We report the large-amplitude coherent soft phonon dynamics dominated by anharmonicity for ferroelectric semiconductor near topological transitions In-doped (Sn,Pb)Te using high-field terahertz spectroscopy. The phonon frequency is dramatically increased through intense terahertz excitation of soft phonons having large dielectric response. The atomic displacement and phonon anharmonicity, which are quantitively retrieved from the field strength dependent response, are the most enhanced near the ferroelectric transition; the displacement is as large as the spontaneous ferroelectric distortion and the anharmonic positional free energy for phonons reaches up to 75% of the total one. The present findings are crucial for nonlinear phononics including terahertz control of emergent topological phases.

• Received 6 June 2023
• Revised 20 November 2023
• Accepted 5 January 2024

DOI:https://doi.org/10.1103/PhysRevB.109.L081102