Competing Lattice and Defect Dynamics Govern Terahertz-Induced Ferroelectricity in Quantum Paraelectric SrTiO$_3$

TL;DR

This study reveals how competing lattice and defect dynamics under intense terahertz pulses induce transient ferroelectricity in SrTiO$_3$, highlighting defect-mediated mechanisms and ultrafast control possibilities.

Contribution

It uncovers the role of defect and lattice dynamics in terahertz-induced ferroelectricity, providing new insights into transient symmetry breaking in quantum paraelectrics.

Findings

Transient inversion symmetry breaking observed via TFISH.

Competing lattice and defect modes suppress long-range ferroelectric order.

Critical temperature for ferroelectric order is approximately 28 K.

Abstract

Intense terahertz (THz) pulses induce transient inversion-symmetry breaking in quantum paraelectric SrTiO$_3$, yet the underlying mechanism remains controversial. Using fields up to $\sim$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 $\sim$5 ps, while heavily damped soft/AFD modes and a defect-induced low-frequency mode ($\sim$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^*\simeq$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.