Intense terahertz (THz) pulses induce transient inversion-symmetry breaking in quantum paraelectric SrTiO3, yet the underlying mechanism remains controversial. Using fields up to ∼1.1 MV/cm, we reveal spatially inhomogeneous THz-field-induced second harmonic generation (TFISH) governed by competing lattice and defect dynamics. Short-lived coherent antiferrodistortive (AFD) modes suppress dipole correlations within ∼5 ps, while heavily damped soft/AFD modes and a defect-induced low-frequency mode (∼0.1-0.3 THz) jointly prevent long-range ferroelectric coherence in oxygen-vacancy-rich regions. Collective modes manifested by oscillatory TFISH components exhibit softening followed by hardening below a critical temperature T∗≃28 K, confirming transient ferroelectric order where defects are sparse. These results reconcile conflicting interpretations, establish defect-mediated competition as a central regulator of light-induced ferroelectricity, and open routes to ultrafast control of quantum materials.
@article{arxiv.2512.01253,
title = {Competing Lattice and Defect Dynamics Govern Terahertz-Induced Ferroelectricity in Quantum Paraelectric SrTiO$_3$},
author = {L. Cheng and K. Hu and S. Yang and Yan Liang and Jiandi Zhang and J. Qi},
journal= {arXiv preprint arXiv:2512.01253},
year = {2025}
}