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<p>A ferroelectric material often exhibits a soft transverse optical (TO) phonon mode which governs its phase transition. Charge coupling to this ferroelectric soft mode may further mediate emergent physical properties, including superconductivity and defect tolerance in semiconductors. However, direct experimental evidence for such coupling is scarce. Here we show that a photogenerated coherent phonon couples strongly to the electronic transition above the bandgap in the van der Waals (vdW) two-dimensional (2D) ferroelectric semiconductor NbOI<sub>2</sub>. 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 the 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.</p>

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Coupling of electronic transition to ferroelectric order in a 2D semiconductor

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年2月23日

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  • Chun-Ying Huang,
  • Daniel G. Chica,
  • Zhi-Hao Cui,
  • Taketo Handa,
  • Morgan Thinel,
  • Nicholas Olsen,
  • Yufeng Liu,
  • Michael E. Ziebel,
  • Guiying He,
  • Yinming Shao,
  • Connor A. Occhialini,
  • Jonathan Pelliciari,
  • Dmitri N. Basov,
  • Matthew Sfeir,
  • Abhay Pasupathy,
  • Valentina Bisogni,
  • David R. Reichman,
  • Xavier Roy,
  • Xiaoyang Zhu

摘要

A ferroelectric material often exhibits a soft transverse optical (TO) phonon mode which governs its phase transition. Charge coupling to this ferroelectric soft mode may further mediate emergent physical properties, including superconductivity and defect tolerance in semiconductors. However, direct experimental evidence for such coupling is scarce. Here we show that a photogenerated coherent phonon couples strongly to the electronic transition above 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 the 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.