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

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.

Figures (4)

• Figure 1: (a) Schematic of time-resolved TFISH. Atomic displacements of soft and AFD modes are also sketched. (b-c) TFISH traces at positions A (low oxygen-vacancy density) and B (high oxygen-vacancy density) at 12 K under multi-cycle resonant (center frequency of $\sim$0.65 THz and bandwidth of 0.25 THz) and single-cycle excitation for different field strengths (100%, 60%, 40% of peak fields $E_m^p\simeq220$ kV/cm and $E_s^p\simeq1.1$ MV/cm). Striking position dependence highlights the pivotal role of defects.
• Figure 2: Temperature evolution of TFISH under resonant multi-cycle excitation ($E_m^p \simeq 220$ kV/cm) with a pulse duration of $\tau^m_{\text{THz}}$($\sim$5 ps). (a),(b) Time-domain traces at positions A and B for several typical temperatures. The solid lines indicate the non-oscillatory components approximated using the exponential decay fitting Cheng2023. (c),(d) The non-oscillatory component after $\tau^m_{\text{THz}}$, attributed to the second-order susceptibility $\chi^{(2)}(t)$, as a function of temperature in the time domain. (e),(f) Fourier spectra of the oscillatory components extracted after $\tau^m_{\text{THz}}$. Position A exhibits clear maxima near $T^*\simeq 28$ K for the non-oscillatory component, and subsequent softening-to-hardening of collective modes, while position B shows monotonic behaviour characteristic of defect-dominated dynamics. Dashed lines are guide for eyes.
• Figure 3: Single-cycle excitation at $E_s^p \sim 1.1$ MV/cm with a THz pulse duration of $\tau^s_{\text{THz}}$($\sim$2 ps). (a-b) Time-domain TFISH traces at several typical temperatures in positions A and B. (c),(e) Fourier spectra after $\tau^s_{\text{THz}}$ (left) and after 5 ps (right) in position A, respectively. (d) Fourier spectra after $\tau^s_{\text{THz}}$ in position B. (f) The amplitude of non-oscillatory component at time indicated by the dashed line in (a) as a function of temperature. Dashed lines are guide for eyes.
• Figure 4: Competing THz-driven pathways in quantum paraelectric SrTiO$_3$. Left (oxygen-vacancy-sparse region, position A): Exciting coherent AFD rotations transiently suppress dipolar correlations within $t_0$ (orange arrow), e.g. $t_0\simeq$5 ps for single-cycle pulse with $E_s^p$=1.1 MV/cm. After decay, transient ferroelectricity driven by coherent soft-mode and THz-induced correlations of PNRs re-emerge (blue arrow). Non-oscillatory TFISH signals include competing contributions determined by the second-order susceptibility tensors: $\chi^{(2)}_{\text{FE}}$ and $\chi^{(2)}_{\text{PNRs}}$. Left (oxygen-vacancy-rich region, position B): Soft and AFD modes are heavily damped; a persistent defect-induced low-frequency mode ($\sim$0.1-0.3 THz, dark-green oscillation) dominates and frustrates global coherence, preventing transient ferroelectricity and suppressing dipolar correlations of local polar clusters (PNRs). Non-oscillatory TFISH signals are dominantly contributed by $\chi^{(2)}_{\text{PNRs}}$.