional (2D) ferroelectric semiconductor NbOI2. Using terahertz time-domain spectroscopy and first-principles calculations, we identify this mode as the TO phonon responsible for ferroelectric order. This exclusive coupling occurs only with above-gap electronic transition and is absent in the valence band as revealed by resonant inelastic X-ray scattering. Our findings suggest a new role of the soft TO phonon mode in electronic and optical properties of ferroelectric semiconductors.
@article{arxiv.2410.09238,
title = {Coupling of Electronic Transitions to Ferroelectric Order in a 2D Semiconductor},
author = {Chun-Ying Huang and Daniel G. Chica and Zhi-Hao Cui and Taketo Handa and Morgan Thinel and Nicholas Olsen and Yufeng Liu and Michael E. Ziebel and Guiying He and Yinming Shao and Connor A. Occhialini and Jonathan Pelliciari and Dmitri N. Basov and Matthew Sfeir and Abhay Pasupathy and Valentina Bisogni and David R. Reichman and Xavier Roy and Xiaoyang Zhu},
journal= {arXiv preprint arXiv:2410.09238},
year = {2024}
}