Coupling of Electronic Transitions to Ferroelectric Order in a 2D Semiconductor

TL;DR

This study demonstrates strong coupling between a soft transverse optical phonon and electronic transitions in a 2D ferroelectric semiconductor, revealing new insights into the material's electronic and optical properties.

Contribution

It provides direct experimental evidence of phonon-electron coupling in a 2D ferroelectric semiconductor using terahertz spectroscopy and first-principles calculations.

Findings

Coupling occurs only with above-gap electronic transitions.

The soft TO phonon is responsible for ferroelectric order.

Coupling is absent in the valence band.

Abstract

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.