摘要 (Abstract) ：

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 [1]. 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.

[1] L. Cheng, K. Hu, S. Yang, Yan Liang, Jiandi Zhang, and J. Qi, arXiv:2512.01253 (2025)